USGS DIGITAL CARTOGRAPHIC DATA STANDARDS 

USGS Circular 895-A: Overview and USGS Activities 

B: Digital Elevation Models 

C: Digital Line Graphs from 1:24,000-Scale Maps 

D: Digital Line Graphs from 1:2,000,000-Scale Maps 

E: Land Use and Land Cover Digital Data 

F: Geographic Names Information System 

G: Digital Line Graph Attribute Coding Standards 

Questions regarding availability and ordering of US GeoData (all 

types of digital cartographic and geographic data produced and 

distributed by the u.s. Geological Survey) should be addressed to: 

User Services Section 

National Cartographic Information Center 

u.s. Geological Survey 

507 National Center 

Reston, Virginia 22092 

Technical questions and comments should be addressed to: 

Chief, National Mapping Division 

u.s. Geological Survey 

516 National Center 

Reston, Virginia 22092 

Any use of trade names and trademarks in this publication is for 

identification purposes only and does not constitute endorsement by the 

u.s. Geological Survey. 
LAND USE AND LAND COVER DIGITAL DATA 

By Robin G. Fegeas, Robert w. Claire, 

Stephen C. Guptill, K. Eric Anderson, and Cheryl A. Hallam 

USGS Digital Cartographic Data Standards 

Edited by Robert B. McEwen, Richard E. Witmer, and Benjamin s. Ramey 

Geological Survey Circular 895-E 

1983 
United States Department of the Interior 

JAMES G. WATT, Secretary 

Geological Survey 

Dallas L. Peck, Director 

Library of Congress catalog-card No. 83-600581 

Free on application to Distribution Branch, Text Products Section, 

U. S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 
FOREWORD 

In recent years, the disciplines of cartography and geography have undergone a rapid and striking reorientation as the 

techniques for digital collection and manipulation of data have 

evolved from fledgling laboratory procedures into dominant and 

driving forces that now pervade the disciplines. Digital techniques have provided a variety of new and powerful capabilities 

to collect, manipulate, analyze, and display spatial data. 

However, this evolution also has introduced a number of new and 

complex problems. One of the most pressing problems, and one 

which is receiving particular attention at present, is the issue 

of digital cartographic data standards. 

The u.s. Geological Survey {USGS) has been actively 

developing digital cartographic and geographic techniques for 

over a decade and has taken significant steps to develop and 

define in-house standards governing the various types of 

digital cartographic data that are being collected and archived 

in a national digital cartographic data base. The in-house 

standards are expressed in the form of specifications documents 

that were prepared to govern collection of the data and in the 

form of user guides that were prepared for distribution with 

the data. 

In an effort to fulfill lead agency requirements for 

promulgation of Federal standards in the earth sciences, the 

documents have been assembled with explanatory text into this 

USGS Circular consisting of separately bound chapters. This 

Circular describes some of the pertinent issues relating to 

digital cartographic data standards, documents the digital 

cartographic data standards currently in use within the USGS 

National Mapping Division, and details USGS efforts to define 

national digital cartographic data standards. 

Chapter A is an overview in which the major issues involved 

in developing digital cartographic data standards are discussed 

and the activities of the USGS related to digital cartographic 

data production and standards development are described in 

detail. Succeeding chapters comprise the pertinent documents 

that establish USGS in-house standards for the various types of 

digital cartographic data currently produced by the National 

Mapping Division--that is, digital elevation data, digital 

planimetric data, digital land use and land cover data, and 

digital geographic names data. 

This compendium of relevant material is prepared to serve 

as a benchmark and to assist ongoing efforts to establish 

acceptable standards and conventions for both Federal agencies 

and the public. 

Dallas L. Peck 

Director, 

u.s. Geological Survey 

Department of the Interior 

iii 

R. B. Southard 

Chief, 

National Mapping Division 

u.s. Geological Survey 
PREFACE 

This Circular is the result of the efforts of numerous individuals who have contributed to the research, development, and 

preparation of various digital cartographic and geographic standards for the National Mapping Division of the u.s. Geological 

Survey. The individuals named as chapter authors represent both 

the originators of the various concepts as well as the writers 

who expanded and clarified these ideas. Their contributions, 

either to the concepts or the writing, are of such magnitude as 

to warrant crediting as authors. 

Atef A. Elassal was largely responsible for the original 

data structures and computer file formats that are used for the 

Digital Line Graphs and Digital Elevation Models. The attribute 

coding scheme was first developed by members of the Digital 

Applications Team under the direction of Robert B. McEwen. The 

Geographic Names Information System was conceived and developed 

by Sam Stulberg and Roger L. Payne. The Geographic Information 

Retrieval and Analysis System was developed by Robin G. Fegeas, 

K. Eric Anderson, Stephen c. Guptill, Cheryl A. Hallam, and 

William B. Mitchell. The small-scale Digital Line Graph data 

structure and attribute coding scheme was developed by Warren E. 

Schmidt and Michael A. Domaratz. 

The Circular was compiled in part from various user guides 

and technical instructions of the National Mapping Division. 

These documents were originally prepared by several individuals; 

credit is acknowledged to G. Michael Callahan, A. Joan Szeide, 

William R. Allder, Vincent M. Caruso, Hugh w. Calkins, Donna 

Cedar-Southworth, and Cheryl A. Hallam. The compilation of the 

various guides, instructions, and other material into the 

Circular format was performed with major assistance by Clark H. 

Cramer, Eloise R. Byrd, and Cynthia L. Cunningham. 

We acknowledge these substantial contributions that have led 

to this publication. 

iv 
CONTENTS 

Page 

Foreword • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • iii 

Preface . • . . . • . . . . . • • • • • • • • • • . . . . . . . . . . . . . . . • . • • • • • • • • • • • • • • i v 

Abstract • • . • • • • . . . . . . • • . • . • • • • • . • • • • • • • • • • • • • • • • • • . • . . . • • • • 1 

Introduction . . . • • • . . . • • . • • • . . . . . . . . • . . . . • . . . . . . . • • . . . . . • • • • 1 

Source data characteristics 2 

Land use and land cover map •••••••••••••••••••••••••••• 2 

Political unit map ••••••••••••••••••••••••••••••••••••• 3 

Census county subdivision map •••••••••••••••••••••••••• 3 

Hydrologic unit map •••••••••••••••••••••••••••••••••••• 4 

Federal land ownership ••••••••••••••••••••••••••••••••• 4 

State land ownership map ••••••••••••••••••••••••••••••• 4 

GIRAS data structure elements •••••••••••••••••••••••••••••• 4 

Creation of the GIRAS files •••••••••••••••••••••••••••••••• 5 

Applications • . . • • • • • • . . . . • . • • • . • • • • . . . . • • • . • . • . . . • • . . • . . . . • 6 

Manipulation and analysis •••••••••••••••••••••••••••••• 6 

Display • . • • • • • . • • • • • . • • • . . • • • • • . • • • • • • • . . • • • • • • • . • • • • • . 7 

Data vol\lllles • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 7 

GIRAS file fonnat • . . . . . • • . . . • . • • . • • . • • • • • . • . . • • . • • • • . . . • . . . 8 

The standard local UTM coordinate frame of reference ••• 9 

Map header . • . • • . • • • • • . • • . • • . . • • • • • • • • • • . . • • • . • • • . • . . . • • 9 

Section header ••••••••••••••••••••••••••••••••••••••••• 10 

Arc records subfile •••••••••••••••••••••••••••••••••••• 11 

Coordinate subfile ••••••••••••••••••••••••••••••••••••• 11 

Polygon records subfile •••••••••••••••••••••••••••••••• 12 

FAP subfile • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 13 

Text subfile ••••••••••••••••••••••••••••••••••••••••••• 14 

Associated data subfile •••••••••••••••••~•••••••••••••• 16 

Composite theme grid (CTG) data file format •••••••••••••••• 16 

Binary CTG data file format •••••••••••••••••••••••••••• 16 

Binary CTG map header ••••••••••••••••••••••••••••• 17 

Character composite theme grid (CTG) file format ••••••• 18 

Standard character-formatted CTG data file •••••••• 19 

Character CTG map header •••••••••••••••••••••••••• 19 

References • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 20 

Appendix. Listing of CTG map header data • • • • • • • • • • • • • • • •.• • 21 

Figure 1. 

2. 

3. 

4. 

5. 

6. 

ILLUSTRATIONS 

Topological elements of a polygon map •••••••••• 

GIRAS file structure ••••••••••••••••••••••••••• 

GIRAS map header structure ••••••••••••••••••••• 

GIRAS section header structure ••••••••••••••••• 

GIRAS arc records subfile •••••••••••••••••••••• 

GIRAS coordinate subfile ·•••••••••••••••••••••• 

v 

Page 

5 

8 

10 

10 

11 

11 
7. 

a. 

9. 

10. 

Table 1. 

2. 

3. 

GIRAS polygon records subfile •••••••••••••••••• 

GIRAS FAP subfile •••••••••••••••••••••••••••••• 

GIRAS FAP subfile creation ••••••••••••••••••••• 

GIRAS text record subfile •••••••••••••••••••••• 

TABLES 

u.s. Geological Survey land use and land cover 

classification system for use with remote 

Page 

12 

13 

14 

15 

Page 

sensor data •••••••••••••••••••••••••••••••••••• 3 

Federal land ownership ••••••••••••••••••••••••••• 

Sample GIRAS data volumes from 1:250,000-scale 

4 

map overlays ••••••••••••••••••••••••••••••••••• 7 

4. Map type codes for GIRAS data base ••••••••••••••• 9 

5. Sample text subfile for a Census County 

Subdivision GIRAS file ••••••••••••••••••••••••• 16 

vi 
USGS Digital Cartographic Data Standards 

LAND USE AND LAND COVER DIGITAL DATA 

By Robin G. Fegeas, Robert w. Claire, 

Stephen c. Guptill, K. Eric Anderson, and Cheryl A. Hallam 

Abstract 

The discipline of cartography is 

undergoing a number of profound changes 

that center on the emerging influence of 

digital manipulation and analysis of 

data for the preparation of cartographic 

materials and for use in geographic information systems. Operational requirements have led to the development by the 

USGS National Mapping Division of several 

documents that establish in-house digital 

cartographic standards. 

In an effort to fulfill lead agency 

requirements for promulgation of Federal 

standards in the earth sciences, the 

documents have been edited and assembled 

with explanatory text into a USGS 

Circular. This Circular describes some 

of the pertinent issues relative to 

digital cartographic data standards, 

documents the digital cartographic data 

standards currently in use within the 

USGS, and details the efforts of the USGS 

related to the definition of national 

d_igital cartographic data standards. It 

consists of several chapters; the first 

is a general overview, and each succeeding chapter is made up from documents 

that establish in-house standards for one 

of the various types of digital cartographic data currently produced. This 

chapter 895-E, describes the Geographic 

Information Retrieval and Analysis System 

that is used in conjunction with the 

USGS land use and land cover classification system to encode, 

and analyze land use 

digital data. 

edit, manipuate, 

and land cover 

INTRODUCTION 

The u.s. Geological Survey (USGS) is 

currently producing land use and land 

cover maps and associated overlays for 

the entire United States. These maps are 

being digitized, edited, and incorporated 

into a digital data base. The data will 

be available to the public in both 

graphic and digital form. 

These maps will help satisfy a longstanding need for a consistent level of 

detail, standardization of categories, 

and consistent use of scales of compilation for a type of data used by government land use planners, land managers, 

and resource-management planners. Once 

this benchmark series of maps is completed, updating of the maps will provide 

a much-needed tool for analyzing trends, 

problems in local and regional areas 

throughout the Nation, and changes in 

land use patterns. 

The set of land use and land cover 

and associated maps consists of land use 

and land cover, political units, hydrologic units, census county subdivisions, 

Federal land ownership, and State land 

ownership (optional). 

The land use and land cover map is 

compiled to portray the Level II categories of the land use and land cover 

classification system documented by 

Anderson and others (1976). The Level II 

categories of this land use and land 

cover classification system provide the 

user with a basic framework to which 

third- and fourth-level categories may be 

added. 
The associated maps portray either 

natural or administrative information and 

provide the user with the opportunity to 

utilize the land.use and land cover maps 

and data either individually or collectively to produce graphic or statistical 

data for the areas portrayed on the associated maps. This mapping system is constructed in such a way that the graphical 

and statistical land use and land cover 

data can be related to other resource 

fields such as soils, geology, hydrology, 

and demography. 

To provide the data in digital form, 

the Geographic Information Retrieval and 

Analysis System (GIRAS) has been developed (Mitchell and others, 1977). The 

data structure used in GIRAS to store the 

information is the end result of a series 

of evolving structures and, as such, reflects the judgment by the USGS concerning the presentation and format of polygonal data. For those users better able 

to handle data in a grid cell form, data 

are also provided in a Composite Theme 

Grid (CTG) format. 

SOURCE DATA CHARACTERISTICS 

The characteristics of the digital 

cartographic data base for land use and 

land cover and associated maps reflect 

the parameters used in compiling the 

maps. The land use and land cover mapping program is designed so that standard topographic maps at a scale of 

1:250,000 can be used as a base for compilation and reproduction. After the 

start of the mapping program, the USGS 

began compiling intermediate-scale topographic maps at a scale of 1 :100,000. 

The USGS anticipates preparing land use 

and land cover and associated maps at a 

scale of 1:100,000 and will release them 

to the open-file system when 1 :100,000-

scale topographic map bases become 

available. 

The 1:250,000-scale mapping format is 

generally a quadrangle unit of 1° of latitude x 2° of longitude. The 1:100,000-

scale mapping format has been established 

as a 30 1 x 60 1 quadrangle, normally a 

quarter of a 1:250,000-scale quadrangle. 

Both series use the Universal Transverse 

Mercator projection. 

2 

LAND USE AND LAND COVER MAP 

The basic purpose of this map is to 

provide land use and land cover data to 

be used either as data sources by themselves or in combination with the other 

data sets produced in the program. One 

of the basic sources of land use compilation data is the NASA high-altitude U-2/ 

RB-57 aerial photo coverage, usually at 

scales smaller than 1:60,000. The 

1:250,000-scale topographic map series 

is used as the base map for the compilation of the land use and land cover maps 

and the associated overlays, with the exception that the 1:100,000-scale topographic map base is used if that base map 

is available at the time the data set is 

released to the open file system by the 

USGS. Although compilation of land use 

and land cover data is performed on a 

film-positive base enlarged to a scale 

of approximately 1:125,000, the' associated overlays are both compiled and 

digitized at a scale of 1:250,000. 

Land use and land cover data compilation is based upon the classification 

system and definitions of Level II land 

use and cover shown in table 1. 

All features are delineated by curved 

or straight lines that depict the actual 

boundaries of the areas (polygons) being 

described. The minimum size of polygons 

depicting all Urban or Built-up Land 

(categories 11-17), Water (51-54), Confined Feeding Operations (23), Other 

Agricultural Land (24), and Strip Mines, 

Quarries, and Gravel Pits (75) is 4 hectare (ha). All other categories of land 

use and land cover have a minimum polygon 

size of 16 ha. Those sizes also are considered the minimum sizes to which polygons are digitized. In the Urban or 

Built-up Land and Water categories, the 

minimum width of a feature to be shown 

ie 200 m; that is, if a square with sides 

200 m in length is delineated, the area 

will be 4 ha. Although the minimum-width 

consideration precludes the delineation 

of very narrow and very long 4-ha polygons, triangles or other polygons are 

acceptable if the base of the triangle or 

minimum width of the polygon is 200 m in 

length and if the area of the polygon is 

4 ha. For categories other than Urban 

or Built-up Land and Water, the 16-ha 
Table 1.--u.s. Geological Survey Land Use 

and Land Cover Classification 

System for Use with Remote 

Sensor Data 

2 

4 

5 

6 

7 

8 

9 

Urban or 

Built-up Land 

Agricultural 

Land 

Rangeland 

Forest Land 

Water 

wetland 

Barren Land 

Tundra 

Perennial snow 

or Ice 

LEVEL II 

11 Residential 

12 Commercial and Services 

13 Industrial 

14 Transportation, Communications 

and Services 

15 Industrial and Commercial 

complexes 

16 Mixed Urban or Built-up Land 

17 Other Urban or Built-up Land 

21 Cropland and Pasture 

22 Orchards, Groves, Vineyards, 

Nurseries, and ornamental 

Horticultural Areas 

23 Confined Feeding Operations 

24 Other Agricultural Land 

31 Herbaceous Rangeland 

32 Shrub-Brushland Rangeland 

33 Mixed Rangeland 

41 Deciduous Forest Land 

42 Evergreen Forest Land 

43 Mixed Forest Land 

51 Streams and Canals 

52 Lakes 

53 Reservoirs 

54 Bays and Estuaries 

61 Forested Wetland 

62 Nonforested Wetland 

71 Dry Salt Plats 

72 Beaches 

73 sandy Areas 

Other than Beaches 

74 Bare Exposed Rock 

75 Strip Mines, Quarries, 

and Gravel Pits 

76 Transitional Areas 

77 Mixed Barren Land 

81 Shrub and Brush Tundra 

82 Herbaceous Tundra 

83 Bare Ground 

84 Wet Tundra 

85 Mixed Tundra 

91 Perennial Snowfields 

92 Glaciers 

minimum size for delineation requires a 

minimum width polygon of 400 m. Line 

weight for delineating land use and land 

cover polygons and for neat lines is 

0.10 mm at the production scales of 

1:250,000 or 1:100,000. 

3 

POLITICAL UNIT MAP 

The political unit map provides a 

graphic portrayal of the county and State 

boundaries and is compiled using base 

maps at scales of either 1:250,000 or 

1:100,000. Source material for the political unit maps is from the Bureau of 

the Census unpublished maps entitled 

"County Subdivisions--Townships and 

Places" and from the Geographic Identification Code Scheme (U.S. Bureau of the 

Census, 197 2a) and the County and City 

Data Book (U.S. Bureau of the Census, 

1972b). The "County Subdivision--Townships and Places 11 maps are used also to 

separate census county subdivisions into 

census tracts. State and county political subdivisions are encoded with a 

five-digit number in accordance with the 

Geographic Identification Code Scheme 

with the exception that nontracted "independent cities" in Maryland, Missouri, 

Nevada, and Virginia are given an eightdigit code reflecting the State and city 

codes. 

CENSUS COUNTY SUBDIVISION MAP 

The census county subdivision map 

depicts census tracts in Standard Metropolitan Statistical Area (SMSA) counties 

and the minor civil division or equivalent division in non-SMSA counties. This 

overlay map is digitized in the same 

manner as the other overlays. The map is 

based on data from a Bureau of the Census 

publication entitled Census Tracts that 

d~scribes the census tracts of the SMSAmapped counties (u.s. Bureau of the 

Census, 19 7 2a ) and on a Bureau of the 

Census publication entitled Geographic 

Identification Code Scheme (u.s. Bureau 

of the Census, 1972c). Each census tract 

is encoded with a four-digit number that 

is unique for each census tract. Minor 

civil divisions are encoded with an 

eight-digit number: the first two digits 

are the State code, the next three are 

the county code, and the last three are 

the minor civil division identifier. 
HYDROLOGIC UNIT MAP 

The hydrologic unit map is based on 

the Hydrologic Unit Maps published by 

the u.s. Geological Survey Office of 

Water Data Coordination, together with 

the list "Boundary descriptions and name 

of region, subregion, accounting units, 

and cataloging unit." The hydrologic 

units are encoded with an eight-digit 

number that indicates the hydrologic 

region (first two digits), hydrologic 

subregion (second two digits), accounting 

unit (third two digits) , and cataloging 

unit (fourth two digits). 

FEDERAL LAND OWNERSHIP MAP 

The USGS has the responsibility for 

researching, obtaining, and formatting 

maps, plots, and other descriptive data 

related to Federal land ownership. Minimum size for the delineation is 16 ha. 

Ownership is encoded according to the 

agencies listed in table 2. 

STATE LAND OWNERSHIP MAP 

In instances in which the USGS has a 

cost-sharing cooperative agreement with 

a specific State, a map overlay showing 

an inventory of State-owned land is produced from data furnished by the State. 

Although this overlay is compiled to the 

same map base used for the other overlays, the polygons are encoded according 

to the referencing system used by the 

State. 

GIRAS DATA STRUCTURE ELEMENTS 

The GIRAS digital data structure was 

designed to handle large quantities of 

m~p data Gf the polygon type. The topological elements associated with polygon 

maps are shown in figure 1. A polygon is 

an area that is homogeneous in the characteristic being mapped. An arc describes a boundary either between two 

polygons or between a polygon and the 

outside of the map. Further defined, an 

arc begins at one node, or point common 

to three or more arcs (that is, an intersection), and ends at another node but 

4 

11 

12 

13 

21 

31 

32 

33 

34 

41 

42 

43 

44 

45 

46 

47 

48 

51 

61 

71 

72 

73 

81 

82 

83 

84 

85 

Table 2.--Federal Land Ownership 

DEPARTMENT OF AGRICULTURE 

Agricultural Research service 

Forest Service (National Forest) 

Forest service (National Grassland) 

DEPARTMENT OF COMMERCE 

National Oceanic and Atmospheric 

Administration 

DEPARTMENT OF DEFENSE 

Air Force 

Army 

Army (Corps of Engineers - Civil Works) 

Navy 

DEPARTMENT OF THE INTERIOR 

Bonneville Power Administration 

Bureau of Indian Affairs (does not 

include Indian lands held in trust) 

Bureau of Land Management 

Bureau of Mines 

Bureau of Reclamation 

Fish and Wildlife Service (National 

Wildlife Refuge) 

National Park Service (National Monument, 

seashore, and Recreation Area) 

National Park service (National Park) 

DEPARTMENT OF JUSTICE 

Bureau of Prisons 

DEPARTMENT OF STATE 

International Boundary and Water 

Commission, u.s. and Mexico 

DEPARTMENT OF TRANSPORTATION 

Federal Aviation Administration 

Federal Railroad Administration 

u.s. coast Guard 

OTHER AGENCIES 

Energy Research and Development 

Administration 

General Services Administration 

National Aeronautics and Space 

Administration 

Tennessee Valley Authority 

Veteran's Administration 

does not pass through any node. A special case is a simple island polygon 

totally surrounded by a larger polygon. 

For purposes of digitizing, an arbitrary 

point on the boundary of the island is 

chosen as the beginning and ending node 

of the arc. Each polygon label on the 

map is a code, not necessarily unique, 

that identifies or describes the polygon 

in which it is placed or to which it 

points. 
21 

42 

• 

12 

M 

@ 

EXPLANATION 

NODE 

ARC 

POLYGON 

ISLAND 

12 

COMPLEX ISLAND 

POLYGON LABEL 

Figure !.--Topological elements of a 

polygon map. 

In the GIRAS structure, the common boundaries, or arcs, are digitized only once. 

The arcs are then linked together by 

editing software to form polygons. 

5 

Within the GIRAS data structure, the 

basic topological elements of a polygon 

map (arcs, nodes, and polygons) are all 

uniquely identified and cross-referenced 

to one another. The spatial location of 

an arc is given by a string of x,y 

points; the first point is its beginning 

(from) node,· and the last point is its 

ending (to) node. The sequence of points 

of an arc defines a direction that, since 

the arc separates two polygons, determines a polygon to the right and a polygon to the left. A single node is the 

endpoint (first and (or) last) of three 

or more arcs (or the first and last endpoint of a one-arc island). A given 

polygon is spatially defined as the sequence of arcs that constitute its boundary, both external perimeter and any internal islands· 

CREATION OF THE GIRAS FILES 

The data capture procedure involves 

the conversion of the source material 

into a digital format. As defined by 

GIRAS, the digitization process includes 

not only the initial conversion to digital form, but also the editing process 

by which clean or logically correct data 

files are produced. 

In digitizing, lines are not tagged 

in any way, and all that is required is 

the capability to recognize line 

intersections (nodes) during computer 

processing. Since land use and land 

cover maps consist entirely of polygons, 

the map is completely defined when each 

arc, including the arcs that are the 

boundaries of the map, has been digitized. Along with the line data, a file 

that contains at least one attribute and 

a point inside the polygon (represented 

by an x,y coordinate pair) for each polygon of the map must be entered into the 

system. These data are not in the GIRAS 

format at the end of the digitizing. 

Data are converted to the GIRAS format 

only when editing is finished. 

After the necessary data have been 

captured, the data go through the 

following steps to produce a GIRAS file: 

1· Conversion of the data file to the 

standard editing format; 
2. Data reduction, by elimination of 

points not needed to define lines 

within a specified tolerance; 

3. Splitting of data sets into 

sections, if necessary; 

4. Limited automatic editing and 

detection for the arc data; 

5. Manual batch editing of line 

with returns to step 4 until 

are error free; 

error 

data, 

data 

6. Merging of labeled polygon points 

with the arc data resulting in 

either further error detection or 

clean files; 

7. Manual batch editing of polygon 

labels data with returns to steps 5, 

6, or 7, if necessary; 

a. Edge matching of each section with 

neighboring sections and of each map 

with every available adjoining map; 

9. Conversion of data files to standard 

GIRAS format; and 

10. Transformation 

coordinates to 

system; where 

equals 10 m. 

of data from table 

the UTM coordinate 

one coordinate unit 

APPLICATIONS 

MANIPULATION AND ANALYSIS 

Spatial data in the GIRAS format can 

be applied to individual problems through 

manipulation and analysis procedures 

such as: 

1. Rotation, translation, and scaling of 

the coordinates; 

2 • Conversion to geographic coordinates 

and from geographic coordinates to 

specified map projections; 

3. Conversion from arc-segment polygon 

structure to grid cells of a specified size; 

4. Conversion to standard polygon 

fonnat; 

5. Production of area summary statistics 

from polygon or grid cell data; 

6. Production of border (perimeter) and 

adjacency lengths of particular polygon types from the polygon data; and 

7. Selection of a portion of a map for 

closer consideration by using procedures 1 through 6. 

6 

The first two procedures in the list 

deal with coordinate conversion. To 

apply the data to needs of various users, 

it is often necessary to be able to 

rotate and translate the coordinate 

system and scale it to the desired size. 

Similarly, a facility to transform the 

data to another map projection is desirable, particularly when supplemental 

data exist on a different projection. 

A number of existing data systems 

utilize data stored in grid cells. Thus, 

the ability to convert the polygon structure to a grid-cell format can be very 

useful. 

The land use and land cover and associated map data files were converted to 

grid cells of a specific size and orientation to permit their addition to a 

existing data base. This permitted GIRAS 

data to be used where the GIRAS data 

structure was inappropriate. GIRAS's 

capability to produce data in both 

polygon and grid-cell formats provides a 

flexibility whereby the needs of more 

users can be met. 

Area summaries for a GIRAS data set 

may be obtained directly from the file. 

However, if a further breakdown of the 

information (for example, land use within 

each political unit for a data set) is 

wanted, it may be derived more easily 

from grid-cell formatted information than 

from an arc-segment file. Once all associated maps as well as the land use and 

land cover map for a 1:250,000-scale 

quadrangle sheet are in GIRAS format, 

they may be converted to one file of 

grid-cell information (an explanation of 

the CTG data file format begins on page 

16) from which the more detailed summary 

described above may be obtained. For 

example, with the grid-cell ( CTG) file, 

summaries of land use and land cover by 

county within a given drainage basin can 

be produced. 

As with the grid-cell format data, 

the GIRAS format data facilitate production of certain types of information. An 

environmental study might find it useful 

to know the length of the border between 

two noncompatible types of land use or 

land cover (procedure 6). For example, 

if an area of industrial land borders a 

lake under study, the amount of lakeshore occupied by the industrial site 
might be of interest. This type of information can be derived easily from the 

arc-segment GIRAS files. 

While GIRAS stores polygons as the 

arcs of which they are composed, many information systems that deal with polygon 

data store those polygons as complete 

entities. To bridge the gap between the 

two formats, conversion to a standard 

polygon format is necessary. Although 

this requires more storage than GIRAS' s 

format, it allows the use of a simpler 

set of software for plotting, perimeter 

calculation, and area calculation. 

Another technique for extracting information from the GIRAS files is the 

selection of an area smaller than the 

standard 1° x 2° map data file (procedure 

7). It is often helpful to select data 

from the files for closer consideration. 

This windowing process reduces the amount 

of information handled by eliminating the 

portions of the map that are of no interest to the study. Once this smaller portion has been selected, any of procedures 

1 through 6 may be applied. 

DISPLAY 

Computer-generated graphics may be 

used to augme~t the manipulations and 

analyses described previously. A 

computer-generated ~haded color plot of 

the Level II land use and land cover for 

a data set along with a summary of the 

land use gives the investigator a spatial 

perspective of the distribution of land 

use and land cover over the area mapped. 

If it would be more helpful to see only 

one or more of the land uses displayed, 

they can be specially selected. If the 

lengths of borders between two specific 

land uses or land covers are under study, 

a plot showing only those two land uses 

or land covers would be helpful. 

The detailed summary of land use and 

land cover by county might be illustrated 

by placing the outline of the county over 

the land use and land cover map. The 

pattern of land use and land cover for 

that county can be seen with the areas of 

the different land uses computed and 

displayed. 

7 

DATA VOLUMES 

Large sets of complex spatial data, 

such as those handled by GIRAS, necessitate an efficient data structure. Table 

3 shows some measure of GIRAS data volumes derived from records of data editing 

procedures. The numbers of coordinates 

(two coordinates, x and y, per point) are 

those defining the arcs and reflect the 

Table 3.--Sample GIRAS data volumes from 

1:250,000-scale map overlays 

Sample size Average (maps evaluated) 

Land Use and Land Cover 

Coordinates (no.) 54 

Arcs (no.) 54 

Polygons (no.) 51 

Arc length (m) 50 

165,384 

7,340 

2,986 

60 

Political Units 

Coordinates (no.) 47 3,526 

Arcs (no.) 47 42 

Polygons (no.) 54 16 

Arc length (m) 34 9 

Census County Subdivisions 

Coordinates (no.) 45 10,283 

Arcs (no.) 43 419 

Polygons (no.) 49 156 

Arc length (m) 33 12 

Hydrologic Units 

Coordinates (no.) 45 4,850 

Arcs (no.) 43 36 

Polygons (no.) 54 12 

Arc length (m) 34 5 

Federal Land Ownership 

Coordinates (no.) 33 5,492 

Arcs (no.) 32 59 

Polygons (no.) 37 31 

Arc length (m) 24 4 

High 

435,540 

27,339 

9,865 

139 

7,834 

101 

37 

13 

29,088 

2,259 

642 

21 

10,966 

72 

23 

8 

13,896 

604 

222 

16 
m1n1mum number of points needed; straight 

two-point line segments have replaced 

multipoint curves within the spatial resolution tolerance of the original graphic 

map. The actual physical size of a GIRAS 

file is basically a function of the 

numbers of arcs (NA), coordinates (NC), 

and polygons (NP) : approximately ( 36NA + 

2NC + 32NP) bytes, plus identifying and 

descriptive data as specified in the map 

header and text subfile sections. Arc 

lengths are given in terms of meters at 

the scale of the base map ( 1 :250,000). 

The greater complexity of land use and 

land cover maps results in a large number 

of arcs and polygons recorded as compared 

to the number recorded for the associated 

maps. 

GIRAS FILE FORMAT 

The GIRAS file format is a specific 

physical implementation of the logical 

GIRAS data structure. The format was 

designed to optimize storage requirements, transfer operations, and sequential processing. As implemented in a 

computing environment based on 32-bit 

words, a GIRAS file is organized sequentially as 32-byte (S bits per byte) 

logical records, blocked by_ whatever 

factor is appropriate for the particular 

storage device (disk or tape) and computing environment use. Each 32-byte 

record may consist of anywhere from 1 to 

16 data-element fields, depending upon 

where, within the file, it is located. 

Each data-element field may contain only 

one of three types of data elements: 

(1) a 2-byte (16-bit) binary integer, 

(2) a 4-byte (32-bit) binary integer, or 

(3) a string of EBCDIC coded characters, 

1-byte per character. 

For ea.se of transfer to other computing environments, a GIRAS 32-byte 

binary file format may be translated into 

a card image character (coded in either 

ASCII or EBCDIC) file format. Each 32-

byte binary record is expanded to an SOcharacter card image; a 2-byte binary 

integer field is converted to a 5-

decimal-digit character field, and a 4-

byte binary integer field is translated 

into a 10-decimal-digit character field. 

The EBCDIC coded characters within a 

binary-formatted file are translated 

s 

without expansion, allowing two sequentially adjacent 32-byte records containing character data to be converted into 

one SO-character card image record. 

A GIRAS file, either in binary or 

character format, logically consists of 

six or more subfiles. The general structure of a GIRAS file is shown in figure 

2; details are shown in figures 3-s. The 

first six subfiles always exist (in the 

order shown in figure 2); the seventh, 

the text subfile, may or may not exist 

for data released by the National Cartographic Information Center (NCIC); the 

eighth, the associated data subfile, is 

meant for user-attached data and is never 

present in NCIC-released data. A map 

~A - - - - MAP HEADER - ~ - - -

8 SECTION HEADER 

c . ARC RECORDS SUBFILE 

D 

COORDINATE SUBFILE 

E 

POLYGON RECORDS SUBFILE 

F FAP SUBFILE 

ITEMS '8' THRU 'F' REPEATED 

FOR OTHER MAP SECTIONS 

TEXT SUBFILE 

ASSOCIATED DATA. SUBFILE 

Figure 2.--GIRAS file structure. 
data file may need to be divided spatially into several parts (sections) for 

processing purposes. For this reason, 

the second through sixth subfiles (fig. 

2, B through F) may be repeated, one set 

of five subfiles per section. A text 

subfile (only one per GIRAS file) may 

follow the final section subfile. 

THE STANDARD LOCAL UTM 

COORDINATE FRAME OF REFERENCE 

A GIRAS file contains a number of 

different types of data elements--text, 

codes, identifiers, counters, poifters, 

and some derived measurement data. Most 

of these data elements will be explained 

under the descriptions of the various 

subfiles below. However, one type of 

data, the coordinate data, needs an overall discussion here. All coordinates 

stored within a GIRAS file are coded as 

2-byte (16-bit) integers. The coordinate 

frame of reference is defined in the map 

header of the file by a projection code 

(MPJ, see fig. 3) and six control points. 

For each of the control points, the internal x,y coordinates are equated with 

the geographic (latitude and longitude) 

coordinates of the point. The resolution 

of an internal coordinate unit is indicated by the map scale (MSC) value in 

the map header. This value is the scale 

denominator of a graphic plot of the 

GIRAS file, if the data were plotted at 

one internal coordinate unit per mil 

(0.001 in.) on the plot. 

A GIRAS file containing data based on 

a standard USGS 1:250,000- or 1:100,000-

scale quadrangle (such as the land use 

and land cover and associated map series) 

is routinely stored in a local (to the 

map) UTM coordinate system (MPJ = "1" for 

UTM). Since 16 bits are not enough to 

store full UTM coordinates (which may exceed 4,000,000 m), the nearest 100,000-m 

UTM grid intersection, which is both 

west and south of all map control points, 

is used as a local origin (x,y = 0,0). 

Further, the resolution of an internal 

coordinate unit is set to 10 m (the MSC 

value "393, 701"). Using this coordinate referencing system, a GIRAS file 

may store data covering a 327,680-m 

9 

square, more than enough for 1:250,000-

and 1 :100, 000-scale quadrangle-based 

data. 

MAP HEADER 

The map header (fig. 3) contains a 

substantial amount of information, including the amount of data in the file, 

the date of the source material, title 

information, and ground control information. The values in variables NA, NC, 

NP, and LFP represent totals of the corresponding values for each section of the 

map data file. For example, NA is the 

sum of the NAS (number of arcs in a section) values for all sections of the map 

data file. These values can be used to 

estimate the length of the file before 

analysis. The point tolerance (PTL) and 

arc length tolerance (ATL) are values 

used during processing and editing of the 

data to eliminate spurious or unneeded 

data. PTL is the width, in internal 

coordinate units, of a corridor that was 

used to delete unnecessary points from 

each arc of the map and reflects the 

relative accuracy of the original graphic 

lines. ATL is the m~n~mum allowable 

length (in internal coordinate units) of 

any arc on the map. The possible map 

type codes (MTP) are listed in table 4. 

Table 4.--Map type codes for GIRAS data 

base 

Code 

1 

2 

4 

10 

20 

40 

Map type 

Land Use and Land Cover 

Political Units 

Census County Subdivisions 

Hydrologic Units 

Federal Land Ownership (optional) 

State Land Ownership (optional) 

The map date (MDA) is the year of the 

source material used to make the map, 

which is usually not the same as the year 
MAP HEADER 

I I I I I 

p A N M L M 

NA NC NP T T s T T p MSC MDA 

L L c p X J 

X y X y s s N N N N N N s s s s 

M M M M w w w w c c E E E E c c 

N N X X X y X y X y X y X y X y 

SWLA SWLO NWLA NWLO NCLA NCLO NELA NELO 

N N L > 

c. SELA SELO SCLA SCLO A c F E JDA IHS 

D H p w 

TITLE 

TITLE 

I I I I I I I I I l I I I I I 

Length 

Name (bytes) Description 

NA 4 Number of arc records in map. 

NC 4 Number of arc coordinates in 

map. 

NP 4 Number of polygon records in 

map. 

PTL 2 Point tolerance (in coordinate 

units). 

ATL 2 Arc length tolerance (in 

coordinate units). 

NSC 2 Number of map sections. 

MTP 2 Map type code. 

LTX 2 Length of text subfile. 

MPJ 2 Map projection code. 

MSC 4 Map scale denominator (1 

coordinate unit per 1 mil). 

MDA 4 Source map date. 

XMN to 2 ea. Minimum and maximum x,y 

YMX coordinates for map. 

SWX to 2 ea. x,y coordinates of control 

SCY points. 

SWLA to 4 ea. Latitude and longitude of 

SCLO control points. 

NAD 2 Number of records of associated 

data. 

NCH 2 Number of characters in TITLE. 

LFP 2 Length of FAP subfile(s). 

JDA 4 Julian Date of file creation. 

IHS 4 Time of file creation, in 

hundredths of a second. 

TITLE 64 Title of up to 64 characters. 

Figure 3.--GIRAS map header structure. 

10 

the map or data set is published. The 

number of characters (NCH) in the title 

gives information to ease the reading of 

the map title (TITLE) that occupies the 

first NCH bytes of the final two records 

of the map header. 

SECTION HEADER 

Because of computer constraints, the 

number of x,y coordinates (twice the 

number of arc points) of a section of a 

map dat? file is held to 32,000; the 

numbet of arcs, to 2,500; the number of 

polyg6ns, to 1,500; and the total length 

of a FAP (file of arcs by polygon) subfile, to 6, 000. Where these limits are 

exceeded within one map data file, the 

map area is broken into more than one 

section. 

Four elements near the end of the 

section header (fig. 4; XMNS, YMNS, XMXS, 

and YMXS) are the coordinate limits of 

the section, and indicate the minimum and 

maximum x and y coordinate values within 

the section. 

SECTION HEADER 

s 

E 

c 

M X y X y I I I I 

N N N L A M M M M N A c p F R N N X X N Empty 

s s s s K s s s s I I I I 

Length 

Name (bytes) Description 

SEC 2 Section number. 

NAS 2 Number of arcs in section. 

NCS 2 Number of arc coordinates in 

section. 

NPS 2 Number of polygons in section. 

LFS 2 Length of subtile assigning arcs 

to polygons. 

MARK 2 Indicates processing history. 

XMNS, 2 ea. Minimum x,y coordinates in 

YMNS section. 

XMXS, 2 ea. Maximum x,y coordinates in 

YMXS section. 

NN 2 Number of nodes in section. 

Figure 4.--GIRAS section header 

structure. 
ARC RECORDS SUBFILE 

Each record of the arc records subfile (fig. 5) contains a pointer (PLC) to 

the x,y coordinates that represent the 

arc in the coordinate subfile. There is 

one PLC value for each arc, and it represents the position within the coordinate 

subfile of the last coordinate of that 

arc. For example, if the first arc contained 6 points (12 coordinates), its PLC 

value would be 12, and if the second arc 

contained 8 points (16 coordinates), the 

PLC value would be 28 (12+16). Following 

this pattern, the last arc would have a 

PLC value equal to the total number (NCS) 

of coordinates in the coordinate subfile. 

Along with the PLC value, each record in 

the arc records subfile contains the 

unique numbers (PL and PR) of the polygons that this arc separates and the 

types (PAL and PAR) of polygons between 

which this arc forms the border. 

ARC RECORD 

A p I I X y X y I 

I L p p PAL PAR M M M M ALEN s F 

D c L R N N X X N N I I A A A A 1 

Length 

Name (bytes) Description 

AID 2 Arc number. 

PLC 2 Position of last arc coordinate 

in COORDINATE subfile. 

PL 2 Polygon number of polygon to 

left of arc. 

PR 2 Polygon number of polygon to 

right of arc. 

PAL 4 Attribute of polygon to left of 

arc. 

PAR 4 Attribute of polygon to right of 

arc. 

XMNA, 2 ea. Minimum x,y coordinates 

YMNA in arc. 

XMXA, 2 ea. Maximum x,y coordinates 

YMXA in arc. 

ALEN 4 Arc length in coordinate units. 

SN 2 Node number at beginning of arc. 

FN 2 Node number at end of arc. 

Figure s.--GIRAS arc records subfile. 

11 

COORDINATE SUBFILE 

The coordinate subfile (fig. 6) is 

simply a sequential listing of every x,y 

coordinate needed to represent the arcs 

of the map section. When a map is digitized, a series of x,y coordinates is 

recorded for each arc of the map. Although the direction taken by the digitizer during recording is not significant, that direction--once determined by 

digitizing--becomes important in the data 

structure. The direction of recording 

is referred to as the positive direction 

COORDINATE DATA SUBFILE 

Hy++++l I I I I I I I I II• 

•IIIIIIIIIIIIIIIJI~I 

Length 

Variable (bytes) 

2 

Yn 2 

COORDINATE 

(PLC(I-1)+1) 

COORDINATE 

(PLC(I)) 

Description 

x integer coordinate value 

of point n 

y integer coordinate value 

of point n 

is the x coordinate value of 

the first point of arc I. 

[PLC(O)=O] 

is the y coordinate value of 

the last point of arc I. 

The order within the COORDINATE subfile of 

the (x,y) points of a given arc determines the 

direction of the arc, and therefore the right 

and left of the arc. The first point in an arc 

(x,y) string is its "FROM" or ''START'' node; 

the last point in an arc (x,y) string is its "TO" 

or "END" node. 

LEFT 

From To 

RIGHT 

Figure 6.--GIRAS coordinate subfile. 
for the arc, and each arc can be referred to by a positive or negative representation of its arc identification 

number (AID). For example, if arc 1 is 

read from its starting node (SN) to the 

node (FN) at the end of the arc, it is 

represented as "+l" or 1. If the arc is 

to be read from FN to SN (in reverse 

order from that stored in the Coordinate 

subfile), it may be represented by "-1." 

The need for this will become apparent 

when the FAP subfile is described. 

To repeat, the order in which the 

arcs are recorded is not significant, but 

their order, once established, is important to the GIRAS structure. The order 

allows each arc to be accessed by using 

only the unique arc number (AID) and its 

PLC value. No pointers to the beginning 

coordinate of an arc exist in the coordinate subfile. For example, if the PLC 

for arc 1 is 16, then the 17th element of 

the coordinate subfile will be the first 

coordinate of arc number 2. This permits 

storage of the x,y coordinates in the 

smallest possible space. 

POLYGON RECORDS SUBFILE 

The polygon records subfile (fig. 7) 

describes each polygon of the map section. The polygon identification number 

(PID) is the unique number by which each 

polygon may be referenced. The PLA value 

(equivalent in purpose to PLC in an arc 

record) points to a list, in the FAP 

subfile, of the arcs that comprise that 

polygon' s borders. As with the coordinate subfile (referenced by PLC), the 

FAP subfile is a sequential list of 

information. For each polygon it contains the list of the numbers (AID's) of 

the arcs needed to create the boundary of 

the polygon. These values are listed 

first for polygon 1, second for polygon 

2, and so on for each polygon. If polygon 1 of a section were the three-sided 

polygon with attribute 76 in figure 1, it 

would be composed of three arcs, and the 

PLA value for it would be "3." If polygon 2 were the polygon in figure 1 

labeled 42, the situation would be 

slightly more complex because the FAP 

subfile entry for this polygon must also 

12 

POLYGON RECORD 

p p I I X y X y I 

c c M M M M N N 

I L X y ATT AREA N N X X PERL I I 

D A w p I I p p p p I 

Length 

Name (bytes) Description 

PID 2 Polygon number. 

PLA 2 Position of last arc number of 

polygon in F AP subfile. 

CX,CY 2 ea. x,y coordinates of an interior 

point (arbitrarily positioned). 

ATT 4 Polygon attribute. 

AREA 4 Area of polygon in coordinate 

units squared. 

XMNP, 2 ea. Minimum x,y coordinates of 

YMNP polygon. 

XMXP, 2 ea. Maximum x,y coordinates of 

YMXP polygon. 

PERL 4 Perimeter length of polygon in 

coordinate units. 

NIW 2 Number of islands contained 

within polygon. 

NIP 2 Number of the polygon containing 

this polygon, if it is an 

island. 

Figure 7.--GIRAS polygon records subfile. 

include the arcs that make up the outside boundary of the island within polygon 2. In this case, the value of PLA 

would be the number of arcs it takes to 

enclose the polygon ( 4) plus one place 

for a zero (indicating that what follows 

is a list of arcs making up an island) 

and one place for an arc number to represent the island (2) plus the PLA of the 

previous polygon (3). The PLA value for 

polygon 2 would then be 9. It also 

follows that the PLA value for the last 

polygon will equal the total length of 

the FAP subfile (LFS in the section 

header). A detailed explanation of the 

FAP subfile begins on page 13. 

The next entries in the polygon 

record are the x,y coordinates (CX and 

CY) of a point inside the polygon. These 

coordinates do not represent a centroid 

or center of mass. They define a point 

inside the boundary of the polygon that 
was recorded during the digitizing 

process and used during the editing 

process to tie the arcs together to form 

the polygon. The polygon attribute (ATT) 

and polygon area (AREA) also are included, as well as the minimum and maximum x and y coordinates (XMNP, YMNP, 

XMXP, YMXP) of the polygon. The area of 

a polygon is useful information and is 

stored to avoid recalculation. The 

limits of the polygon allow the user to--

in a simple way--isolate a polygon from 

much of the rest of the map section. The 

perimeter length (PERL) of the polygon 

can be used along with the total area 

occupied by the polygon to compute measures of the compactness of the polygon. 

The final two entries in the polygon 

record are the number of islands (NIW) 

contained within the polygon and the 

number (NIP) of the polygon that contains another polygon as an island. The 

number of islands is helpful when used 

along with PLA to read the island entries 

in the FAP subfile. The NIP entry identifies a polygon totally surrounded by 

another polygon whose attribute can 

easily be obtained by looking at the 

value of ATT in the record for polygon 

number NIP. If the data were to be compressed and small polygons to be eliminated or combined with larger surrounding 

polygons, NIP would show quickly which 

polygon number and type that small island 

would become. 

FAP SUBFILE 

The FAP (file of arcs by polygon) 

subfile (fig. 8) is the last subfile that 

exists separately for each map section. 

It is accessed by way of an entry (PLA) 

in the polygon records subfile. The 

total length (LFS) of the FAP subfile is 

stored in the section header. To understand the contents of the FAP subfile, it 

is helpful to know how its contents are 

used to construct a polygon. The FAP 

subfile consists of lists of arcs, one 

arc-list per polygon. The arcs for a 

polygon with no islands are listed in 

clockwise order around the polygon from 

an arbitrary starting point. This starting point can be either the starting node 

(SN) or final node (FN) of the first arc 

13 

FAP SUBFILE 

l~lflflil llllllllllll• 

•11111111111 ~~11111 

Each record of the F AP subfile consists of 16 

elements (2 bytes each) for the FAP array. 

Each element contains an arc identification 

number pointing to the arcs that make up a 

specific polygon. 

FAP (PLA(I-1)+1) is the first arc bordering 

polygon I. 

[PLA(O)=O] 

F AP (PLA(I)) is the last arc bordering polygon I. 

Within the F AP subfile, the identification 

numbers of the arcs constituting a given 

polygon are ordered clockwise around the 

perimeter of the polygon and counterclockwise 

around interior islands of the polygon. 

A positive arc identification number indicates 

the polygon is to the right of the arc. 

A negative arc identification number indicates 

the polygon is to the left of the arc. 

A zero FAP element indicates that the arc 

identification numbers following it,. and before 

the next zero in the subfile or the end of the 

F AP list, point to the arcs that constitute an 

interior island of the polygon. 

Figure 8.--GIRAS FAP subfile. 

in the list. If the starting point is SN 

for the arc (the polygon to the right of 

the arc), then the number of that arc is 

recorded in FAP as "+AID" or ''AID.'' If 

the starting point is FN (the polygon is 

to the left of the arc), then that arc is 

recorded as "-AID." In figure 9, the 

direction in which the arcs of a polygon 

were digitized (positive direction) is 

indicated by the filled arrows. The list 

of arcs composing the polygon begins at 

the node closest to the internal point 

(CX, CY), and the FAP entry for this 

polygon is "-1 4 3 -2." 
.... _____ _ 

* (CX,CY) 

4 

.... ____ _ 

EXPLANATION 

• NODE 

FAP DIRECTION 

ARC DIRECTION 

1 ,2,3,4 ARC NUMBERS 

* (CX,CY) INTERIOR POINT 

REMAINDER OF MAP 

Figure 9.--GIRAS FAP subfile creation. 

The FAP subfile contains a series of 

elements for each polygon whether or not 

that polygon is an island. If the polygon is an island, a list of the arcs that 

14 

make up its outside boundary will also be 

listed in counterclockwise order in the 

FAP entry for the surrounding polygon in 

which it is an island. 

This points out one factor not yet 

mentioned in describing the FAP subfile. 

In many maps there may be two types of 

islands--simple and complex. Examples of 

both island types can be seen in figure 

1. A simple island is a polygon that 

stands alone, totally surrounded by one 

other polygon and directly bordered by 

only that polygon. A complex island is a 

cluster of adjacent polygons that, as a 

group, are totally surrounded by one 

other polygon. When the arcs that make 

up a complex island are listed--in 

counterclockwise order--in the FAP subfile, only the arcs that compose the outside boundary of the entire cluster are 

recorded. The individual identities of 

the polygons that make up the cluster are 

not maintained because the cluster is 

considered to be an area to exclude from 

the polygon being described. 

The FAP subfile is crucial to the 

GIRAS data structure because it is the 

way individual arcs can be described as 

part of the polygons they compose. From 

this file the entire outline of a polygon 

can be obtained, including island areas 

within its boundary that are to be excluded. This information is used, for 

example, when computer-generated shaded 

color plots are made. 

TEXT SUBFILE 

The text subfile in a GIRAS file is 

reserved as a place where numerical polygon codes are assigned textual labels. 

An individual GIRAS file includes only a 

single text subfile that physically 

follows the data for the various map 

sections. 

Every record in the text subfile consists of three elements--an attribute 

code, a hierarchy code, and a descriptor 

(fig. 10). 

A text subfile record format of (A4, 

A2, 13A2,/16A2) was designed to be compatible with the standard binary representation of GIRAS files. Each logical 

text subfile entry comprises two GIRAS 

32-byte records. 

Included in the text subfile are 

records for general codes that provide 
TEXT RECORD 

Length 

Name (bytes) 

ATT 4 

HIER 2 

DESC 58 

DESC 

Description 

Polygon code, right justified in 

a 10-digit field. 

A 5-digit parameter to indicate 

polygon code type (that is, 

specific, general, or special). 

A label or descriptor defining 

the polygon code, left justified, in a 58-character field. 

Figure 10.--GIRAS text record subfile. 

labels or descriptors for the more 

general categories under which specific 

polygon codes come. Consider references 

made to Level I and Level II land use and 

land cover categories. A polygon coded 

as "13" is known to be Industrial. The 

digit '1' indicates that it falls under a 

broader category of Urban and Built-up 

Land. General code records also appear 

in the text subfile to provide definitions for these more general categories. 

For the above example, a text subf ile 

record with "10" in the polygon code 

(ATT) field would have the descriptor 

Urban and Built-up Land. (The "0" is for 

positional purposes only and can be considered insignificant.) 

The hierarchy code ( HIER) indicates 

the degree of generality of a text subfile polygon code (classification level). 

Specifically, the hierarchy code is defined as the number of insignificant 

digits on the right-hand side of the 

polygon code. Our sample land use and 

land cover text subfile records would 

thus appear : 

___ ....;10 lURBAN AND BUILT-UP LAND 

___ ....;13 OINDUSTRIAL 

The role of the hierarchy code is more 

appreciated when dealing with other 

15 

classification schemes utilized by GIRAS 

that involve multiple levels. 

Text subfile records are arranged in 

ascending order by polygon code. Consequently, more general codes precede 

general codes of greater detail, which in 

turn precede codes explicitly referenced 

in the file. This ordering can be considered an indentation-type structure, as 

illustrated in table 5 for a sample 

census unit subfile. 

Special polygon codes are required 

where available codes are not suitable. 

The set of special codes used in GIRAS is 

defined at the end of the subfile, as 

also shown in table 5. Special polygon 

codes were intentionally assigned extreme 

values (greater than 2,000,000,000) to 

force them to the bottom of the text 

subfile. 

The type of map stored in a GIRAS 

file has an impact on the nature of the 

text subfile. Land use and land cover 

and Federal ownership maps reference 

classification schemes that are attribute in nature. That is, there are a 

relatively limited number of possible 

codes (37 for land use and land cover and 

26 for Federal ownership), and many polygons may have the same codes. The text 

subfiles for these types of GIRAS files 

list text records for all codes in the 

classification scheme. Consequently, all 

land use and land cover text subfiles are 

identical, as are Federal ownership text 

subfiles. 

Census unit files, political unit 

files, and hydrologic unit files, alternatively, utilize codes that serve as 

unique identifiers. Two polygons rarely 

have the same code. Furthermore, it 

would not be feasible to list all possible codes. Consequently, only codes 

explicitly referenced in the file (and 

respective general and special codes) 

are defined in these types of GIRAS text 

subfiles. 

The text subfiles for State ownership 

maps are not processed because coding 

schemes vary from situation to situation. 

The length of the text subfile is 

given by the LTX parameter in the GIRAS 

map header. LTX refers to the number of 

logical text subfile records (that is, 

polygon code definitions) and includes 

general and.special polygon codes. 
Table 5.--Sample text subfile for a 

Census County Subdivision 

GIRAS file 

42000000 

42051000 

42051095 

42059000 

42059005 

42059127 

42059130 

54000000 

54001000 

54001005 

54001015 

54001025 

1628000000 

1628732000 

1628732000 

1628762000 

1628762000 

1900000000 

1900020100 

1900020100 

1900020200 

1900020200 

1900020900 

2000000000 

2000000100 

2000000101 

2000000102 

2000000200 

2000000201 

2000000202 

6PENNSYLVANIA 

3FAYETTE COUNTY 

OLURERENE 

3GREEN COUNTY 

OALEPPO 

OWAYNE 

OWHITELEY 

6WEST VIRGINIA 

3BARBOUR COUNTY 

OBAKER 

OELK 

OPHILIPPI 

6PITTSBURGH 

2WASHINGTON 

OWASHINGTON 

2WASHINGTON 

OWASHINGTON 

6WHEELING 

2MARSHALL 

OMARSHALL 

2MARSHALL 

OMARSHALL 

0 •••••••••• 

9SPECIAL CODES 

2AREA UNDEFINED BY CODING 

SCHEME 

OLAND AREA UNDEFINED BY 

CODING SCHEME 

OWATER AREA UNDEFINED BY 

CODING SCHEME 

2UNMAPPED AREA 

OUNMAPPED U.S. AREA 

OUNMAPPED NON-U.S. AREA 

ASSOCIATED DATA SUBFILE 

The final GIRAS subfile is the associated data subfile that allows the user 

to store information of his own that 

16 

pertains to the file (for example, population information that pertains to a 

particular census map). A pointer (NAD) 

to this subfile provides a place in the 

map header subfile to store the number 

of records in the associated data file 

once it is created. 

COMPOSITE THEME GRID (CTG) 

DATA FILE FORMAT 

Digital data from all the overlays of 

a given quadrangle also are combined in a 

raster or grid-cell format as a Composite 

Theme Grid (CTG) file. 

A CTG file is available in either 

binary or character format. In either 

format, the files are sequential and consist of fixed-length logical records, all 

of identical internal format, one grid 

cell per logical record. The grid cells 

are actually a regular point sample. The 

attribute codes at the center point of 

each cell are recorded from each overlay. 

The points are oriented to the UTM projection and are usually spaced 200 m 

apart in both east-west and north-south 

directions. The cell records are first 

ordered in the file by row from north to 

south, then within each row, by column 

west to east. 

Header data, usually physically separate from the CTG data file, contain the 

following data necessary to complete the 

description of an individual CTG data 

file: 

1· Title; 

2. Number of overlays and which 

overlays are represented; 

3. Numbers of rows and columns; 

4. UTM zone number; 

5. UTM Easting of the west edge of 

cells; 

6. UTM Northing of the north edge of 

cells; and 

1. Cell width in meters. 

BINARY CTG DATA FILE FORMAT 

Each logical record of a binary CTG 

file is 52 bytes in length. A record 

<:onsists of 13, 32-bit (4-byte) binary 

integers in the following order: 
Bytes 1- 4 

5- 8 

9-12 

13-16 

17-20 

21-24 

25-28 

29-32 

33-52 

Row index, where 1 is the 

index of the northern-most 

row and index numbers increase by one for each row 

moving south (NOTE, due to 

a processing error, CTG 

files in which the State 

ownership is not coded will 

have all zero row index 

numbers 1 the row index is 

then a function of the 

sequential position of the 

record within the file}; 

Column index, where 1 is 

the index of the westernmost column and index numbers increase by one for 

each column moving east; 

Land use and land cover 

code; 

Political unit code; 

Hydrologic unit code; 

Census county subdivision 

or SMSA tract code; 

Federal land ownership 

code; 

State land ownership code 1 

and 

Null (binary zeros} fields. 

If a given overlay has not been digitized, the codes for that overlay will 

all be zero. To be sure that a regular 

. grid of cells (form-ing a UTM rectangle} 

covers the entire base map quadrangle, a 

"buffer zone" of cells with all zero 

attributes has been included in the 

binary CTG data file. 

Binary CTG Map Header 

The header data associated with a 

binary CTG data file are stored in a 

physically separate sequential file, 

modeled after the GIRAS fonnatted map 

header. The header consists of six, 32-

byte logical records. All elements in 

the first four records are coded as 

either 16-bit (2-byte} or 32-bit (4-byte} 

binary integers. The title in the fifth 

and sixth records is coded as a string 

of EBCDIC characters (one character per 

byte}. 

17 

Record 1: 

Bytes 1- 4 

5- 8 

9-12 

13-14 ::; 

15-16 

17-18 

19-20 

21-22 

23-24 

25-28 

29-32 

Record 2: 

Bytes 1- 2 

3- 4 

5- 6 

7- 8 

9-10 

11-12 

13-14 

15-16 

17-18 

19-20 

21-22 

23-24 

25-26 

27-28 

29-30 

31-32 

Record 3: 

Bytes 1- 4 

Number of rows; 

Total number of cells x 

2; 

Number of columns; 

Meaningless field (duplicate point tolerance}; 

Cell size (width and 

length) in meters; 

Number of overlays 

merged; 

Map type 

below} 1 

code (see 

Projection zone number; 

Map projection code 

(should be "1" for UTM)~ 

Scale of a plot at one 

mil per cell width; and 

Source date of the land 

use overlay. 

Minimum column index; 

Minimum row index; 

Maximum column index; 

Maximum row index; 

Column index for SW control point; 

Row index for SW control 

point; 

Column index for NW control point; 

Row index for NW control 

point; 

Column index for NC control point; 

Row index for NC control 

point; 

Column index for NE control point; 

Row index for NE control 

point; 

Column index for SE control point; 

Row index for SE control 

point; 

Column index for SC control point; and 

Row index for SC control 

point. 

Latitude of SW control 

point; 
5- 8 

9-12 

13-16 

17-20 

21-24 

25-28 

29-32 

Record 4: 

Bytes 1- 4 

5- 8 

9-12 

13-16 

17-20 

21-24 

25-28 

29-32 

Records 5 and 6: 

Bytes 1-32 = 

Longitude of SW control 

point; 

Latitude of NW control 

point; 

Longitude of NW control 

point; 

Latitude of NC control 

point; 

Longitude of NC control 

point; 

Latitude of NE control 

point; and 

Longitude of NE control 

point. 

Latitude of SE control 

point; 

Longitude of SE control 

point; 

Latitude of SC control 

point; 

Longitude of SC control 

point; 

UTM Easting value of 

west edge of cells; 

UTM Northing value of 

north edge of cells; 

File creation date (a 

Julian date); and 

Meaningless field (time 

of file creation). 

Title (text characters 

in EBCDIC coding). 

Some further explanation · is needed 

for some of the elements in the CTG map 

header: 

1. The map type code (in bytes 19-20 of 

the first record) indicates which 

overlays have been included in the 

CTG data file. The code is formed 

by the addition (in base 10) of the 

separate GIRAS map type codes for 

each of the overlays: 

01 Land use and land cover; 

02 

04 

10 

20 

40 

Political units; 

Census county subdivisions 

and SMSA tracts; 

Hydrologic units; 

Federal land ownership; and 

State land ownership. 

18 

For example, the map type code for a 

combination of the first four 

overlays above would be 17; all six 

overlays combined have a map type 

code of 77. 

2. The UTM Easting and Northing values 

given in the fourth record (bytes 

17-24) are in whole meters and are 

values for the west and north edges 

of the cells, rather than the center 

point of the first (northwest 

corner) cell. The Easting and 

Northing values for a given cell may 

be calculated thus: 

Easting 

Northing 

(XORG-CW/2) + (column 

index)*CW 

(YORG+CW/2) - (row 

index)*CW 

where XORG and YORG are the Easting 

and Northing values in bytes 17-24 

of the fourth header record, and CW 

is the cell width in bytes 15-16 of 

the first header record. 

3. The control points usually define the 

1° x 2° (for 1:250,000-scale base 

maps) or 30 1 x 1° (for 1 :100,000-

scale base maps) quadrangle on which 

the overlay data are based. The 

latitude and longitude values are 

given as positive integers of the 

form DDDMMSS, where DDD is degrees, 

MM is minutes, and SS is seconds. 

Western longitude values are given as 

positive numbers, increasing in value 

from east to west. The row and 

column values given for the control 

points are the indices for the cell 

whose center point is closest to the 

true position of the control point. 

CHARACTER COMPOSITE THEME GRID (CTG) 

FILE FORMAT 

For ease of transfer and readabiltiy, 

CTG files also are available in character-coded (ASCII or EBCDIC) format. 

Each binary integer value or code has 

been converted to a number, right justified (with leading blanks if needed) 
within a fixed-length field of an socharacter record. All character CTG 

data files consist of fixed length socharacter logical records, one grid cell 

per logical record. Two internal record 

formats are available--one with row and 

column index numbers such as the binary 

CTG format and the second (and standard) 

format with UTM zone, Easting, and 

Northing values in place of row and 

column indices. 

The row and column number format is a 

direct copy of a binary CTG file with 

each 4-byte binary integer represented 

as a 10-digit character-formatted integer. The null fields (bytes 33-52) of 

the binary CTG data records are not 

copied. Each character CTG data record 

in this format has eight, 10-digit 

integer fields--row and column indices 

followed by the six attribute codes in 

the same order as in the binary CTG data 

file. 

Standard Character-Formatted 

CTG Data File 

Each logical record of a standard 

character-formatted CTG data file is SO 

characters in length and consists of nine 

decimal integ~rs, right justified (with 

leading blanks) within fixed-length 

fields: 

Bytes 1- 3 

4-11 

12-19 

20 

21-30 

31-40 

41-50 

51-60 

UTM zone number (this 

value should be the same 

in every record of a 

given CTG file); the 

first byte will always be 

a blank for zones in the 

northern hemisphere; 

U'IM Easting value, in 

whole meters, for the 

sample point of the cell; 

UTM Northing value, in 

whole meters, for the 

sample point of the cell; 

Blank; 

Land use and land cover 

attribute code; 

Political unit (FIPS 

State/county) code; 

USGS hydrologic unit code; 

Census county subdivision 

or SMSA tract code; 

19 

61-70 Federal land ownership 

agency code; and 

71-SO State land ownership code. 

If a given overlay category has not 

been included within the file, the codes 

for that category will be zero (O). 

Since some misregistration of map overlays occurs, some of the cells along. the 

edges of the 1:250,000- or 1:100,000-

scale quadrangle may have codes for some 

overlays, but not others (the "other" 

code(s) will be zero). The standard 

character CTG data file will have only 

those cell records for which at least one 

of the categories is coded. This means 

that, since the 1:250,000- and 1:100,000-

scale quadrangles do not form perfect 

rectangles in the UTM projection (lines 

of latitude curve and lines of longitude 

converge), a variable number of cell 

records will exist for any given row or 

column. 

Character CTG Map Header 

CTG map header data associated with 

a character CTG data file may exist in 

one of three states: (1) as a separate 

file, ( 2) as the first five records of 

the character CTG data file; and (3) as 

a printed listing. In either of the 

first two forms, the data exist in five 

SO-character records. The first four 

records are exact copies of their binary 

CTG header counterparts, but with the 16-

bit ( 2-byte) integers of the first two 

records converted to five-digit integer 

fields and the 32-bit ( 4-byte) integers 

of records 1, 3, and 4 converted to 10 

digit integer fields (right justified 

with leading blanks). The title is 

carried as the first 64 characters of the 

fifth record (with 16 blanks to fill the 

end of the record). 

A listing of the CTG map header data, 

which is meant to be delivered to a user 

with the CTG data file on tape, lists all 

the elements (see the list under the description of a binary CTG map header) of 

the header (but not in exactly the same 

order as in an actual CTG map header). 

A sample listing of CTG map header 

data (as produced by the binary-tocharacter conversion program CTGBTA) is 

shown in the Appendix. 
REFERENCES 

Anderson, J. R., Hardy, E. E., Roach, 

J. T., and Witmer, R. E., 1976, A 

land use and land cover classification system for use with remote 

sensor data: u.s. Geological Survey 

Professional Paper 964, 28 p. 

Mitchell, w. B., Guptill, s. c., 

Anderson, K. E., Fegeas, R. G., and 

Hallam, c. A., 1977, GIRAS--A geographic information retrieval and 

analysis system for handling land 

use and land cover data: u.s. 

Geological Survey Professional Paper 

1059, 16 P• 

20 

u.s. Bureau of the Census, 1972a, Census 

tracts--1970 census of population 

and housing: U.S. Bureau of the 

Census Final Reports PHC(l)-1 

through PHC(l)-241. [One volume for 

each SMSA.] 

_____ 1972b, 

book, 1972: 

Census, 1020 P• 

County 

u.s. 

and city 

Bureau of 

data 

the 

----~-----1972c, Geographic identification code scheme--1970 census of population and housing: u.s. Bureau of 

the Census Reports PHC(R)-1 through 

PHC(R)-4. [One volume for each of the 

u.s. regions: NE, NC, s, and w.] 
APPENDIX 

Listing of CTG Map Header Data 

COMPOSITE THEME GRID CHARACTER DATA OUTPUT: 

C T G B T A RUN: JUNE 3, 1982 TIME 19:23:06 

GRID CELL MAP HEADER INFORMATION: 

TITLE: LAWRENCE, MO KS 1:250,000 QUAD LU PB CN HU FO SO 

FILE CREATION DATE: 81084 TIME 0: 0 

MAP TYPE: 77 PROJECTION: 1 SCALE 1: 7874016 MAP DATE: 1973 

NUMBERS OF FILE ELEMENTS: 

CATEGORIES CELLS ROWS COLUMNS ZONE WEST & NORTH EDGES: 

NUMBER EASTING NORTHING 

6 485368 575 884 15 236900 4321100 

DUPLICATE POINT TOLERANCE = 0 CELL SIZE IN METERS = 200 

MIN COL = 1 MIN ROW = 1 MAX COL = 884 MAX ROW = 575 

CONTROL POINT INFORMATION: 

LONGITUDE LATITUDE COL ROW 

SOUTH WEST 960000 380000 -1 557 

NORTH WEST 960000 390000 17 2 

NORTH EAST 940000 390000 883 21 

SOUTH EAST 940000 380000 877 576 

CHARACTERISTICS OF THE CHARACTER CTG FILE: 

THE FILE CONTAINS ONLY GRID CELL (AND NO HEADER) RECORDS. 

THE FILE CONSISTS OF 80 CHARACTER RECORDS, ONE GRID CELL PER RECORD. 

UTM ZONE, EASTING, AND NORTHING VALUES ARE PART OF EACH CTG DATA RECORD 

AS THE FIRST THREE INTEGERS, RIGHT JUSTIFIED IN BYTES 1-3, 4-11, AND 12-19. 

BYTES 21-80 OF EACH RECORD CONTAIN THE USGS 10-DIGIT INTEGER CODES, 

RIGHT JUSTIFIED WITHIN 10-BYTE FIELDS, FROM THE FOLLOWING OVERLAYS, IN ORDER: 

LAND USE/LAND COVER, POLITICAL UNIT, HYDROLOGIC UNIT, CENSUS SUBDIVISION/TRACT, 

FEDERAL LAND OWNERSHIP, AND STATE LAND OWNERSHIP. 

ONLY RECORDS WITH AT LEAST ONE NON-ZERO ATTRIBUTE ARE PART OF THE FILE. 

(A VARIABLE NUMBER OF RECORDS EXIST FOR A GIVEN ROW OR COLUMN.) 

21