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Cofrin Center for Biodiversity

•  SUBMETER GPS  •

There's more to submeter GPS mapping than buying an expensive receiver. A number of factors have to come together.

Receiver Technology
In spring of 2011, the Biodiversity Center purchased GPS equipment with submeter mapping capability.

  • The guts of the system is the Trimble GeoXT 6000 receiver-datalogger. It combines a handheld GNSS receiver and a Windows PC in a single weatherized instrument package.
  • The receiver includes Trimble's "Floodlight" technology which is claimed to improve performance around trees and buildings more info.
  • Trimble's Terra-sync software handles communication between the receiver and datalogger.

Reference Station(s)
One or more reference stations must be sufficiently close by to permit post-processing to the desired accuracy

Data Collection Methodology
How the data is collected impacts final accuracy:

  • Receiver configuration settings should reflect project requirements.
  • Data collection can be scheduled to avoid unfavorable satellite geometry.
  • Longer sessions are more likely to achieve carrier-phase acquisition.

Data Correction
The GeoXT's data-logger stores collected data in a "rover" file. Trimble's Pathfinder Office software is used to download and post-process the rover file. The table below describes the accuracy associated with various correction options:

Correction Method Performance in Open Conditions Performance In Moderate Tree Cover
No correction Might as well use recreation-grade
Differential correction via code-phase post-processing
Specified accuracy (horizontal RMS) is 50 cm + 1 ppm
Specified accuracy is reliably achieved when session lenth, distance to a reference station and constellation geometry are favorable Specified accuracy is occasionally achieved. Session lenth, distance to a reference station and constellation geometry are factors
Differential correction via carrier-phase post-processing
Specified accuracy (horizontal RMS) is 1 cm + 2 ppm
Specified accuracy is reliably achieved when session lenth, distance to a reference station and constellation geometry are favorable Specified accuracy is occasionally achieved. Session lenth, distance to a reference station and constellation geometry are factors

User Notes
2-nov-2013 - When a rover file is post-processed, the more positions that are corrected using carrier processing, the more accurate the result. It appears that some of the factors that affect this are beyond the technicians control. One factor that can be controlled is the length of the field session. It looks like the longer the session, the better the chances that a lot of positions will receive carrier processing - Mike Stiefvater

2-nov-2013 - We wanted to use the DePere WISCORS for post-processing positions collected on and around the UWGB campus. We had trouble figuring out how to obtain the desired correction files and how to use them in the PFO differential correction utility. Seiler Instrument tech support directed us to a video that explained things very nicely - Mike Stiefvater