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Cofrin Center for Biodiversity

•  RECREATION-GRADE GPS RECEIVERS  •

The majority of the Biodiversity Center's GPS mapping work is done with Garmin hand-held receivers. We have around thirty of these units of varying age and technical capability. They are in constant demand for classes and research projects. However, there are usually a few available to be loaned out. Biodiversity Center Program Assistant Kim McKeefry manages equipment sign-outs.

Recreation-Grade Receiver Options and Performance Information

Equipment Descriptions Open Conditions Moderate Tree Cover
Garmin hand-held receivers with high sensitivity receiver technology and WAAS differential correction enabled by the user
GPSmap models 60csx, 62s, 72csx, 76csx
Specified accuracy with stable 3d lock is < 3 meters
Specified accuracy is reliably achieved Unknown
Same equipment as above with WAAS differential correction disabled by the user
Specified accuracy with stable 3d lock is < 15 meters
Specified accuracy is reliably achieved Specified accuracy is frequently achieved
Garmin hand-held receivers with obsolete receiver technology and WAAS differential correction enabled by the user
Models GPS 60, GPS 72, GPS 76
Specified accuracy with stable 3d lock is < 3 meters
Specified accuracy is reliably achieved na - seldom able to maintain stable 3d lock
Same equipment as above with WAAS differential correction disabled by the user
Specified accuracy with stable 3d lock is < 15 meters
Specified accuracy is reliably achieved na - seldom able to maintain stable 3d lock
Garmin hand-held receivers with obsolete receiver technology and lacking differential correction capability
Models GPS 12, GPS 12XL
Specified accuracy with stable 3d lock is < 15 meters
Specified accuracy is reliably achieved na - seldom able to maintain stable 3d lock

15 Meters - Are They Serious ?
The specified accuracy for 3d position fixes without differential correction is < 15 meters. Our experience tells us that the reality is much better than that.

2d vs 3d Position Fixes
To produce a robust position fix, a receiver must be locked on to four satellites (the 3d condition). If it can only lock on to three satellites, it produces a rough estimate called a 2d fix. If satellite conditions and/or cover are so unfavorable that stable 3d lock can not be maintained, mapping work should be discontinued. Sometimes a better result can be obtained by rescheduling for a time when the geometry of the satellite constellation is more favorable more info.

Garmin High-Sensitivity Receiver Technology
About half of our receivers have Garmin's high-sensitivity receiver technology. The improvement over the old technology is very apparent, especially when working in tree cover

Wide Area Augmentation System (WAAS)
WAAS is one of several differential correction schemes devised to tighten up the accuracy of satellite navigation systems. Check out Garmin's What is WAAS? website. It states that WAAS drives position accuracy down to < 3 meters. That sounds pretty cool until you consider that airport traffic control operations are the only users that it MUST work for. So WAAS does not have to take obstructions like buildings and trees into account and we know that it doesn't like trees.

WAAS technology has been standard on recreation-grade receivers since c. 2008. It's up to the user to decide whether to enable it on his/her receiver. In open conditions, the system appears to work as-advertised. Even in moderate tree cover, however, we've found that our receivers have a tough time staying locked on to the WAAS correction signal. So at best WAAS is no help for a lot of our work. But we've also observed that WAAS never stops trying and our receivers are constantly losing and regaining lock as cover conditions vary. Does that instability result in erratic location estimates? We don't know, but we generally disable WAAS unless we are working in very open conditions.

What About the Weather ?
Alert GPS user's notice that the ability to achieve stable 3d lock varies throughout a field session. I've run into folks who blame this variability on the weather, but aside from solar storms, atmospheric conditions don't play a role. Variabilty in the geometry of the satellite constellation is a better explanation.

Waypoints
Recreation-grade receivers have a MARK function that writes selected latitude/longitude coordinate pairs to the receiver's memory. These coordinate pairs, called waypoints, are the basis of almost all mapping work. Raw GPS waypoints are not particularly useful as-is. A number of processing steps are required starting with transfer of the waypoint data to a Windows PC. Check out our downloading options page for more info.

Tracks Vs. Waypoints
Recreation-grade receivers take position fixes every few seconds and store them in a data file called a track. Users are sometimes tempted to use tracks for mapping linear features (trails, edges of vegetation patches, etc). This is generally not good practice. There is no way to control the accuracy of the individual points that make up the track. Linear features are captured more accurately when they are mapped as a series of carefully-marked waypoints.

GPS Accuracy
For as long as GPS has been around, techy types have been interested in determining whether published accuracies for the system are believable. There is also the concern that an individual receiver might be defective. That could result in costly do-over or undetected data errors. Here are some articles on the subject:

  • Testing GPS Units is a protocol for verifying that a recreation grade GPS receiver is behaving normally. The protocol is published by the Natural Resources Research Institute at the University of Minnesota Duluth and is used by field staff working on the U.S. Environmental Protection Agency-funded project called the Great Lakes Coastal Wetland Monitoring Project.