# Presentation: 649729

## Tools of the Trade 

**Notes:**
Introduction:

This is a continuation of yesterday’s “Tools of the Trade” series.  Parts I,II and III covered the GPS, GPS communication software and GIS side of things.  Today, as a wrap up will be a hodgepodge of things to watch out for when you get your next assignment.

Called Tarpits, and Tips

Part 4 - Tarpits and Tips

## Tools of the Trade - Part IV

**Tarpits and Tips**

- TIPS

## Outline

- Datums
- Error, Accuracy, and Precision
- Map Scale and Errors
- Tips for Managing GPS Error
- Better Maps for GPS

## GPS Datum Tarpit

- What’s the big deal
- A football field of difference if you are wrong!
- Ask the 3 soldiers in Afghanistan

## Florida Example

**Florida Example**

## GPS Datum Tarpit

- Summary:
- Know your Datum at all times
- When you hear WGS84, it’s the same as NAD83

- Aviation GPS is always WGS84

## Error, Accuracy, and Precision

**Error, Accuracy, and Precision**

- Importance
- Safety
- Make or break a GIS project
- Learn ways to keep GPS errors to a minimum
- Be aware of the issues

- Kenneth E. Foote and Donald J. Huebner, The Geographer's Craft Project, Department of Geography, The University of Colorado at Boulder.

## 1.) Accuracy: how close to truth or accepted values.

2) Precision: how repeatable or consistent

- 2) Precision: how repeatable or consistent

**Basic Definitions**

**Notes:**
It is important to distinguish from the start a different between accuracy and precision

1) Accuracy is the degree to which information on a map or in a GPS matches true or accepted values.  In discussing GPS, it is possible to consider horizontal and vertical accuracy with respect to geographic position.

2) Precision refers to repeatability or consistency of a GPS.

High accuracy does not  imply high precision, nor does high precision indicate high accuracy. But high accuracy and high precision are both expensive. 

## 3.) Error: includes both the imprecision of data and its inaccuracies

**Basic Definitions**

-  = Low Error !

- Accurate + Precise

**Notes:**
It is important to distinguish from the start a different between accuracy and precision

1) Accuracy is the degree to which information on a map or in a GPS matches true or accepted values.  In discussing GPS, it is possible to consider horizontal and vertical accuracy with respect to geographic position.

2) Precision refers to repeatability or consistency of a GPS.

High accuracy does not  imply high precision, nor does high precision indicate high accuracy. But high accuracy and high precision are both expensive. 

## False Advertising

- Beware of False Accuracy and False Precision
- UTM = 1 meter
- DMS.S = Translates to          about 3 meters

**Notes:**
GIS users are not always aware of the difficult problems caused by error, inaccuracy, and imprecision. They often fall prey to False Precision and False Accuracy, that is they report their findings to a level of precision or accuracy that is impossible to achieve with their source materials. 
Since a Garmin III+ reports down to 1 meter, it makes no sense in reporting locations down to a tenth of a meter.
A Garmin III+ in DMS.S mode translates to about a precision level o 3 meters.
Just because computers can store numeric figures down many decimal places does not mean that all those decimal places are "significant." It is important for GIS solutions to be reported honestly and only to the level of accuracy and precision they can support. 

## Map Scale

- Accuracy and Precision for a map are a function of the scale at which the map was created
- Applies for digital or paper
- National Mapping Accuracy Standards employed by USGS

## Accuracy Standards for Various Map Scales

- 1:2,400 ± 6.67 feet
- 1:24,000 ± 40.00 feet
- 1:63,360 ± 105.60 feet
- 1:100,000 ± 166.67 feet

**Notes:**
Here is the NMAS standard:
"requirements for meeting horizontal accuracy as 90 per cent of all measurable points must be within 1/30th of an inch for maps at a scale of 1:20,000 or larger, and 1/50th of an inch for maps at scales smaller than 1:20,000." 
The next set of slides show a graphical representation of this for a 1:24,000 scale map 

## Map Scale

## Map Scale - Points

**Notes:**
When we see a point on a map we have its “Probable” location within this area.

## Map Scale - Lines

**Notes:**
When we see a line  on a map we have its “Probable” location within this area.

## Map Scale and the Garmin

- Question?  Will your Garmin III Plus meet Mapping Accuracy standards for a 1:24,000 scale product?
- How about a 1:63,360?
- Today, we will answer that question?

**Notes:**
Question to ask the class.  Following today’s Navigation Exercise when you will navigate to “Truth”, you should be able to answer that question.

For a 1:24,000 that means 40 feet from Truth
For a 1:63,360, that means 105 feet

## Sources of Map Errors

- Age of Data

- Map Scale

- Digitizing Errors

- Mapsource data is 1:100,000 scale

**Notes:**
Age of DATA:  An interpretation of a Topo map in relation to your GPS data may reveal the datedness of the map.  

Map Scale:  The ability to show detail in a map is determined by its scale
Scale restricts type, quantity, and quality of data (Star and Estes 1990). Enlarging a small scale map does not increase its level of accuracy or detail. 

Digitizing Error:
Maps are created by humans - or at least human input/QC.  Errors can include digitizing a line, the process of generalization or booboos.

## Tips for Managing GPS Error

- Record Metadata:
- Who, What, Why

**Notes:**
Here is the NMAS standard:
"requirements for meeting horizontal accuracy as 90 per cent of all measurable points must be within 1/30th of an inch for maps at a scale of 1:20,000 or larger, and 1/50th of an inch for maps at scales smaller than 1:20,000." 
The next set of slides show a graphical representation of this for a 1:24,000 scale map 

## Managing GPS Error

- Try to average points

**Notes:**
Here is the NMAS standard:
"requirements for meeting horizontal accuracy as 90 per cent of all measurable points must be within 1/30th of an inch for maps at a scale of 1:20,000 or larger, and 1/50th of an inch for maps at scales smaller than 1:20,000." 
The next set of slides show a graphical representation of this for a 1:24,000 scale map 

## Managing GPS Error

- Try to use Antenna when possible

**Notes:**
Here is the NMAS standard:
"requirements for meeting horizontal accuracy as 90 per cent of all measurable points must be within 1/30th of an inch for maps at a scale of 1:20,000 or larger, and 1/50th of an inch for maps at scales smaller than 1:20,000." 
The next set of slides show a graphical representation of this for a 1:24,000 scale map 

## Make Better Maps for GPS

- Basic Premise:
- Maps and GPS are made for each other
- Location finding requires map grids, labeled tics and graticules
- Ingredients Required:
- ArcView
- Coordinate Grid Maker extension*
- Map Data

- *Christoph Feldkotter Version 2.270  Available from www.esri.com/arcscripts

## Importance of Datums
How error free are you?
By managing error you can beat  the odds

- Importance of Datums
- How error free are you?
- By managing error you can beat  the odds

**Summary**

**Notes:**
Well, this is the last presentation.
Today, I hope you can use some of this information to better ascertain the issues out on the fire.

How important Datums and Projections are
The differences between Accuracy, Precision and Error and how they relate with GPS and Maps.

Finally a message on how you can best beat the odds and manage the common sources of GPS error in the field.