Cofrin Center for Biodiversity

GIS at the Biodiversity Center
GIS/GPS Techniques for Field Work
GPS Reporting Error
Reference Datasets on UWGB "Ecodata" Drive
Latest Satellite Predictions for the UWGB campus (.pdf)
Useful Links for GPS Users

GPS Users Should be Aware of the Reporting Error Inherent in the System

When using GPS (Global Positioning System) for recreation or navigation, it is useful to know how close the observed position readings are to the true position. When GPS data becomes part of a scientific report, knowing the error associated with location observations is essential. The methods section of the report should state the maximum positional error for the data along with the equipment and techniques used to ensure that the stated error is not exceeded.

The long-term goal of the CACB is to produce a template for quantifying GPS error in scientific reporting. Research into how the rest of the world is doing this is just getting started (June 2001), so it may be several months before a formal report is published. In the meantime there are several things that the growing number of GPS users on the UWGB campus can do to deal with the issue of GPS error.

The first thing that all of us need to do is become better informed. Read everything you can find about GPS in general and read the manuals for the particular units that you are using in your work. If you need to control positional error to 100 feet or less, find out about Differential GPS and how it can be obtained at your particular study site.

If you find a good article or web page, SHARE IT. The ultimate solution would be to find a methods write-up so comprehensive and reputable that we can adopt it as our standard and reference it in our reports. Forward abstracts and links to stiemg10@uwgb.edu. The best sources will be added to the reference section of the GPS web page.

GPS Reference Point in Front Lab Sciences.

Get to know how the particular equipment you are using handles error reporting and whether or not the built-in error reporting is reliable. The CACB has established a referencepoint on the campus to assist GPS users who want to test the performance of a GPS unit. The point is located on the sidewalk between the Laboratory Sciences Building and the Lab Sciences parking lot and is marked by a piece of orange tape and 2 permanent cuts in the concrete.

Mike standing with right foot on reference Point. While standing at the reference point, record the location displayed on the GPS screen. Forms to assist data-taking are available on the ecodata drive in both UTM format and decimal degrees. The UTM format is suggested to simplify interpretation.

If available on the unit being used, the PDOP (or DOP) should be recorded. Both acronyms stand for Position Dilution of Precision which is the Defense Department's standard method for expressing the accuracy that is possible with a particular satellite configuration (the lower the better). Some units also display Estimated Positional Error or EPE which should also be recorded if available. However, we need to know more about the reliability of this number before accepting it as a direct measure of observational accuracy.

The distance between the observed coordinates and the true coordinates listed below is a predictor of the offset you will experience in the field under similar conditions. To simulate less favorable conditions, which will be indicated by higher values for DOP and EPE, shade the GPS unit while it is computing your location.

Coordinates for the GPS Reference Point:

System	Brown County NAD 83
Northing	572,276.050 feet
Easting	124,369.682 feet
Source	Robert E. Lee & Associates -- Registered Land Surveyors


System	UTM NAD 83
Northing	4,930,982.66712 meters
Easting	426,846.18103 meters
Source	Computed from survey data using ArcView and TNC Tools


System	Latitude/Longitude WGS84--Decimal Degrees
Latitude	44.52846 N
Longitude	87.92065 W
Source	Computed from survey data using ArcView and TNC Tools


System	Latitude/Longitude WGS84--Degrees, Minutes
Latitude	44° 31.7076' N
Longitude	87° 55.2390' W
Source	Converted from decimal degrees


System	Latitude/Longitude WGS84--Degrees, Minutes,Seconds
Latitude	44° 31' 42.456" N
Longitude	87° 55' 14.340" W
Source	Converted from decimal degrees

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