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GPS Communication Software. Mapping Program. Tarpits and Tips. Tools of the Trade . GPS. 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.

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Tools of the Trade

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GPS CommunicationSoftware

Mapping Program

Tarpits and Tips

Tools of the Trade


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Tools of the Trade - Part IV

Tarpits and Tips


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  • Datums

  • Error, Accuracy, and Precision

  • Map Scale and Errors

  • Tips for Managing GPS Error

  • Better Maps for GPS

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GPS Datum Tarpit

  • What’s the big deal?

    • A football field of difference if you are wrong!

    • What if you’re on the wrong side of the ridge?

    • Or worse…the valley!!


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Datum Shift


Shift Happens - Florida Example

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Shift Happens - Redmond OR 2005 Example

710 Feet

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In the Field

Comms between GPS users

Comms between GPS and Maps

Example: dispatch in one datum, you read off in another

In the GIS

Projecting data on top of GIS data

Example: You set DNRGarmin Projection to UTM 10 NAD83, but your GIS is in UTM 10 NAD27

When Datums Matter

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or Spoken




400 feet off

Display set to NAD27

They Expect WGS84

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Transfer to


Set Projection in DNR Garmin to NAD27



Always Stores in WGS84

Working in NAD27

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GPS Datum Tips

  • Summary:

    • Check, check, check

    • When Downloading with DNR Garmin, Always check “Set Projection” to match your GIS

    • ASK!

    • When you hear WGS84, with your scale, it’s the same as NAD83

Aviation GPS is always WGS84

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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.

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Basic Definitions

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

2) Precision: how repeatable or consistent

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Basic Definitions

Accurate + Precise

= Low Error !

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

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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

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Map Accuracy Standards

  • 1:2,400 ± 6.67 feet

  • 1:24,000 ± 40.00 feet

  • 1:63,360 ± 105.60 feet

  • 1:100,000 ± 166.67 feet

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Map Scale and the Garmin

  • Will your Garmin GPS MAP76 meet Mapping Accuracy standards for a 1:24,000 scale product (+/- 40’)?

  • How about 1:63,360 (+/-105’)?

  • Yesterday, we answered that question?

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Sources of Map Errors

  • Age of Data

  • Map Scale

Mapsource data is 1:100,000 scale

  • Digitizing Errors

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Tips for Managing GPS Error

  • Metadata

  • Average all Points

  • Fresh batteries

  • External Antenna

  • All else fails

    • Map and Compass

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Tips for Managing Error

  • Staying As Accurate as possible

+/-26 ' (open)

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Tips for Managing GPS Error

  • Record Metadata:

    • Who, What, Why

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Managing GPS Error

  • Try to average points

  • 60 points

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Managing GPS Error

  • If in the trees, turn off WAAS

  • Position can be even worse.

  • Always take a 3D position over a 2D Differential!

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Managing GPS Error

  • Try to use Antenna when possible

  • A Map76 under Canopy gets twice as accurate with an external antenna

  • Use Sleeve mounts if you have to

  • When mounting in vehicle, use brackets

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GPS Accessories

  • Maps

  • Compass

  • GOTO:

    • Distance

    • Bearing

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Ideal GPS Accessories

  • Extra AA batteries

  • Case ($14.00)*

holds cables,




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Make Great Maps

  • Practice Collecting Good Data in the field

  • Pass onto others only important stuff

  • Make your data secure

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Future Firefighters

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  • Importance of Datums

  • How error free are you?

  • By managing error you can beat the odds

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