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In order to effectively use GPS, certain tasks had to be completed. These included: querying the existing USEPA Database to generate a list of all um-napped groundwater sources with associated contact information, preplanning data collection, in-field lat/long data collection, post-processing of location data, and finally transferring the data from GPS to an acceptable database format.
GLOBAL POSITIONING SYSTEM
GPS is a radio based satellite system developed by the
Department of Defense for accurately locating positional data
(Kennedy, 1996). This system has four major components: 24 orbiting
satellites, data collection receivers, base station receivers, and
the user. Recent advances in this technology and decreases in price
have allowed for GPS to reach a wider audience of users. The
accuracy, ease of use, efficiency, and GIS compatibility of GPS
technology was fundamental to this project.
The accuracy of GPS for locating real world features is the single biggest reason many natural resource managers have employed this technology. There are a variety of factors that influence the accuracy of the System: type of equipment, data collection methodologies and settings, inherit GPS errors, post-processing techniques, and data exchange procedures. The type of receiver used for collecting data however, directly influences all the other factors. There are three basic grades of receivers; recreational, mapping, and surveying. Accuracy and cost increase when going from the recreational grade receiver to the surveying grade. Additionally, the type of procedures available to the user also changes as receiver upgrading occurs.
For this project, a Trimble Pro XL receiver was used in gathering locational data for each PWS groundwater source. This is a eight channel, mapping grade receiver that maintains accuracy standards of approximately one meter after applying post-processing techniques. This receiver is also accompanied by a data logger that allows for ease in attributing and describing features.
The remainder of this chapter briefly describes the methodologies employed in using GPS, specifically Trimble GPS products, throughout the project. However, prior to traveling to data collection sites, a number of pre-planning tasks had to be accomplished relating not only to GPS but also to establishing a tentative schedule. The pre-planning steps allowed for the data to be collected as accurately and efficiently as possible.
DATABASE QUERY
The initial phase of this project involved first querying the
system and source tables in order to create a list of collection
sites. It was necessary to determine which systems were still
active, and of these systems, which had groundwater sources that had
not yet been located. Once this was completed, a list was created
for each county containing contact, system, and well information
necessary to find the source.
PRE-PLANNING
This phase was two-fold: 1) contacting owners/managers to
obtain permission and help in locating the source, and 2)
establishing standards for certain critical settings within the
receiver. Initially field personnel would contact PWS system
owners/managers prior to going to the site. This quickly became
economically unfeasible due to the number of calls necessary to
contact people. The most efficient way proved to be direct
communication with owners/operators onsite. Usually the Chamber of
Commerce for each city became a valuable asset since they could
direct personnel to the general locations.
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). All of these are fairly self-explanatory except for PDOP which is a measurement of the number and geometry of the satellites being used for data collection. Finally, a data dictionary was created, requiring the user to input the unique identifiers for each source (i.e. system identification number, source identification number, and groundwater source (spring or well)). AR pre-planning activities were completed prior to leaving the office for data collection.
GPS DATA COLLECTION
Each groundwater source required that 30 positions be logged at
one second intervals. Additionally at no time could the Precision
Dilution of Position (PDOP) fall above six. All positions were
collected using the 3-D setting with no less than four satellites
being used. Each day, all collected data were downloaded to a
laptop PC for safety and storage.
POST-PROCESSING
Once the data had been downloaded, project accuracy
requirements made it necessary to post-process or differentially
correct the data. GPS points collected without applying this
process can have worst case accuracy errors of 100 meters. However,
data that has been differentially corrected usually falls within one
to two meters of its true location (specific for Trimble Pro XL
receiver). 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.
For this project all base station data were obtained from the Casper BLM/UW station. 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. Correction files can be downloaded via a modem and use of a bulletin board. 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.
DATA TRANSFER
The final step in data collection is to transfer all the
groundwater source locations back to the original table to be
included with all PWS sources. This was accomplished through the
use of PFINDER's export command and then combining all
database tables to form a single table relating only to the
locations collected by WWRC. This table was then related to the
original source table allowing for all collected
latitudes and longitudes to be placed respectively within the table.
PROJECT SUMMARY FOR DATA COLLECTION
From USEPA's Safe Water Drinking Act Program Database
source table 511 active groundwater sources were
lacking latitude and longitude descriptors. Of these, 419 were
located by this project with an additional 59 new sources added to
the table. These additions to the database were operational or
partially completed wells not listed within USEPA's database and,
according to the PWS contact, were or going to be part of that
system.
There were a number of different reasons why 92 of the 511 listed sources were not located, however the biggest factor was dictated by the time and travel schedule during the data collection phase of the project. Every county throughout Wyoming except Campbell County was visited by a VVWRC employee. Campbell County was excluded due to the number of mining PWSs (mines require safety training be completed before data can be collected within the permit area) and preexisting data in the Gillette area which was collected by the State Engineer's Office. This county contributed to nearly half of the groundwater sources that currently lack locations and should be the focus of future data collection efforts.
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