fUSACERL Special Report N-90116

OTiCl FILE COPY September 1990

of~ EnlneZr

Rowit LbohlryAD-A227 582

Cartographic Issues in the Development

of a Digital GRASS Database

byDT

Marilyn S. Ruiz D T

Jean M. Mossorsmith -' ELECTE. "71

The process of building a complete and useful database OCT 111990

for the Geographic Resources Analysis Support System (GRASS), a geographic Information system (GIS). Is complox and costly. If data are entered or manipulated incor- 1

rectly, the result can be a largo, costly body of useless

goographic Information. The likelihood of such mistakes

arises In part from a users misunderstanding of basic

differences between digital map data and conventional

analog paper maps. Additionally, a number of environmental planners have been faced with the necessity of

creating or editing GRASS databases without substantial

training in cartographic concepts or the use of a GIS.

This report provldes Information that will help mrnmizo

mistakes comim'nly made In the creation and modification

of GRASS databases.

EZ~ASS now used widely at military Installations, and

by numerous other public and private egencies. This

report answers some of the questions most frequently

asked by GRASS users throughout the public and private sectors. The contents of this report are drawn largely from the collective experience of researchers at the US.

Army Construction Engineering Research Laboratory (USACERL), where GRASS was develooed. Issues

addressed Include soltware-specific matters, the characteristics of digital cartographic databases, the relationship between digital and analog maps In general, and the pre- paration of analog map Information for conversion into

digital data. u r

Approved for public release; disir'ibution is unlimied. 90 io -i~ ()(
The contents of this report are not to be used for :dvertising, publication,

or promotional purposes. Citation or trade names does not constitute an

oflicil indorsement or il lrovai or the use or such commercial products.

The findings of this report are not to be construed ais an official Department of the Army position, unless so designated by other atuthorized

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REPORT DOCUMENTATION PAGETo ptd

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Septemiber 199 Final_ _ _ _ _ _ _

4 11LE AND SWSIRLS 5, FLQ4OINO NOWNJS

Cartographle issues in the Deveopment of a Digital GRASS Databaise WU-ZP9:R-IT-D-SCS

0 ALI19OIIS)

Mtarilyn S. Ruiz andl Jean N1. Mesmittt

: ~RP4A O WOrGANIZATION NAME(S) ANO ALX1(S*ES) 1, !CIWOE1MINORWIZtAI

U.S. Army Construction EnginecrIng W MkM

Researc~h Luboratory SR N-W016

P.O. B~ox 4005

Cliampaign, IL 61824-4005

9- SPOr4SOflINGMOWTOING AGENCY NAMC,(S) AND) A1COESES) t0i 6SWM4.MWeflT0"G

U.S. tDcpanment or Agriculiurc Soil Conservation Service AkYIErfWN DR

P.O. 2890

Washington, DC 20013

11 *UrPLEM'TM1YNOTES

* Copies are available froniliae National Technical lnrormation Service, 5285 Pori Royal Road,

Springrild, VA 22161 ___

I~.' 01IRIWOTIKAVAUAWITY sTA1CMC~r ISROIhCN

Approved for public release; distribution is unlimited.

TABSITIACI (Am.00 wv)I

'h11 proce~ss of Wuilding A completeW and u$Crull tlatabw fosr tid Gahc Resources Analysis Suport -System (GRASSI. a

gogr-iphic inrormation sysicm (CIS). Is complex and cosily. If data are entered or' matnipulated Incorectly. d~ie res$ult can$ -e it large.

vostly body ofr10 gsel oraphic Information. The likelihood or such mistakes arises in part from a wer's misiultcwanding of

b'sslc diffrendces between diitlS map data and conventional analog paper maps. Additionally, a nuniber of CuSvilptalJIN planners

have beent raced with the necesity of cresting or Wlilng GRASS databases withiout substantial training in cautographic conCepts or

the use of a CIS. Mils report ptovides infrmnation tImat will help minize mistakces comnmonly made. in the creation a=d modilcation or GRASS databasecs.

GRASS is now used -widely ti military instillations. arid by numerous other public and 1xivate agenics. This toext answers some

of the quesions most rrcqudinmly asked by GRASS users; thfou&).out the public and private sectors. Thw contents of this rcport are

drawn lirgely (ton the collective experwice or researchers at the U.S. As4Iny construction Engincering Research Laoaory

(LtSACEFRL i GRASS wats develop~d. Issues addressed include software-specifl. matters. the characteristics of diglial

ctogrpldic a. the relstionship between digital ond smilog-maps in general, and the preparation of analog map Inrortion

fo~r con'ersta AigitrJ-data.Djuipaient shops in the U.S. Army Directorates or &Entinecding and Housing (DElls) normally use

%Vritten iles t%, 1nir vcords alud inventories. and to author reotis. TMils report describes criteria fr selection and

m111pkennt:0 or car, Aly available. coznputcr-based vehicle and cquipineni maintenance management systems that.can

Ant(1t11,t1 dito~ tasks. Cap1abilities of uppropnate mianggememit systemis are dlescrjed. including the use of bar code technology, Two

syeWns 03l1 Symetcs' Shoplas. and Plrotoq-pe's EMSIPC) were selected fr~ test imiplementation at three Army installatioll .7011

L4w. VA. Fort Riley. KS; and Fwxt Meade. M4D). The report details the experiences or' cach, installation with the introducti,, a

eomputer-bised naongement system. and the benefits derived from that introduction. Recommendations awe made as to the

circumstantces unider which each system is more suitable.

14. SUaJECT lERMS; 15. NUJMBER OF PAGES

GRASS 49

Geogramphic Resources Analysis Support System 1&, PRICE CODE

gcogra phic information systems

17, SE~CURITY CLASSIFICATION it8 SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATIONI 20, UIMITATK)N OF ABSTRACT

or- REPORT j OFi-MIS PAGE I OF ABSTRACT SAR

Unclassified I Unclassiicd Unclassified

NSN 7SiO*O1-280-5500 St 8adForm298 pov. M9

Pmwactod by ANSI SWd 23o-I8

2WI-02
I

FOREWORD

This report was pn pred by the Environmental Division (EN) of the U.S. Army Construction

Engineering Research Laboratory (USACERL).

This work was sponsored by the U.S. Department of Agriculture Soil Conservation Service (SCS)

under its program, "Cooperadve Development and Support or GRASS Between USACERL and SCS';

Work Unit ZP9:R.FED-SCS, "GRASS-SCS." The SCS technical monitor was Dr. Richard Liston. The

USACERL technical editor is Gordon L. Cohen, Information Management Office.

Dr. Ravinder-K. Jain is Chiler or USACERL.EN. COL Everett R. Thomas is Commander and

Director or USACERL, and Dr. L.R. Shaffer is Technical Director.

2
CONTENTS

SF 298 1

FOREWORD 2

LIST OF FIGURES AND TABLES 4

1 INTRODUrc TON ...................................................... 7

Background

Objectives

Approach

2 GRASS CELL MAPS . .. . ................. ....... .. ......... 8

Cell and Ve tor Data Formats

Data Formats and Conversion Issues

Chosng the Correct Cell Resolution

3 D)tGITIZINGAS FOR A GRASS DATABASE ........ 14

Getting Started

Preparing an Analog Map ror Digitizing

Map Registration Special Is.suts in Digitizing

4 CARTOGRAPIIIC CONSIDERATIONS ....... 25

Map Scale

Coordinate Systenis and Projections Accuracy and Quality in a GRASS Database

5 CONCLUSION ....................................................... 39

Important Technologies ror Database Development

Importing Digiial Data in Otler Formals

Conclusion

REFERENCES 41

AP1ENDIX A: Drafling Materials 42

APPENDIX B: Conversion Factors ror Representative Fractions

(Map Scales) 44

APPENDIX C: A Checklist for Maintaining a High Slandard ror

Data i)evelopnent 45

DISTRIBUTION

3
FIGURES

Number Page

I Cell Map 8

2 Vector Map 8

3 The Same Arch Shown as Vector and Cell Data 9

4 Convening Linear Dataqo Cell Format 10

5 Two Ways To Represent Sites in Cell Format Ii

6 Map Informaion 16

7 Category Information ror Geograpic Features 16

8 Map Inrormation on Overlay 17

9 Coding Linear and Areal Attributes 18

)0 Reference Points 20

11 Extension or One UTM Grid Zone Into Another 21

12 Digitizing Threshold 23

13 Visual Depiction of Scale 25

14 Bar Scales 26

15 UTM Coordinate System 31

16 Aligning Overlay to Mhp 34

17 Aligning Adjoining Aerial Photographs 34

18 Reliability Diagram 36

TABLES

I Suggested Range of Resolutions for Common Map Scales 13

2 Acceptable Range of Residuals When Digitizing From Maps 22

3 Sci , and Resolution of Common Photographic Products 28

4
T'A III.,, - (C'is .d)

Number Page

4 Rcsolution or Common Satdllite Products 2'9

5 UTM Refcrence System Spacing for USGS Topographic Maps 32

6 Map Scale and Horiontal Naional Map Accurcy Standards 35

_- l') TIw: I.BC

Aly -itily d~

D | Av iI !iii u',d/oi

5,
I

CARTOGRAPHIC ISSUES IN THE DEVELOPMENT

OF A DIGITAL GRASS DATABASE

I INTRODUCTION

Background

The Geographic Resources Analysis Support System (GRASS) is an image processing and

geographic information system (GIS). It is used to manage, analyre, input, and display digital

represcntations of geogiaphic data. Initially developed by the U.S. Army Construction Engineering

Research Laboratory (USACERL) forenvironmental planners at miltlay installadons, GRASS is now used

in both Ihe public and private ectors to assess environmenal impats, evaluate site suitability, decect

change over time, manage resources, and model the -eTects of cnvironmcntal phenomcna acros a

landscape.

Building a complete and useful GRASS database is complex and costly. If not done properly, this

work can produce a body of expensive, but useless, digital data. Additionally, digital cartographic data

often is not well understood by the people who use it. Printed maps have always been an essential tool

in environmental planning, but a GRASS digital database is quite different from-the maps it will replace.

Many of the questions atise concerning the building and understanding of a GRASS database.

Objectives

Three major objectives of this report are:

1. To promote a clearer understanding of GRASS databases among users

2. To provide guidance to GRASS users who need to add map data to an existing database

3. To organize and make readily available at USACERL information on the construction or

GRASS databases.

Approach

The information in this report has been drawn from the collective experience of researchers at

USACERL involved in the creation and use of GRASS databases, the litcratuit of digital cartography, and

thc expertise availablc at USACERL in the areas of cartography and GRASS programming.

7
2 (RA'SS CHU:1,, MAINS

In GRASS, all analyses are performed on cell maps, oir- of the basic formats used to present digital catographic data. The user must thoroughly understmnd how cell maps portray geographic information in

order to make appropriate choices about cell resolution and understand the output of GRASS analyses. Some data, however, may be input as vector daa; this must be convened to cell format for analysis in

GRASS. The user must consider a number of Issues relating to the transformation of analog data into a

form that GRASS can analyze.

Cell and Vector Data Formats

All GRASS analyses arc perforni,.d on cell maps, which consist of cells anayed in a grid pattern, each containing a value to represent a corresponding geographic location (Figure 1). The terms "pixel"

and "raster data" arc sometimes used to describe this data format. The terms "cell" and "cell map" arc

used in this document for consistency with other GRASS documentation.

In addition to the cell map format, geographic data may also be stored in a vector format. In this

formal, th. data is stored as arcs, each or which begins and ends with a special point called a node (Figure

2). GRASS uses cell maps in its analyses, but vector maps arm used primarily for graphic purposes.

Vector analysis is not currently possible in GRASS.

GRASS converts data to cell maps in several ways, depending on te type or data. Geogmphic dta

is often classified as an area, a line, or a point. The following sections examine how these types of data

arc convened to GRASS cell maps. The user is then given general guidelines for choosing the proper

resolution for a cell map. Finally, a discussion or conceptual issues in cell data is presented.

Data Formats and Conversion Issues

Much cartographic data is entered into GRASS already in cell format. Examples include satellite

imagery, scanned aerial photographs, and elevation data in the form of a digital terrain modcl.

22 1 4444414 o

22 1 1 4 42444 4"

4 4 1 1 1 1 2 2 4 14 14_ i/

24444444444. 4 4 1 44 41414

12 14 4 4 4 44 4 4 4 4 "

Figure 1. Cell map. Figure 2. Vector map.

*Otlwr compfcrized systems dealing with carographic dra may use different terms for describing these concepts.

8
As prcvioly mentioned. doat will also come to the user in a vector format. 1hetm am a number

or vector formt produced by various systms.

The GRASS 3.1 vector format Is known as the digit formal. For the puposes of this report, vector

data Is assumed to be In the digit format. T may equtIr the user to cony n other vector form~as to

digit, or to use GRASS to digitize analog Information from printedmaps. GRASS 3.1 ouputs data In the

GRASS digit format.

* Another Importam format of GRASS data is called t "ites list." A site Is a eographic location

where "it feature is found; a sites list Is a list of coordinate pair. describing the locaion of several

features. The form*- of a sites list is described in the GRASS User's Reference Manual entry sitesforma(3):

To be used in GRASS analyses, vector and sitcs data must be convened to the appropriate cell

format.

* Converting Areal Data to Cell Format

Examplcs or arcal dala include such themes as clssificationu of geology, land use, and vegetation

cover. The GRASS program vect.w.cell Is used to make cell maps from vector maps coptining areat

data. Figure 3a shows a portion of a soils map, with a value in each polygon indicating the type of soil

it represents. Figure 3b shows the sam Informadon as it would appear in a GRASS cell map. The

overlaid grid illustrates how GRASS might divide the map to store it as cells. On a color monitor or

printout, the different values in the cells would be represented in different colors. The original polygon

boundaries arc included in Figure 3b to Illustrate t:it cell values art assigned ccording to the polygon's

value at the center of each cell in the overlaid grid.

1 4 101 5 5

1 " 4 11 5 5 5 22 22

10101 4 1 5 5 22 222

221010 10 1 5 5 2222 22

10 2 2- 10 10110 1 5 222

10011 70 222

a. b.

Figure 3. The same area shown as vector and cell data.

For more inrormation on the digit formai, see "GRASS 3.0 MAPDF.V Vector Formal," "GRASS MAPDEV Atwibuto Fil

Fonnam," and "GRASS MAPDEV Digit File Forma" in James Westervel. Michael Shapiro. William D. Goran. et Al., GRASS

UO¢r's Rrference Afanual, Automatic Da Processing (ADP) Report N-97/22 (USACERL, SeXember 1988).

For more information on sites, see the entries for .TiiesI), sil.occuK3), siles.report(3), and sitesS(3) in James Westervelt.

Michael Shapiro, William D. Gorm, eg al.; and Michael Shapiro, James Westervclt, et al., GRASS 3.0 Programwer's Mwwal,

ADP Report N-89/14 (USACERL, Septembrr 1989).

9
Converting Liar Data to Cell FormW

Linear map daia includcs such themes as roads orasreams. lkcause GRASS perrorms wAlysez. only

on cell data, lines must often be con eisd to cell maps. Unear data starts as a vector file, mn is

convened with the GRASS pmrgrm yeico.¢ 1, just as *real data Is. The Bresenham algorithm, which

converts linear data inhowedl or~eiu,- -ifffercn, howver, from the one od to convert ar al data. It

creates a cell map in moch tl. sa way a line is dist-lyed using the pixels of a color graphics monitor. The algorithm makes cc~isIo about the eie loo-of the line based on which cells will most closely

represent the original lie (Filmly 4). A. a s. sorccells that actually contain a pMn of a line will not

be activated if that would:x,),tod-t. cscicNt of-ihe line. This algorithm is not perfect, but it gives a

better rendition of liear dwik-fhtn would the t.lcr point plotting method used for weal data.

Converdng onW*t DNoi to Ceil Fornvrt

Archaeological sites, heticopter pads, and windmills am examples of pointal data. Pointal data Is

convertcd to cell format, by the GRASS sties program.

The sites program allows the user to choose one of two methods by which points are converted to

cell tromiat. One method is based on 4 simple binary pr sencetabsence concept. Empty cells ar assigned

a value of 0. Any other value indicates ihat a site is present. rigure 5: rep sents the location of sites

in the original sites list. Figure 5b illustrates the prseclabsence concept. The value is assumed to bc

I unless a pound symbol (0) is placed berore the description number in the sites list ile, which Indicates

that the site number should be used as the cell value. The other method of conventing pointal data to cell

format assigns values that indicate the number of sites located in a cell. A cell without a site would get

a value of 0, a cell with one site would get a value of 1, two sites would be represented as a 2, and so

on (Figure 5c).

Choosing the Correct Cell Resolution

Choosing the resolution of a cell map can be a complex process. First, and most importantly, the

user must understand exactly what is meant by cell resolution. Then, when a new cell map is created, a

number of conceptual and practical issues must be addre&ed.

,--- 1 1~

\ /1 1_

.1"1111"' 1 1 1111

Figure 4. Converting linear data to cell formal.

10
1 1

,, '1 1 1 1

-- - - ---

0. ANALOG SITES b. PRESENCE /ASSENCE . UI MBER OF SITES

DATA WMTHO IN CELL

Figure S. Two ways to represent sites in cell formt.

General Issues in Cell Resolution

\Vhen tho resolution of a cell is measured In meters, the number or meters-specified on the GRASS

window statement indicates how many meters one side of a cell wilr represent. A low-rcsolution map might use a cell to represent 100 meters or more. A high.r.solution map may use a ceil to represent about

10 meters per side. Esentially, the ictl of resolution determins the area or the piece or the earth to be

represcnted by one cell.

'Ih GRASS user selects the desired resolution before creating a new map. Iis is done by designating the desired resolution for the current GRASS window. In most cas, tle reso.tion or tl

current window dictates the resolution or the cell map. (An exception to this is the program slopeaspwct, which uses the recoution of the original elevation file to set the resolution of ncw cell files regardless or

the current window setting.) Unless the user actively changes the resolution, it retains its value rrom the

pitvious session. Thc current winidow resolution may equal the d.,fault resolution, but it may also be st

unusually low because somebody wanted to see a rough view of a cell map. The user must always be

aware of lie current window settings before creating a new cell map.

Once the resolution of a new cell map is set, the geographic detail available % that map is fixed.

"lie user may subsequently use lhe GRASS window command to change :he working resolution of a cell

map. but dhe original reolution remains intact. Increasing the resolution of an existing cell file will only suldivide the information from the existing cells. It is important to understand that increasing the

resolution of an existing cell map cannot actually access more data than was originally present.

Comceptual Issues in Cell Resolution

When creating a new GRASS cell map, ome data may already be in cell format, and some may con: from printed maps. Each kind of data must be treated differently.

ThlC to most important UxatmplCs of data existing in cell fonnat are satellite images and digital krr, n minols. rie resolWlion or these types or data is generally dictated by their original resolution. In

satellite imagery, tie original resolution is the result of that satellite's technical limitations, In digital terrain models, the original resolution is based on the scale or tl source material and the sampling density of the points taken from that source? Outside of resampling to limit the size of the fi.,ral cell map, little

is gained by changing the original resolution on tlie finished GRASS cell map.

Sec Stuart Bradshaw and Pam Thompson. Options for Acquiring Ell'aaion Data, Technical Manuscript (TM) N-89/20

ADA220934 (USACERL, January 1988). for a description of some standard digital elevation model (DEM) products.

I1
The choice of cell reolution is not clear cut in the case of daN from analog maps. There are at

least two ways to decide, but t user must determine which alternative is best.

First, the user might use the map's scale and overall quality to determine the appropriate resolution.

Large.sciic maps may be accurately preaented by relatively high-resolution cell maps; small-scale maps

should be represented by velatively low-resolution cell rmap, (Sec tlb', section entitled Map Scale in

Chapter 4 for a ditcussion of map scale.)

Table I may be used as a general pideline for considering map scale, The resolutions suggested

in tl, table are based on the assumption that a printed map of moderate scale may be resolved to a level

of between 0.73 mm and 1.5 mm. That is, an area of about one square millimeter, as measured on k

printed map, would be Included as a single cell in a cell map. These measurements are not fixed, but they

have been detcrmint- d p-nially by experience. Also, it has been said that quality American maps am

accurate to about I millimcier.4

The table values were determined as follows. If, for example, the scale of a map is 1:24,000, a

distance or 0.7S mm (the lower end of the resolution range suggested above) would represent I meters

(0.75 mm X 24,000 x 18,000 mm). The value nr 18 metcrs is rounded up to 20 for use In the table.

Therefore, the cell resolution of a btter-than-avetage digital representation of a 1:24,000 map would be

20 metcrs.

Using the guidlines from Table 1, the resultant cell maps would be more or IC "chunky"

depending on the scale and quality or the soure map. It can be argued that this roughness might be a

fairer representation of the feature being depicted than would a smoother one. In the case of ambiguous

boundaries, such as those on a soils map, the rough cell version may actually be conceptually more

accurate than a smooth line,

The user's other option is to follow the original map as closely as possible by choosing a highresolution cell grid. It can be argued that this option is better because there may be no good reason to

add ambiguity to an already imperfc represntation. The scale of the map would thus be a factor

prininrity in a map-user's assessment of the data's value for a particular application.

Although these two methods of setting cell map resolution are conceptually different, in practice the

final result is often similar. However, a smaller scale map is more likely to be represented at a higher

resolution by the second method than the first.

Practical Issues in Cell Resolution

Several other practical considerations affect cell resolution decisions. One is that GRASS cell files

can be quite larg%, and the higher the resolution, the larger the file will be. The user may find that a

dalaset large enough to reach the limits of the computer's memory and disk storage may be cumbersome

to use. GRASS can compress cell maps, however, to take up less computer space. Once compressed,

high-resolution cell maps with a lot of "empty" space (e.g., linear themes) will take up little more space

than they do at a lower resolution.

'Andrew M. Glusic, The Poslionl Accuracy of Maps, Teci ,nal1 Report (TR) No. 35 (U.S. Arny Map Service [AMS],

Wkshingion, D.C.. 1961). p 15.

12
TAbtW I

Sigted Rangt or Rtsolullons For Common Map Scles

Map Scole Re*ult on Range

1:15,840 10.-2 meters

1:20,000 15-30 meters

1:24,000 20-35 meters

1:31,680 25-50 meters

1:50,000 40-75 meters

1:63,360 50-95 meters

:75M0X0 55-110 meters

l:l00,000 75-1.0 meters

Another practical consideration is tha(- all maps used in a single analysis must be read at til same

cell resolution. Many analyses may require cither satellitc imagery, a digital terrain model, or both. As

discussed above, data from these types or maps is best used at its original resolution. In such cases, data

from printed maps should probably be rmade to match the resolution of lhe satellite image or terrain model

used in the same analysis. The re solution of satellite images and terrain models tends to range from 20

to 30 meters, so this is oflen the practical limit of resolution at which any layer of map data should be

set. (A map layer Is a set or canographic data representing common geographic features found within a

mip's boundarie.s)

Somtimes tie designation or a lower resolution is useful to give a quicker(but rougher) view when

dkplaying or printing a map layer. Ilie user should be aware that using a lower resolution may cause

cerimin features-)anicularly small polygons and horizontal or venical lines-o disappear.

In certain cases it may be desimble to represent linear themes and sites by a resolution that reflects

Mhe tne measure of the 1eature's width. A road that is about 10 meters wide, for example, could be

represented-by cells at a resolution of 10 nccrs. Sites may be convened to cells at a resolution that

reflects the average measure ofr the sites involved, if they are all about the same size. Another approach

would be to set cell resolution to equal a standard unit of mcasure-Im, for example. This approach

would be helpful for estimating linear distances. Because, in this example, the cell size would represent

one square mcer, the area given for the road or stream (as listed on a repon of areas) would provide a

good approximation of the feature's length.

WiCn converting sites data to cell romat the user should be aware of how the choice of resolution

and mehod of representing the data will affect the character of the resuhant cell file. Using thc

pre,,ence/absence method or representing sites, a lower resolution might cause two sites to fall within the

same cell, and consequently they would be counted as one. On the other hand, using the sites-in-cell

method, a lower resolution would cause the grouping of two (or more) sites in one cell, but its value

would actually reflect the number of sites present. The user must choose the method and resolution most

appropriate for a particular analysis.

13
3 DIGTZING MAPS FOR A GRASS DATABASE

Getting Started

Before any catogrphic dau Is xally digitized. prtparatlo oft dusa is n"qired. T1e user

must also decide whethr t digitizing will be done manually or by an electortescannn device.

Creating a Dambase

:A database Is a collection of map layers cotaining various geographic themes for a given location.

At USACERL, databae have been developed primarily for environmental offices at military Installations.

Thee map help military environmentalists make decision about land management.

When creating a database, several isstu must be resolved. Firm, it must be decided which map

layers will be needed in the database. In general, the choke of layers depends on the location for which

the database is being developed. In the case of databases created for military environmental offices, it

depends on the duties of the environmental office at the installation. A list Is made of all map layers that

would be useful for the environmentalist's applications. Second, the GRASS data developer must determine how much of this information Is available in digita or nondigital formalO If the information is not

available, the user must consider the practicality of creating the map layer. Finally, the user must decide

how many laycr will be created and In how much detail. Thes decisions must be based on the purposes

for which the layer is to be used. For example, if the environmentalist wants to know where roads on the

installation ar located, but is not concerned with what types of mads ar there, all mads would be put into a single layer called "roads." If, however, the environmentalist needs to know whether the roads are

primary, secondary, or trails, a separate category would be developed for each type.'

,Manul Dight:ig vs. Scanning

When it has been determined which nondigital map data will be added to a database, and how

detailed the categorization of the data will be, decisions must be made about how to convert the data into

digital form. Either a manual digitizing device or an automatic electronic scanner may be used to convert

analog data into digital form. The manual digitizing unit is a computer input device consisting of a table

and a cursor. This cursor, also called a "uck," somewhat resembles the well known computer "mouse."

However, the resemblance is superficial; the operator uses the cursor's crosshairs and specialized input

keys to tracc features'from an analog map and convert them into digital data. Any labeling of traced

features must be done manually.' The automatic electronic scanner converts the line information into

digital format by scanning the map and recording locations where it reads black lines. As with the manual

digitizer, labeling must be done by hand.

Several factors determine whether a map will be manually digitized or scanned. Generally, a map will be manually digitized if it contains a low density of geographic features. Also, manual digitizing may

For sources of digital data. see Mark D. Johnson and William D. oman. Sources of Digital Spa ial Data for Geographic

Infoormaton Sydo,. Technical Report (TR) N-880/ADA189788 (USACERL, December 1987); for sources of nondigital data,

see William D. Got and R.E. Riggins, Geoqrephk Materials to SPport Biophysical Quamnlative Envirmenlal Impact

Analys.is.ourcei ofEiiq Materials, TR N.68ADA069097 (USACERL, March 1979). 'For mote information abont the creation of a GRASS database, including charts of possible map layers and applications, see

Mary V. Matin et 4. GRASSIGIS Implemeaation Gid"e. Special Report (SR) N.90tO2 ADA214623 (USACERL, October

1989).

For a complete descripon of the manual digitizing process, see the GRASS tutorial "digit; Its Use and Its Features" in James

Westervelt, Michael Shapiro, William D, Goran, et ad.

-14
be required when the project deadline does not pemrii a Scanning contract to be awaxrd and computcr

time is availablc on thc digitizing unit.

A map will usually be scanned If them is a high deity of cographic fealue to be recorded and

t'ere is enough tme to award a scanning conract, or them is no manual dig-tizing unit available at the

site. These am only genrl guidelines, so special circumstance may require difrlrel'O* approaches.

Preparing on Ana!og Map for Digitiaing

Manual digitizing Is not usually done directly from a paper map. The following steps may be

necessary before digitizing a map by either method.

The Need for Rcdrating

In many cawe a map's featms must be rdrafted before the map can be digitized or scanned. When

a map is to be scanned, the main technical reason for redrafting is that many electronic scanners can read

only solid black lines. On most prinwd maps, features are represented by different colors, line weights,

or line patterns that a scanner cannot interpret. For U.S. Geological Survey (USGS) topographic maps,

it is possible to purchase in overlay (called a map separate) that shows a single gcograpitic theme in -black

ink. In many cases, however, the separate will no( be in a readyto-scan fonnat. For example, unwanted

linear or areai data may be included. When redrafting the features onto an overlay, the goal is a clean,

uncluttered map showing only one geographic theme. (As scanning technology improves, however, this

limitation is becoming ics.s important.)

Another reason for redrafting is the opportunity it provides to compile map information from

different sources onto a single overlay, thus requiring that only one mylar overlay need digitizing. During

the redrafting stage, decisions about the project area can be made. For example, if a database is being created for an installation, the drafter must decide whether data is to be taken only from within thc

installation boundaries, or up to a certain distance beyond the boundaries. Decisions about how the

features will be categorized can also be made at the time of redrafting. When aligning two adjoining

source maps, any discrepancies whemr they connect can be corrected. Finally, because any map is likely

to contain errors, the u:cr must make sure that corrections are made so they will not be required during

the digitizing procmss.'

Initial Documentation

During thc initial stages ora project, certain information should be recorded for later use during map

preparation. The dota developer should first list all source maps and documents provided for the project,

and who provided them. For maps, this list should include the map's name (c.g., watershed map, streams

map, etc.), scale, Universal Transverse Mercator (UTM) zone, source (e.g., USGS, Defense Mapping

Agency IDMA}), date, and other inrormation of importance (Figure 6). If other documentation is

provided, the user should record its tide, date, author, and briefly tmrarie its contents.

Next, record the geographic feature categories represented on the map, and the source from which

nhe categorics were taken, Each category must then be given an attribute code number. To assign these

codes, list all data categories, beginning with "no data." Then number each category, with zero

rcprcscnting "no data." Make sure the codes are consecutive numbers (Figure 7).

'Fleet, Harvey, Chief, Branch of Digital Cartography. U.S. Deparunent of the Inierior National Park Service (unpublished

interview, 29 May 1990).

15
FRoa Map

1:24,000

UTM zone 13

Source map from USGS topraphic map Dadwood, 1982

(Information taken from a myla smate rate than J pape map)

Solt Map

1:24.000

Source: from SCS Solis Sury foor Lawrence ,ourty , 1976

(FW soils sheets we no availe)

Figure 6. Map information.

Road CGttgodios Soo Categories

(using USGS oad dassilications) (using SCS soils classticaticons)

0 no data 0 no dati

I p!imaly highway i AB

2 scondary highway 2 Ba

3 rght duty toad 3 Bb

4 unimproved mad A BcA

5 trail 5 BcB

Figure 7. Category inrormiation for geographic features.

Map and Mylar Layout

After the initial work is complete, use drafting tape to secure the source map on a table, with the

frosted side facing up If the source map is on mylar. It is important to tape down the map as smoothly

as possible to avoid distortions while drafting. (Appendix A lists materials necessaty for the map

prepartion steps described here.) Next, cut a rectangular piece of mylar that will cover the project area

and Its reference points. (For mor inrormation about refetrnce points and registration marks, sce the

section Map Registraion later in this chapter.) If the map is to be electronically scanned, make sur heem

is enough room on the mylar, beyond a corner registration mark to record de legend information. Tape

the overlay to the table and gently clean it with rubber cement remover or alcohol on a soft cloth.

Tile UTM reference points am drafted on the overlay first. Using a 0 or 00 technical pen. trace the

registration marks from the map onto the mylar. Assign a unique number or letter to each reference point.

"rhen, in a comer of the mylar, record the following information: (1) title of the map project, (2) name,

date, scale, UTM zone, and source of the map, (3) the mapmaker's name and the date, (4) the Nouthing

and Easting coordinates for each of the reference points, and (5) any other important information. If

multiple sheets am needed to complete the overlay, use the bottom of each sheet to sketch the relationship

of that sheet to the entire project area (as in Figure 8).

16
Section IVA'

Soils Map io o

U.S.G.S Spearish quad 4

1:24,000, 1982

JTM Zone 13

Mcssersmith - April 1988

Reference Points:

N E

(1) 4515(XX) 760000

(2) 4499000 76000

(3) 4515( X) 772000

(4) 4499(00 772W0

Figure S. Map infornalion on overlay.

Draf ing the Iancr

11u: technical pen is used to carefully trace map features onto tl overlay. if a line is not drawn

properly, It can be erased with the gentle rub of a molsietd pen erser. Execesive eraser pressure,

lowever, may remove tie mylar's frosted coating, redering that porlion or the surfice unsufiable for

furtiar drafting. A razor blade or knife can also be used to comect lines, but these, too can daniage the

mylar. "leli.se tools are best used to touch up lines that will not be redrafted.

Asighning Attribute Codes

Cut a second sheet of mylar to approximately the same dimensions as the first.shet, and tape it over

the completed overlay. This top overlay is for coding the attributes recorded in the first overlay. Using

the technical pen. trace hli UTM intersections from tih four corner of the completed overlay onto the

new mylar. Label tlhcs intersections with their respective coordinates: thq- will be used to align the two

overlays Then assign each feature on the map its proper attribute code from the list completed earlier.

If the feature being labeled is a line, write the code on the line. If, however, the feature being labeled is

*in area, write the code insfide the polygon (as shown in Figure 9).

lhitiing

The final stage or maip prepar.aion involvcs editing the feature and attribute code overlays.

Although this stage can be tedious, it is necessary to ensure accuracy. It is often helpful to establish a

ltrge grid system on a sheet of clear acclate, and place it on the mylar overlay for use as an editing guide.

l'or editing a complex overlaty, the best approach is to first align and affix the feature overlay atop the

source map. and place die acclaie grid on top ofcvcrything. Editing section by section, make sure every

line has been drafted correctly. Next, remove the feature overlay and secure the attribute code overlay

on top of the source map. Check to ensure that all areal and linear features have been correctly labeled.

For a simple map, place both the feature and attribute code overlays on top of the map and edit both at

thc same time. When every feature is properly drawn and categorized, the map is ready to be registered.

17
+ + + + + + +

S 2 

23

3 6 4

LO NAR ATTRISUTE ATTRIBUTE COOK AREAL ATTRIBUTE ATTRIBUTE COCE OVER.AY OVIERLAY OVE" OVERLAY

+. + +

SUPEFRIMPO OVERLAY LINEAR SUPCRIMPOSMO OVERLAYAREAL

Figure 9. Coding linear and areal attributes.

Map Registration

lDcfore a map can be digitized, it must be registered so the digitizing table "knows" how the map

is oriented on the table, and to establish a relationship between the coordinates of the sounce map and

those recorded by the digitizing table. This step is especially important because it connects the graphic representation to an actual geographic location, which is the advantage or a GRASS database over a

simple picture, such as those created by non-GIS grphics software.

Mtarking Reference Points

Before the map is registered, referencc points must be marked on the source map. This process differs slightly depending on whether the map is to be manually digitized or electronically scanned.

For manual digitizing, reference points must be in the same units as the database. For example, the

unit will be meters when the UTM system is used. (See The UTM Coordinate System in Chapter 4 for

a description of the UTM system.) For the many standard topographic maps that already include a UTM

grid system, simply copy the intersection of a Northing and Easting grid line in each comer o' the map. While only four points are needed to register the map, up to 10 points dispersed over the map may be

used. Using a pencil, make a small cross mark at each comer of the map to show the intersection of the

grid lines. These points are then traced onto the mylar overlays (Figure 10a).

18
For automatic scanning of ap, te UTM reference points must be established in the four comners

outside the project area Most scanning compMnics rcqust only four points. If the entir map is the

project area, the coordinate symm must k extended beyond the map's borders. This is done by

measurlng with a niler the distance between two siccessMiv Northing grid lines, and tVo successive

Eat ing grid lines. At the northern edte of the map, measure one UTM grid dissne north from the

outermost Nonhing grid line. Using a pencil, mark that disiance at two or three points parAlel to th*

outermost Northing line. Align the edge ofa long rr with the pencil marks, and draw the new Northing

linc. Repeat this procedure at the wmuthcm, astem and western edges of the map. The refernce marks

are then established in each corner, outside the promjc area, by tracing the Intersections of the hewly

established Nonhing and Easting grid lines (igure 10b).

Sometimes the project a'a tr ,ces two UTM ons. On a large.scale map, there will be a 25.mil

overlap bctkvci zones (I mile x 1.609 kilometers). GRASS is not cv.,ble of interpeting maps from

more than one U1I zone unless one Yon's coordinate gdd is extended to labd the portion of the map

from the other zonc. This is illustrated by the dashed lines extrapolated frtom the left halfof the map into

the right (lgur Ii).

Reshlals and Map Registradon

* 'When the map is ready, it is set up on the digitizing table. When prompted by the GRASS digit

program, the operator entcs map coordin,.tes for the reference points, then uses the digitizer's cumor to record their table locations. The locations of these points ar processed to establish the relationship

Lbtween the two coordinate systems. The differnce between the values that the table coordinate system

expccts" from the map and the actual values entered for the registration points result in residuals, whicl

Are listed kt.ide each registration idn. Thwse residual values, which indicile the degree of a map's inaccuracy, ar givcn in map units (meters). The value of residuals varics with the scale of the map.

Thc digit progrm can accumtcly register a map with four points, one frnm each corner of a

relatively accurte map. In cases wher the map to be digitizcd is a composite of several source maps,

however, it is a good idea to take one or two points from each component or the composite.

When registering a map using the GRASS digit program, residual values should be kept as low as

possible. Since residuals are presented in map units, the acceptable value will vary with scale. The best

maps generally give lower rsiduals than less accurate maps at the same scale.

Table 2 gives some gencral guidelines for evaluating rcsidual values. le first column ofsuggestcd

values is suited to maps that are nearly ideal. Tlhe best source maps are of known accuracy and in

excellent condition-not folded or worn. An example might be a recent 1:24,000 USGS paper quad sheet

in good condition, or an acetate separate of it. The table's second column of values is suited for the cases

in which the source materials are less idcal. Pedaps several maps have been used to create the drafted

copy to be digitized, or maybe some of the source material is or dubious accuracy. During map

rcgir.fion, it is best to continue trying diffcrent registration points until the residuals reach acceptable levels. Digitizhg a tisp without proper registration will cause serious inaccuracies in the coordinate

rahes cf the resultant iigital map.

A uscful rule of thumb is to keep the averge of all residual values to the middle or lower end of

the range indicated in the table. No single residual should cxcccd die upper range. If it is-impossible

to get Ihe residuals- to this level, thc map may be too inaccurate to include in the database in its current

form. No amount of.atijusting the registration will bring an inaccurate map any closer to representing true

ground coordinates. (See Improving Map Data in Chapter 4 for ways of improving the quality of map

data before it is added to a GRASS database.)

19
1- -1- - - -. - - - - - - - - -1 -I --I -1 I - 1 I 1

I 1i in I t I -

I I V

:i1 In 1 i i n In1 In 1 I v I

i~~~~~ _~il~ltl,. 4 ~~ 

I I J : f0 0 IIIIM=IFT' OW, on 

-- . - ---- a o

in In In in i n Nn L V) J iI_2_ 

-I-I! --------------- I----"-_-7- I N. . . .. 0 , • ,-- _ - _- ,

~~0 m] m 0

0 ~ 0 ~ 0 0 0
230 Tj52 1 3 736 1137 2439240 241 2412 43

~451T

4Sjj4

44515

G5521
Table 2

Acceptable Range of Residuals When Digitizing 1rom Maps

Residuals for Residuals for Map Scale Most Accurate Less Accurate

Maps Maps

1:100,000 20 - 30 mcte 45 - 60 metcrs

1:5,000 5 - 10 meters 20 - 30 meters

1:24,000 1.2 - 2.4 meters 7 - 12 meters

:400 .5 - 1.0 meters 2 - 4 meters

h should be noted tht is possible to achieve a low residual averge and still have inaccurate

registration. To check for this possibility, the GRASS digit program provides an option tha allows the

user to move the digitizer cursor around the map and preview the coordinate values anywhere on the map.

If tls points seem inaccurate, the user should try to find better registwraion points.

Another way to rest the soundness of the map's registration is to make a test printout, called a "proof plot," which is compared with the map data being digitized. The user can digitize a few long lines

from the map data, then plot them and comrpar them to the soure. if they exhibit noticeable distortion,

the registration i' suspect. Otherwise, digitizing can continue using the currmn registradon points.

Tlhe procedures may sound too difficult to be practical, but need to be done only one time for each

map. Once the registration points have been established and verified, the same points arc used each time ie map Is digitized.

Spcial Issus in Digitizing

This section discusses some special aspects of capturing digital data from printed maps, including

thrnnsolds, snapping, and residuals.

DiVgid:Ing Threshold

Digitizing with the GRASS digit program may be done in either "point" or "stream" mode. In point mode, a point is recorded ;t each location specified by the operator. In stream mode, points ar- recorded

automatically and continuously as the operator traces the line. When digitizing in stream mode, the

equipment often picks up more points than are needed to adequately depict a line. To keep the size of

data files recorded in stream mode manageable, a "pruning" of unnecesury points occurs before the data

is saved. The term digitizing threshold describes a measure.'nent that GRASS employs to decide which

points to save and which to delete.

Pruning occurs each time the operator "accepts" (saves to disk) a line. GRASS achieves this by setting up a directional corridor the width of the digitizing threshold (Figure 12). The direction or this

corridor is determined by the first two points collected by the digitizer. GRASS examines all subsequent points to determine whether they fall within the corridor's limits. When a point lies outside the corridor, GRASS interprets this as defining a new direction for the line. The program saves the first point of the

22
cum nt line segment and the lst point that fills inside the same corrWor. The incrvcning points are

corsdered unnecsary to deflne the tine segment and ae discarded. A new corridor Is then estwblished

between the second saved poin: and the point collected Immediately after It. This process continues as

necessaty.

As a sult of this process, a cured porton of a linear ureru will contain mom points tha a

straight line segmem. In cae where the straIght line scpmeDnts ame lun and imnerspcmd with curves,

digiiing in stram mode Is about as efficient as In point mode. However, it Is not mrommandd that

long, straight line segments be digitized in stremm mode, because they are Oe more efficicntly In pow

rnode.

The default digitizing thmihold Is 0.03 Inches, or about 0.75 mm (I Inch -25.40 millimcier). This

thrshold works quite well for digitizing complex curved lin". If the defiult thrshold is not satisfactory,

it may be changed using the digit "Customize menu. A larger ftshold (e.g., 0.05 inches) will result

in fewer points being saved; a smaller thresotld (e.g., 0.02 inches) will result in mor points being ,ved.

resultant line

00

00

digitizing 0 Discard

threshold 10 Keep

Figure 12. Digitizing threshold.

23
Mop Threah

TIV tenn m w thres describes the sam parameter as digitizing thr shod, bt because it Is

measured in map units, It s directly related to the map's scale. For eample, suppose a map is being digitized using the UTM coordinate system. Mop units would, by definition, be meters, The default

digitizing Ot ld of 0.03 inches equals 0.O0762 mctr If the scale of the map being digitized Is

1:24,000, the map thrcshld would be 0.000762 multiplied by 24,000, or about 18.29 meters A map scal

of 1 :50,000 would give a default map threshold of 38.1 meters.

The map threshold may not be changed directly. It changes automatically with the digitl.ing

thrmshold.

Snapping Threshold

In vector maps, by definitio(i, two arcs am connected by a node. If two nodes are sufficiently dose

to one another, they will automatically merle, or "wsfp," Into a single point. The widest distance at which

snappin5 will occur is called the swipping thra.dd. The two ares connected to snapped nodes will join

at those nodes. In general, the node tha is second In order in the file will sM* to the one tha is irt.

The default snapping teshold is 0.04 Inches, As with the digitizing threshold, the snapping threshold

Is usually measured In distance as measured on t m.V. Occasionally, however, it is expressed as

distance on the ground. Whn expressed n distance on the ground, the number given fox the snapping

threshold dlepends on the scale of the map.

The main purpose or the snoapping threshold is to m2kkiaIt possible for the operator to end one line

and start a connected one at the "same node even though that point on te digitizing table activally

represents two nodes in the database. For the purposes of digitizing, GRASS treats tny two nodes within

the snapping threshold as a single node.

In most cases, the default snapping threshold is sufficient, but it may be chaoged using the digit

"Customize" menu, it necessary. TIere may be a situation when two nodes must be closer than the

snapping threshold allows. The user may want to capture the data die two nodes represent, but digit wants

to m M. them. Lowering the snipping threshold keeps the nodes separate. In such a cas, it is best to

leave the snapping threshold at the lower value when the file Is saved.

The snapping threshold may also need to be changed when importing 2 foreign vector file into a

GRASS database. Different-systems handle snapping differently, so a new file from an external source

may not snap according to GRASS standards. GRASS requires shared nodes to have exactly the same

coordinate values. If It is known that a vector file is not snapped, the user may allow It to snap when

running tl GRASS program impor.ro.vcct. Snapping in such circumstances occurs at the default snap.

ping threshold.

If the exact snapping status of a foreign vector file is not known, the user should first run

wJkrt.tovect without allowing tie nodes to snap. Then, if problems that might be related to snapping

are observed, Iie GRASS program support.vecr may be run. With this program, the user may assign a

smipping threshold or usc the cdcfull.

Caution: When running s.ppor.vccr, the user should be carcful about setting a snapping threshold

higher than ih default value. An overly high threshold could cause nodes to be inappropriately snapped, seriously distorting thc data file. This could be especially prpblcmatic if the file has not been backed up.

24
4 CARTOGRAPHIC CONSIDERATIONS

To dev~elop an accurate and usefu cartographic databae, GRASS users must know the basics of

mop scale, coordinate systems, and map projections They should also know the opnlons for Improving

the horizontal accuracy of Inaccurate so"~data.

MINP Scale

The proprton of real distance on a map to actual ground dLutanc is called mv scale (Figure 13).

Dita developers -nust refer to the map scale several times during the developi'ent or a GRASS database.

Map scalo can be expressd several difterent ways. Scale as a repreenativc fraction defines a ratio

between the map's dimensions and the carth's surface, but employs no unit of measure. For example, the

map scale I :24000O means that I unit of measure on the map equals 24,0 of the same units on the earth.

The ratio applies to any unit of measure. Thus, 1 in., I cm, or I mm-on this map Is equivalent to 24,000

In., 24,000 cm, or 24,000 mm, respectively, on the earth.

Another way of expressing scale Is with a graphic, or bar, scale. A Wa (or line) Is drawn on the

map und calibrated to show how many units or a specific measre it represens (Figure 14).

A -third way to- rpresent. map scale is-with a simple statement, such as "I inch equals 10 miles."

one Inch an the rMp replesents

63,360 knh., on tow toad.

SCALE OF MAP IS: 1:63.360

Figure 13. Visual depiction-of scale. (Source: Armny Map Service [AMSJ Training Aid No. 6, Map Intelligence. 2d Edition [U.S. Army Corps or Engineers, 19541, p 72.)

25
Scale 1:250,000

I,0 $ 10 11} Iw

-- I. - . ' 10 I I

1 t I 10

Figure 14. Bar scales. (Source: 1:250,000 map of Brunswick, GA, prepared by U.S. Army Corp

of Engineers AMS [USGS, revised 1968).)

TrWforMln MOp Scales

While al1 three forms of map scale are commonly used on printed maps, the GRASS digitizing

program requires the user to supply a mpresemative fraction. Therefore, any other type of scale

reprexstation must be convened to a representative fraction. (See Appendix B for a table showing

conversions for different map scales.)

Ira conversion table Is not available, the user must calculate the conversion, This is simply a matter

t transforming an expression of scale into a fraction that applies to any unit of measure. The scale "I

inch equals 10 miles," for example, is first expressed as a fraction:

I inch

10 mils

'he next step is to convcrt the numerator and denominator into common units-inch-miles, in this

example. It is easily established that I mile equals 63,360 inches. This relationship expressed as a

fraction has a net value of I. Therefore, this second fracton can be multiplied by the first without altering the ratio expressed by the first:

I inch I mile I Inch mile I or 1:633,600

10 miles 63,360 inches 633,600 miles inches 633,600

Notice that the second fraction is expressed with miles in the numerator and inches in the

denominator. This neutralizes both units of measure when the fractions are multiplied and the product

reduced. The result is a fraction with no units of mcasure.'

Large Scale vs. Small Scale Maps

The terms large scale and small scale ar often misunderstood. They refer to the relative size at

which geographical features are represented on a map. A large-scale map shows relatively more detail

for a relatively smaller geographical area. A small-scale map shows the opposite: less detail over a larger geographic area. There are no fixed standards for what is large scale or small scale. The Army suggests the following rules of thumb:

"A. Robinson, R. Sale, and J. Morrison. Elents of Cartography, 4th ed. (John Wiley and Sons, Inc., New York, 1978). p 45.

26
• large scale-larger than 1:7S 50

• medium scale-smaller than 1:75,000 and larger than 1:600,000

Ssmall scald-smaller than 1:600,000

On the other hand, the USGS classifies its maps like this:

* large scale-i :24,000

* teninediate scale-t:50,000 and I:OO,000

* small Xsae-.l:250,O00, 1:1,040,000, and 1:2,000,000

Engineers or city-planners may consider a map scale smaller than 1:4800 (1 inch X 400 feet; I foot

0.3 meters) too small for their purposes, while many foresters, by custom M convenience, reel most

at home with a scalc or 1:15.,840 (4 inches a I mile).

Scale and Digital Databases

Although no Ideal map scale exists for all purposes, some Important principles about map scale for digital databases should be emphasized. A major criticism or digitizing map coordinates is that it may

be too easy to misuse the data out of ignorance. For e"ample, when source material is compiled during the manual construction of a map, a cartographer usually tries to select material for the base map at tile

same or, preferably, larger scale than the compilation. This way, when the final map Is printed, its level

of detail is consistent with its scale. It is accepable to generalize from more detail to less, but it is not

a good idea to generalize from less to more, because that creates the 2ppearance of more detail than the

data holds, and can result in serious inaccuracies. Yet, once map inrration is digitized, the computer

offers limitless possibilities for this kind of inappropriate manipulation.

Consider a hypothetical GRASS database in which several layers - roads and streams, for

example - are based on USGS 1:24,000 quad sheets. Other layers that define the boundaries of a military installation and its training areas, come from maps at a scale of 1:50,000. Combining map sources of

these two scales will cause problems. The boundaries may actually follow the roads or streams, but when

the data from the two sources are overlaid, their lines do not match.

In some GlSs it may be simple to "force" the lines to match by directly copying them from one

layer to another. This Is not easily done in GRASS, in part because it is not a good idea. In GRASS,

the user or this hypothetical database may use the layers as they are, ignoring the sloppy appearance of

lines that don't match. Another option would be to back up a step and redigitize the boundaries from the

1:24,000 quads. If the boundaries are not available there, they might be recreated on the 1:24,000 base

map from all information available from other sources. When the newly compiled map is digitized, it will

lit with the ottr layers.

The problem of mixed scales can get even more complex. Suppose the map of soils for the

hypothetical subjcc' area above is available only at a scale of 1:100,000. Creating a new soils map is

prohibitively expensive. Even recompiling the map from other sources (including a soil scientist familiar

with the subject area), and at a larger scale, may not give acceptable accuracy at a scale of 1:24,000. And

even this approach may be possible only as a long-term option. As undesirable as this situation is, it is

entirely possible.

It is important for the user of the data to be aware of such problems. Information about the original soils map scale is supposed to be available by running the GRASS program layer.info (and will be there

if the crcator of the map originally entered it). Based on this information, the user should realiz.cthat a

hard copy of that layer, printed at 1:24,000, would not contain the detail or degree of accuracy that is

usually expected of a soils map at that scale. Also, if that layer were used with others at the larger scale,

27
the soil Infomation would comlate only roughly t the information from the larger-scak maps. Any

application of this data must consider the inconsistcncy of scale built into the databas Unfortunately,

the computer allows many more possibilities for mmipulating the data than does sound judgment.

Map Scalk for "Moplike" Products

A different way to consider map scale arises with aerial photography and satellite imagery.

Although these products ar created at a specific scale, they may be safely blown up to the limits of the

original. Unlike a primed map, the graphic details exist at the original scale; they simply need to be

brought out by enlargement. Thene Is a limit, of counic, to how much an Image can be enlarged. in

photography, for example, the limit will be determined by the resolution and quality of the film, the focal

length of the lens, atmospheric condition%, and flying height of the aircraft,

With satellite images, scale has meaniL.g only in relation to the printed forms, not the digital data.

Because of Its nature, digital imagery is best discussed in terms of its spatial resolution, or pixel (cell) size.

The cells repmeset a specific sixe, cx"psed in meters, that indicates the area of ground depicted by that

cell. The resolution of h ;cll map-in GRASS works on similar principles.

Tables 3 and 4 oudine some Important parameters for several common photographic and 'rcllitc

products. The altitude of any craft, whether airplane or satellite, tends to vary slightly in de course of

a flight. The numbers given In the table arm for the programmed altitudes.

Obviously, one must know more about the products in the table to make good dccisions about their

use. This explanation is only intended to show the relationship between map scale and a GRASS

database.

Table 3

Scale and Resolution of Common Photographic Products

Aerial Photography from the U.S. Geological Survey

Product Scale Focal Length Flying Height

NHAP CIR 1:58,000 8.25 in. (210 mm) 40,000 ft (12,192 m)

NHAP B&W 1:80,000 6 in. (152 mm) 40,000 ft (12,192 m)

NAPP CIR 1:40,000 6 in. (152 mm) 20,000 ft (6096 m)

NAPP B&W reproduced from NAPP CIR

NHAP CIR = National High Altitude Program Color Infrared

B&W Black and White

NAPP = National Aerial Photography Program

28
Table 4

Resolution of Common Satellite Products

Satellite Imagery

PrOuct Spa0Al Resolution Altitude oer Equator

SPOTI &W lom 832 km

SPOT 3 band 20m 832 km

mullispcctral

LANDSAT 1,2,3 MSS 79 m 917 km

LANDSAT 4,; MSS 79 m 705 km

,NDSAT ,- 30 in 705 km

SPOT = Satcllite Probatoim pour L'Observation de ]a Tcrre

LANDSAT = Land Satellite

MSS = Multispcral Scanner

TM = Thematic Mappcr

Coordina(e Systems and Projections

Coordinate systems and projections are both essential components or any map. A projection is a

method of transicrring geographic information from the canh's curved surface to a flat map. A coordinate

system provides a means for establishing thc location of any point in relation to other points. Examples of such systems includc the UTM grid system, the State Planc Coordinate systcms, gcographic coordinates

(latitude and longitude), and a simple cartesian coordinate (x,y) grid. A lcss common system, used by several GRASS programs, is the geocentric coQrdinatc system. Thesc coordinates describe a geographic location in dimensions x, y, and z, with the origin at the center or a sphcroid representing the approximate

shpe or the earth.

Different coordinate systcms have been worked out using different projcctions, but the most common

one used in GRASS is the UTM grid, based on the Transverse Mercator projection. The Slate Plane

systems arc based on cithcr the Transverse Mercator or Lambcr's conic projections. The geographic

coordinatc system of latitude and longitudc may be used with any projection. In general, a projection

must first establish how the curved earth will bereprcscnted on a fiat surfac before any coordinate system

can be applied.

GRASS does not fully support either State Plane systems or geographic coordinates, but eWforts are

being made in that direction. Two GRASS programs (M1l2u, Mu211 convert UTM coordinates to latitude

and longitude, and vice versa. Other programs (Mll2gc, Mgc2l1) convert both ways between geographic and gcoccntric coordinates. Most GRASS programs work with UTM grids, however. (One notable

exception is a caricsian coordinate system used for raw imagery data, where x indicates the row and y indicates the column of a grid cell, with the origin in the lower left comer.)

29
The UTM Coordiate System

Because the UTM grid system is the predominant coordinate system Asd by GRASS, it is worthwhile to describe It In some detail. The UTM grid system Is keyed to the earh between 84 degrees N

latitude and 80 degree S latitude. It longitudinally divides the earth into 60 zones, each 6 degrees wide.

The zones am numbered I through 60 starting at the 180h merIdian. Each zone is divided into 20 eight- degree sections, each or which is letterd alphabetically C through X from south to north (excluding I and

0). F.ch 6- by 8.degme rectangle of earth is uniquely identifled by its column (74n) number and row letter (Figure 15a). Each no-th.sourh and we..east UTM grid line is referred to as an Masing" and a

"Northine respctvey, and is assigned a value In meters. Disti.,s using a UTM grid system arm always measured left to right and bottom to top (Figur 15b). Thi spacing interval or= am grid on a standardi

I USGS topographic map depends on the sc:ale of the map (Table 5).

The UTM system was chosen to be the dominant system in GRASS for two measons. First, the

system was already familiar to personnel at Army installations, who were the first users of GRASS. This

familiatity has helped link the new GIS technology to the methods it Is replacing. Second, but equally significant, the UTM system Is expressed as a plane. The distance between two grid ticks is constant In

both directions x and y. For practical purposw, the section of the Earth being studied Is i:onsidered to

bea plane. This practice Is acceptable for one relatively small area at a time. Note that the geographic coordinate system, which is designed for a sphere, does not sham this characteristic. A degre of

lo, *ide at the equator represents a much larger distance than a degree of longitude near a pole. Plane

geonictry is much simpler than spherical geometry, so using UTMs as the primary map unit allows

GRASS programs to be simpler, 'These factors, as well as its relatively easy usage and widespread

acceptance, made the UTM grid system the natural first choice for GRASS.

When the UTM system is used, the user is expected to choose source materials that use a projection compatible with the UTM coordinate system. The GRASS user, in turn, usually expects the creator of

the source map to have chosen a projection responsibly before placing the grid on top of it. The user

should be wary of a UTM grid drawn on acrial photographs uncorrected for distortion or small-scale maps. The grid is not likely to indicate accurate ground coordinates, and this can cause trouble-especially when

more accurate maps are overlaid on It.

When the UTM system is used with the GRASS a special problem can arise. A GRASS database

itay not extend cast to west beyond one UTM .one, or 6 degrees. This problem is not easily solved. If

the study area extends only a relatively small distance beyond the zone, the grid numbers can simply be extended. This approach is unacceptable beyond a limited area, however, because distortion caused by the Earth's curvature increases as one moves farther from the central meridian of any zone. Another

possible solution is to make separate databases for the parts in different zones.

Maps of large areas have sometimes been digitized using an artificial UTM grid. In such cmass, the study area was simply overlaid with a grid, measured in mecters, that bears no relationship to actual UTM

coordinates. TI'is method results in a map without true ground coordinates, making it difficult to add data

from other sources.

It'/'el file Source Has No UTI Grid

As stated in Chapter 3, the process of map registration establishes the UTM coordinates for a map. If there is no UTM grid on a map that is needed for a GRASS database, however, the user has several

alternatives.

30 -inn
Pej

311
I

Table S

UTM Referene Sytem Spadng

for USGS Topographic iaps

Map Grid Spacing Spacing (on map)

Scalk (m on Irumad) In. mm

1:20,000 1,000 1.97 50.0

1:24,000 1,000 1,64 41.6

1:25,000 1,000 1.57 40.0

1:50,000 5,00 3.94 100.0

1:62,500 5,000 3.15 80.0

1:63,360 5,000 3.10 78.7

1:10,000 10,000 3.94 100.0

1:250,000 10,000 1.57 40.0

1:500,000 50.000 3.94 100.0

1:1,000,000 100,000 94 100.0

SOurte: Motils M. Thom~pton, M Af{ .oe Amrkv (U.S. Geologkal .utvc y. 19"79). It 24.

If another type or coordinate system has been established on the map, there are several ways to

establish aUTM coordinate system. For a map that uses only latitude and longitude, the user can employ the GRASS program M112u to enter the latitude and longitude coordinates. GRASS then assigns UTM

Northing and Easting equivalents to the grid. GRASS can currently provide UTM conversion for latitude

and longitude only. Ifa State Plane coordinate system is the only system established on the mn-p, the user

can obtain software that converts that system into latitude and longitude, or find someone to perform the

conversion. The user may also use a UTM map covering the same area to determine the UTM coordinates

for points on the State Plane map.

A different approach is needed to register an aerial photograph without a UTM grid, such as an SCS

soil survey. Often the scale is inconsistent across such photos. Some distortion is due to relief displacement, and some is due to the nature of aerial photography. The following approach should be used only in cases where the terrain is relatively flat and the photographs are recent, or when an orthophotograph

was used to map the soils. (An orthophotograph is a photo including all the graphic information of an

aerial photograph with the constant scale of a map.) In other cases ti user should recompile the soil

information on a base map with an accurate x,y relationship among features. (See the section Improving Map Data, later in this chapter. for information on recompiling soil maps.)

There are two-ways to identify UTM reference points on aerial photos without a UTM grid. For

one procedure, the user needs a map that covers the area of interest on the photo. The user needs to find

reliable landmarks on the map that can be recognized in the photograph. Reliable landmarks are features

that ar accurately placed on the map, such as a road intersection or a sharp bend in a road. Unreliable

landmarks include features such as a well, water tower, church, or stream. The first three examples are

32
unrcliablc because they are genendly not placed on a map with geat accuracy. A stream Is nOt reliabl

because its shape changes with time, and much time may have passed between the date the map was

drafocd and the dae the photos were acquired. When at kam six dispersed mfrence points on the

photograph art identified on the map, determine fte UTM coordinmes or the points on the photo. An .asy

way to do this Is to ue the manual digitizing unit and GRASS's sits option. (Tbe tutorial "digit; lIt Use

and Its Fatures" offers A detailed explanation of how to use this softwae.) Wen the coordinates for the

points have been determined, they can be trmfcrred onto the photogrAph.

When working with erial photographs, there a two reasons to chooseat leant six reference points.

First, due to the Inherent nature of phoograph the picture is more distorted at Its edge than Its middle.

By choosing at least six points, one can be more certain that the reficince points are a fair representation of the photograph. Second, during the manual digitizing process, at leant four points ar needed to regiser

a photograph or map. By using more than four points from the photogralph during registration, the user

can remove the points with the highest registration residual values. (Se the section Residuals and Mp

ReSO.(tradon, in Chapter 3, for more Information on residual values.)

A second way to establish registry points on an aeria1 photograph requires a map covering the same

area, and of the same scale, as the aerial photo. Tape a piece of mylar on top or the photograph. Ten, in blue pencil, trace some or the roads and intercdons from the photo. There Is no need to trace :ll of

the roads, only enough to cover dispersed areas on the photograph. Then remove the mylar and tape it

on top of the map, aligning the roads traced from the photograph with the roads on the map. The roads

will not align exactly, so "spread out the error" by slightly offsetting all roads on the overlay with the

ronds-on the map (Figure 16). Establish the registration points using the grid system from the map under

the overlay. Finally, take the overlay. realign It to the roads on the photo, and tape It down so the features

on the photo can be traced and manually digitized.

In many cases, the user will need to create a mosaic of photographs to completely cover the area

of interest. As mentioned earlier, photographs ar most distorted along tleir edges, so they will not match

up perfcctly. In these instances, "spread out the error" along the common edge (Figure 17). Once the "misaic" Is completed, locate the reference points needed to regimer the composite. If the features taken

from the photograph are to be scanned. the reference points must be e-stablished outside the area of

interest.

Accuracy and Quality in a GRASS Database

It is important that the data used to build a GRASS database be as free from error as possible. The

required degree of accuracy varies, however, depending on the application of the data. Map accuracy is

a complex topic. The GRASS user must avoid introducing error when possible, and appreciate the

limitations of map data in general. These limitations involve the source material and tl way data is

digitized and entered into a database.

The Accuracy vf Source Materials

Never assume that any source, whether a printed map, aerial photograph, or saiellite image, is free

of error. Each source has problems affecting its suitability for any given application. The size or tle

paper on which maps are printed, for example, is not stable over time. It can expand in humidity as much

as I percent30 It is always best to obtain map information on a stable base material such as plastic when

'Np. Snyder, Map Projections; A Working Manual, U.S. Geological Sen'ice Prorcssiondl Paipr 1395 (U.S. Government rinting

Office, Washington, D.C.. 1987). p 3.

33
451 4513 Roads on overlay

4513 Roads on map

484
ponible. In pcncral. the accuracy of i map's c st i never guar-n4ed. So ix maps Am uw:ly coMidrd k) WOtaln "iout 15 pcrcm error due to the ambiuily or divislon bctwccn van imi soil

PXonlcs,"

Another I~or |mp I the hot tal ¢ctrmy o( locational dta. It depcds on the quality of the orlginal conol survey and the qualky of map pmductio ad rmpduction Moces. Stme mas

have been tested and Wnd t be within the marIn of en allowoed by National Map Accurcy Standards (NMAS). If NMAS esting has been done, a taemen to Oa eff5ct is poimed In the marti of lh map. If thee standards have been me, the map was almoll certainly produced under very strict conm, Infoatmmio ten fam It Is Mobaby as elabk a con be expected frm y map at the s sa. If

no NMAS statemen is primed on a m , t map has either ro met Ox xanda , or I has not been

tested.

The United Sttes NMAS for horizmal accuracy state hat no mor than 10 percon of well defined

points shall be In emor or mor hn a specified amow* (depending on map scale), as mca.red on ft

map. The amount is 0.03 inches forma* of cle larer than 1:20,000 and 0.02 Inches form*$ of'sctae

smaller han 1:20,000. Thex amowts reprscm dlfteret grmond distmes, depending on map sct (sce Tsblt6). Note that these andards ame meaningful for a fairly limited range or scales, At very lrge scales, the error allowed is grater than what- might be aceptab!e; at smaller scales, maps with pwise horizontal accuracy car no be expected. For vetical accuracy, NMAS smate that, tor any map scale, no

mor than 10 percent or the tested elevation points shall be In error mom than the valueo orne half or

a contour interval.".

Another way to define map accuracy is by use of a reliablity diagram. The example In Figurt 18 shows such a diagram from a USGS i:0,000 scale map or Georgia. It does not give a qusnihative

mcasure. but -povides a qualitative index for assessing the potential usefulness of the map.

Table 6

Map Scale and Htorizontal National Map Accuracy Standards

Distance Distance

Map Scale on the Map on the Ground

1:15,840 0.03 in. (0.76 mm) 40 ft (12 m)

1:24,000 0.02 in. (0.51 mm) 40 ft

1:50,000 0.02 In. 83 ft (25 m)

1:100,000 0.02 in. 166 ft (50.8 m)

"P.A. lunough, Principles ofGeorsphkaca hVomwnoe, Sys,for Land R..o i"we Assneomnt (Clarendon Press, Oxoid,

i986), p 108.

'A. Robinson, R. Sale. and 1. Morrison, pp 8.9.

35
AD

D 

I

1917-T419

wUMMO 1 mh 19% .~1 qw

Figure il. Reliability Diagram (Source: 1:250,00 map of Hfunswick, GA, prepared by U.S. Amy

Corps of Engineerfs AMS (USGS, revised 1968),)

lit -many cases, no statement of accuracy is available. "This should be noted in the documentation

for the GRASS map layer. It Is helpful to Include any other information about the source data that would

help the end user accuratcly judge iLs value. Such a report might include justification for the use of a

source of questionable accurcy.

Iiilring MAp Data

Sometimes the sole source of certain map information will be known to have poor horizontal

accurcy, SCS soil atlas sheets, for example, contain very valuable data collected at considerable

expenditures oftime and money, but they have a serious weakness: the soil polygons tre usually drafted

directly onto aerial photogrphs uncorrected for distortion, and offer no guarntec of horizontal accumcy. Data developers have two options that may itiprove the information berore it is entered into the database.

One mcthod involves mechanical rwctification of the polygons with ihe aid of special software. The

lines must first be traced from soils sheets, preferably from film positives rather than paper. (Film

positives of soils shets are available from USDA SCS, National Tcchnical Center, Port Worth, TX.) The

tr.ced lines must thcn either be digitized manually or electronically scanned. The soils map must include

about 10 control points that represent identifiable points. Th1e coordinates for these points may be taken

from USGS 1:24,000 quads or an onhophotograph. or some other reliable large scale source.

Once le map information and control points have been digitized, they are processed by a program that ndjusts the polygon lines to conform with the control points. This process, sometimes called "rubber

sheeting," is not supported by GRASS, however. The user will probably need to contract the work to a

firm that has the necessary software.

36
Although the corrcted soils map lines mor accurately coniform to the actual ground locations the)

r tscin, te ihanlal prNos that corrected tem cxa I always follow certain Storaphic teures

tha usually dellneate certain sM.itypn. The inmformed data should be examined by a soils expert, and

A betwen nat.1l fe*uiS (rfver ridps, and coasines, for example) nd ol polysons should be noted. When the 1tal fleit ediwd to m t!ejudgmnms of the soil expert, the rirJhd

map layer may be convenctd vgl format and added to the GRASS databs.

1t1 GRASS user's o to .. s tO manually adjus the tncurae sil polygons. For this process,

too, the developer needs M m,.fposvcs of the appmpriatc soils shms containing the aerial pholographs

on which the polygons w.ertwn. It is also necessary to obtAin (or have made) othophotographs at the

samc scale as (hht soil$ s*1e(wi

The onhopho Is laid ver the film positive on a large light table. On top of the cis pld a clean

pitcv or mylar drAfting film. Although the data developer may note that, for any one portion orthe map,

features such as rods and -stims will fit accurately with the onhophoto, tf soils map wil exhibit an

overall distortion. The data:dvleper registers the onhophoto to the map so their featurms fit ucross a

small portion or the soil map. 'llw polygons on that portion of the map are redrawn onto t myir.

When tls portion Is compleic, the onhopoto is mmregistered at a diffieren location and t appropriate

Noil polygons am traced. This process coninuxs until the soil polygons across the enti map have been

redrawn. This work should be caried out by a person with graphic skills and training In soil science.

Once the soils maps are redrawn, they are eiler manually digitized or electrically scanned. TIh

new map layer Is then realy t0 dd to the GRASS database.

Each method has advms,.g;s and disadvantages. The mechanical mcthod may be, quicker, especially

in case. where the data is-cimpfex or Is already in digital form, and it may also be less expnjive. 1e

manual method, howtvcr, whar properly executed, gives more depentx!ble results. Decisions on the

proper localion of the polygmi-lines are made by a scientist raier thut computer program. The matual

metlod dso provides the opponuiity to improve and add to the soil data. If updating tle content of the

soil survey Is important. t,'4 rmnual method is probably the preferable choice.

IngraIducti n of Error Through Other Procese

As previously discussicd, map data may be digitized manually or electronically scanne." Both

metfhods can add some eror to the final digital product.

In manual digitizing, an operator traces lines by-haMnd, and defins coonliiwtes to reprsent those

lines digitally. Error introduced at this stage adds-to that in the source material. Such errorofien arises

from dte tedious and fatiguing nature of manual digitizing. The operator should avoid digitizing for more

than four hours a day to reduce fatigue-related error.

It is Important for an operator to be fully trained to digitize before being entrusted with a project.

'he process Is somewhat like drafting: a good eye and a steady hand are, as in drafting, valuable traits

for lhe operator to possess. Nobody can digitize perfectly, but practice and knowlcedge about the procss

can significantly reduce emrrs. It has actually been observed that individuals tend to fwv. an "error

signature" that may be used as a tool to improve an operator's quality consciousness, thus reducing

errors.

1

3

"George F. Jenk "L Lines, Computers, and HIuman Frailties," Annals ofJthe Association ofAmerican Geographers. Vol 7. No.

I (March 1981). pp 6-7.

37
I

Checking the raithruln or the digital pMduct to Its analog CoDOunrp.t Is bKA accompll Sfd by

plotting the digital map a tle scale of the Source, then laying the plotted map over the odlnal. The plo

mug be done on a devi" c of known reliability. Tis process mllows both accuracy and completenes to

be chocked.

Vlectronkcally scwaied map data Is us uly recorded as rasr data or a very One Image reoludton

(at leaIt 100 microns), which Is then convened to the GRASS digit (ormat. When done properly, the

dgre to which the digita dagt differs from the original lines Is eady Indiscernible. One problem worth

noting, however, may occur at sharp conr. The softwarc used to process the scanned data may not be

c€pbe of placing a point exacty at the comer. The corner thus ends up being rounded In appe3rance. Scanned files must also be checked carefully for ewors introduced becauw of mislabeled polygons,

unsnapped nodes, or some misnderstanding by th opermor about the job's requircm .

When digitized vector data is convened to ccl format it must be processed by the GRASS program vect.to.cell. As mendoned previotsy, any vector-torater conversion will introduce enrn" In the form of

the jagged lines charactristic of ratcr da

Another process that may introduec error is the tracing or map dati from an original in pparation for cidt-r manual digitizing or scanning. Enrn is introduced at this stage whenever the person digitizing

deviates from the line being traced. Thes crrors can be kep to a minimum through training and practice,

as well as by using a fine technical pen for the drafting,

Docwnc nntaion

The final stage of making a GRASS cell file is to document it. In GRASS, the command support

gives the data developer the opportunity to enter information about the cell file that will be helpful to the

user, including information on the source map, its accuracy, the accuracy or the registration, special

problems with the data, the digitizing threshl d, the intended application for the data, and anything else

that might help the uwer accurately assess the data. Without full documentation, the usefulness of data is

greatly Impiircd. Appendix C gives a checklist for maintaining high standards in the construction of a

digital database.

38
S CONCLUSION

Imporant Te hnologkis for oalobw De lopment

Throughout tM report, an emphasis has been placed on the digitixing of data from existing maps,

usually in pirited form. But this Is not the only way, or even the bes way, to develon a GRASS

dotabase. Whe time md funding permit, map data can be collected from appropriAte aerial photographs

or the Pdy area. Ground control surveys can be performed, and planimetric and terrain feturms may be

collected pholrammetrically from the photography. This would avoid corrupting the data with the error

already present in existing map sources. and overlays taken from the same photography would be

corsIstem among themrsovc,

Although a photogramnmctric method of collecting map da can reduce the load or in-house

digirbjng, this method Is not without problems, too. For example, map developers using this method of

data collection must consider the concept of digitixing threshold (n described in Chapter 3), The number

of points used to depict a line must be adequate to depict that line smoothly on a plotter at the desired

output scale. Additionally, such data must also be translated into GRASS digit format. Finally, when an

outside finn handlcs the collection of map dwtconrtrol of the results can be difficul. A person who

knows nothing about the study ai-ank trails on an Army installation, for example-cannot make an

Informed decision about which lines on the photographs actually depict tank trails.

Orthophotography, at a large enough scale to give adequate detail of the study area without

excesive budgiit requirements, can be a valuable addition to # database. It can serve as a reliable base

from which the GRASS user can recompile any information that needs to be added to a database. In this

way, the orthophotograph can servo as the control that verifies accuracy among different map layers.

Scanning technology is Improving to the point that scanned images may soon require less redrafting than they now do. Scanning of colored maps is also advancing. It may be more efficient in the future

to remove unneeded features from a scanned map than to isolate the features of interest by redrafling on

an overlay.

Some experimental work has been done in collecting map coordinate points directly from the ground by waking ordriving along the path to be recorded. This technique involves the use of global positioning satellite telmology, however, and questions about accuracy and cost must still be answered.

Importing Digital Data in Other Formals

In some cases it is most efficient to use existing digital data than to create it, so it is desirble to

use such data when possible. The USGS, for example, can provide certain digital map data. Also, as the

number of GIS and computer-aided drafting systems increases, more data will be created for these systems. Workis proceeding toward the conversion, of data created for Intergraph workstations for use in a GRASS

database. Translation between Digital Exchange Formal (DXF) and GRASS formats is already possiblc.

39

-- - - -
Conclusion

GRAMS softwanr cngincrs ar curretly working on new progrms Involving the storage and

collection of map da(a, and m~w cartography technologies an being explored. Th pioblems of providing

accurate maps In a timely manner will always exist, however. In order to make the best or what will

alwa) be a lrwssh3n.ideal situation, both the creators and ONe users of digital map datA must be aware

of the possibilities and limitations or digital canographlc technology.

40
REFERENCES

Aptru Handbook~ 294 A~rrWu.I'Aoq Imeooim k Chu4syb &,d Mqpp&kg Soils (U.S. Dqpwmen otApicultum Sal

Consvtiap Servico (USDA SCSI. 1966) p 1.

Army Msp Sarik. (AMS) Trah"S~ AMd No. 6, Mope ~wg~me, 2Wd ed. (U.S. Army Co4jw of Engirwers, 1954).

tsdihaw, Stuart. wtd Pamn Thompwn Op~onsfo Ao.adrkq Ekeiou Dois, T~Omical Mwmacuip (TM,% N.4120 ADALV034

(U.S. Amny Corottucton Eatirwitins Roanck Labormory tUSACEAL), Jantamy IM).

Hurrough. P.A. ~ eg~M e ro~wo amo~w s~~ssr*(lrdnPas Oxfwd, 1986).

Field Manual 21-26, Mop Re.I.'q (tHadqoaairs. Deporunmn ofthe Army, March 1965).

I'lect. Harvey. Chief Of Digital Caimogay, U.S. IDspaimomn of dw fnterio National Park Sorvie (wurAbsHOW interview. 29

May 199).

Giwsic. Anrew M., The Pid~ondl Accurac of 4Mq, AMS Technical Repmn 35 (Wasington. D.C., 1961).

Gotrn, W., aM R. Riggin, GrqAk MarWS to Saiport 84kiat Qmanrlativ FEvirowwwe~at kn~wo A jysi-so~w of

AxIsnt Maer:. Tiechnicil Report N-MtADA069097 (USACERL. March 1979).

Jcriks, Georle F., 'Llnr, Comptcts. and Hum~an Ftsiliks,' Amokl of tihe Assoeladon of Awrkwi Gegraihes, Vol 71, No.

I (March 1981).

Johnsn. Mark, and William Gorxi, SoucesoDigl Sparilat Dawafor Geotrapimi hj/ormwl~om Systcin. Technics] )Rcpcn N.

SSM0l466908 (USACE)U.. Decembe 1987).

Martin. Mary V.. William U. Coran. ci a&I, GRASSIGIS Inrmienai Gidde, Special Report N.90dV2 ADA214623 (USACERL., October 1989).

Robinson. A.. R. Sale, and J. Morrison, Elexierds ofCanroropy. 4tih ad. (John Wiley a;d Sons, Inc.. New York, 1978).

Shapiro. Michael. James Westen-elt. c aL., GRASS 3.0 Profrainma' Mamvxt. ADP Report N-89/14 (USACERL. Sepcnbc

1989).

SnyNkr. P., Mp Pwojecdons;A lokn VRa.USGS Prorcsslonal Paper 1395 (U.S. Governmnent Printing Office. Washington,

D.C.. 1987).

Thompson, Morris NI.. Maps for America (U.S. Geological Siarey, 1979).

U.S. Army Curps or Engineers Army Map Service. map of Bruwick, GA (USGS, revised 1968).

Westervelt, James, Michael Shapiro. William Goman. et a&., GRASS Use?:s Reference Man-vat, ADP Report N-87/22 (USACIERL

September 1988).

41
APPENDIX A:

DRAFTING MATERIALS

The following is a basic list of drafting supplies needed for techniques described in this report. CTis list

may vaty among cartographers.)

Mylar (3.0 mil thickness)

Mylar is a thin plastic with a frosted matte finish on one side for drafting.

Technical drafting pens Although technical pens ar available in a wide range of line thicknesses, the weights 00, 0, and I are

sufficient to prepare maps for manual digitizing and scanning.

Pn cleaner

Technical pens clog when ink dries in their tips. This cleaner is a .. i solvent that dissolves clogs.

Waterproof black ink

Waterproof ink adheres to mylar better than does water-soluble ink, but it can be removed from mylar with a dampened craser.

Nonphotogrophic blue pencils These are useful because their imprint is invisible to poitography and scanning. They may be used

to draw guides for ink lincs without worry that they will be recorded unintentionally.

Trianglev (with raised edges)

Thesc arm for drafting short traight lines. Raised edges prevent ink from smearing underneath the

triangle during drafting. Drafting triangles come in two types: the 30.60-90 type and the 45-45-90

type. Triangles of varying sizes are useful.

T-square This is used wit1h a triangle to produce a perfect 90.degree angle to an established line. It is also

useful as a long straightedge.

Long meal straightedge A metal straightedge is useful for making straight cuts in mylar and acetate. Plastic ones should not

be used as cutting guides becausc they ar easily gouged.

French curve

This is a curved plastic tool that is invaluable for drafting a variety of curves. A variety of sizcs is

useful.

Templaex,

Templates arc guides for drafting standard geometric features, such as squares, triangles, circles, and

ellipses. A template usually features a shape in several different sizes.

Ruler

Rulers should be finely calibrated so precise measurements can be made.

42
Ink eraser (forplaitic)

Thene rasers are designed especially to remove ink, Pressing too hard during crasu., may damage

the coating on mylar, however.

Kneaded eraser

Thes don't shed bits of rubber like standard pink pencil erasers. They work well to remove

fingerprints from the drafting film.

* Eraser shield

This is usually a flat piece of metal with diffcrent small sharwes cut out or it, used to erase : small

portion of a line. The shield protects the portion of the line not needing correction.

Mineral spirits (or alcohol)

Thse liquids, used to clem drafting film, remove skin oils from mylar and evaporate almost

immediately. Do not randomly wipe the mylar; either wipe up and down, or from side to side.

Excessive pressure during cleaning may remove the mylar's coating. Skin oil prevents ink from

adhering to the mylar properly. Once the mylar is clean, only handle it by its edges. Shield the mylar

from hand smudges with a piece of paper.

Ra:or or knife

Use a single.edged blade or a razor knife with rcplaceable blades, such as an X-3Cto knife. It may be used to cut mylar or paper, and to touch up small inked lines. Use caution because blades can

easily damage mylar. Large lines should be edited with an ink eraser.

Drafting brush

Used for brushing foreign debris from the mylar. It help, pmventmarring the surface with skin oils.

Drafting tope

This special tape is not as sticky AI ,Hn , and is easily removed from paper or mylar without

causing damage,

Clan rags

Use them for cleaning mylar and pens.

43
APPENDIX t:

CONVERSION FACTORS FOR REPRESENTATIVE FRACTIONS (MAP SCALES)

Ratio Seka Iok Per hImen perMilo ptr Mers ptr

Feet per flch IW t Mile h llock

1: 5w 41,667 24,000 126.720 0.006 12.700

t: 600 50.000 20,000 105.60 0.009 15.240

1: 1,000 13333 12.000 63360 0.016 2.400

1: 1,200 100000 10.000 52100 0.019 30.480

1,500 125.000 8.000 42.240 0.024 31.100

1: 2000 166.667 6.000 31.6110 0.032 0,800

1: 2400 200.000 S,000 26,400 0.0311 60.96

1: 240 206333 4100 25344 0,039 63.0

1: 3,000 230.000 4.000 21.120 0.047 76,20

1: 3,600 300.000 3333 17.600 0.057 91.440

1: 4,000 333333 3.000 1S840 0.063 101.600

1: 4.100 400.000 2.500 13.200 0.076 t12.920

1: 5.000 416.667 2.400 12.672 0.079 127.000

1: 6.000 500.000 2,000 10.560 0.095 15.400

I: 7,000 513.333 1.714 9.051 0.110 177.800

1: 7.200 600.000 1.667 8.800 0.114 182.880

1: 7.920 660.000 1.515 1.000 0.125 201.168

1: 8.000 666.667 150 7.920 0.126 203200

1: 1,400 700.000 1.429 7.543 0.133 213.360

1: 9,000 750.000 1333 7.041 0.142 228.600

1: 9,600 800,000 1.250 6.600 0.152 2,43.840

1: 10,000 833.333 1.200 6336 0.158 254.000

1: 10,800 900.00 1.111 5.867 0.170 2743.20

1: 12.000 1,000.000 1.000 5.280 0.189 304.801

1: 13,200 1.100.000 0.909 4.800 0:208 335.21

1: 14,400 1,200.000 0.833 4.400 0227 365.761

1: MS,000 1,250,000 0.800 4.224 0AZ7 381.001

1: 15.600 1,300.000 0.769 4.062 0.246 396241

1: 15,840 1320.000 0.758 4,000 0.250 402337

1: 16.000 1333.333 0.750 3.960 0.253 40(.400

1: 16,800 1,400.000 0.714 3.771 0.265 426.721

1: 18.000 1,500,000 0.667 3"20 0.284 457.201

I: 19.200 1,600.000 0.625 3.300 0303 487.681

1: 20,000 1,666.667 0.600 3.168 0.316 508.002

1: 20,400 1.700.000 0.588 3.106 0.322 518.161

1: 21,120 1.760.000 0.568 3.000 0333 536.449

1: 21,600 1.800.000 0.556 2.93 0341 548.641

1: 22,800 1.900.000 0.526 2.779 0.360 579.121

1: 24,000 2,000.000 0.500 2.640 0379 609.601

1: 25.000 2,083333 0.480 2.534 0395 635.001

1, 31.680 2.640,000 0379 2.000 0500 804674

1: 48,000 4,000.000 0.250 1320 0.758 1,219.202

1: 62,500 5.208333 0.192 1.014 0.986 1,587.503

1: 63,360 5.280.000 0.189 1.000 1.000 1.609347

1: 96.000 8.000.000 0.125 0,660 1.515 2.438.405

1: 125,000 10,416,667 0.096 0.507 1.973 3,175.006

1: 126,720 10,560.000 0.095 0.500 2.000 3.218.694

1: 250,000 20,833.333 0.048 0.253 3.946 6,350.012

1: 253.400 21,120.000 0.047 0.250 4.000 6,437.389

1: 500.000 41,666.667 0.024 0.127 7.891 12,700.025

1: 1.000,000 83,333.333 0.012 0.063 15.783 25,400.050

Source: Agriculture Handbook 294, Aerial.Phaoo lnerpreaon in Classifying and Mapping Soils (USDA SCS. 1966). p 80.

44
I

APEINI)IX :

A CHECKLIST FOR MAINTAININO A IGIM STANDARD FOR DATA DEVELOPMENT

1. Do not assumc that source data Is accurate. Check out and document its usefuln". Remember that

the final digital map will never be more Accurate than the sourcc

2. Obtain map information on plastic separates whenever possible.

3. Work with map data t eor entering it into a database to ensure that it Is as accute as possible, and

that it will line up as xcuratly as possible with related overlay,

4. Recompile map data, when necessatry and feasible, using eliher manual or computer-aided processes.

S. Do not underestimate the potential ror creating new error when manually digitding. Make certain that

pople doing the digiting are trained for the job before undertaking a project. Don't expect anyone

to digitize more than 4 hours per day.

6. Print a proof on a plotter to check the accuracy and completencss of" dlgiticd data against mialog

sources.

7. Provide complete documentation for each map layer to help tlh user asses tlh usefulnes, of the data

for a given project.

45
DISTRIJTON

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AITN S~IIIIIgiu n lcr 09055 A1TN: Facilities Hngr Coin' (9)

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US Militity Acadiemy 1099 Engineering Societies Library

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Comnput &Sitoeces Nations] Guard llireu 20310

AITN: IAliNA Installation Division

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FORSCOM (28) US Army Rny. Hygiene Agency

FORSCO0M Engineer. ATTN-. Spt Dci. 15071 ATTN: 1ISlIB.hE 21010

XrrN. DuL-1

Defense Technical Inko Center 22304

"lsc ATTN: D'11C.FAII (2)

Mt. Sam Houston ANIC 78234

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