There are a variety of factors that influence the accuracy: type of GPS equipment,
data collection methodologies and settings, inherit GPS errors, post-processing
techniques, and data exchange procedures. A Trimble Pro XL receiver was used,
an eight channel, mapping grade receiver that maintains accuracy standards of
approximately one to two meters after applying post-processing techniques.
Certain critical settings insured that the data would be collected as accurately
as possible. All of the manufacturer's recommended settings were followed in
configuring the receiver, including the number of positions collected for each
source, the logging interval of the positions, the number of satellites available, and
the Position Dilution of Precision (PDOP). The PDOP is a measurement of
the number and geometry of the satellites being used for data collection. Each groundwater
source required that 30 positions be logged at one second intervals. Additionally at no time
could the PDOP fall below six. All positions were collected using the
3-D setting with no less than four satellites being used.
POST-PROCESSING: Once the data had been downloaded, to improve the accuracy it
was necessary to post-process or differentially correct the data. In order to
differentially correct GPS data collected in the field, the user must have access to
base station GPS data and post-processing software. A base station is an eight to twelve
channel GPS receiver placed on a known location which produces correction files that can
be applied to any data collected within a 300 mile radius of the station.
Through the use of
differential correction software these files can be applied to field data. Trimble's
PFINDER software with the MCORR400 algorithm was applied for all collected
data. Finally, all corrected positions were averaged to produce a single latitude and
longitude for each groundwater source.