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Some notes from: “ A Comparison of Autonomous, WAAS, Real-Time, and Post-Processed Global Positioning Systems (GPS) Accuracies in Northern Forests ”. By Paul Bolstad, et al. Northern Journal of Applied Forestry. 2005. 22(1):5-11 . Receiver types by accuracy.
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Some notes from: “A Comparison of Autonomous, WAAS, Real-Time, and Post-Processed Global Positioning Systems (GPS) Accuracies in Northern Forests” By Paul Bolstad, et al. Northern Journal of Applied Forestry. 2005. 22(1):5-11 GPS Accuracy in Northern Forests
Receiver types by accuracy • Survey grade – cm or less. (carrier based signals) • “Course acquisition” or C/A code receivers. Smaller, lighter, less expensive, don’t require open sky conditions. • C/A receivers range from a few feet (sub-meter) to several tens of feet (5 to 10 meters). • Manufacturer reported accuracy may be optimistic or might be the expected accuracy. GPS Accuracy in Northern Forests
Improving GPS accuracy • Improvements in hardware and software • Differential correction • Altering data collection methods GPS Accuracy in Northern Forests
Differential correction types • Post-processed. • Real-time. Coast Guard or other beacons. • WAAS GPS Accuracy in Northern Forests
What they tested • Accuracy evaluation of two inexpensive, WAAS capable receivers and three differential-capable receivers. • Quantify the realized accuracy to establish average accuracy of WAAS receivers relative to non-WAAS receivers in open and forest conditions. GPS Accuracy in Northern Forests
Testing, continued • Establish the frequency distribution of errors for a single fix. • Quantify the relationship between the number of fixes averaged for a point and positional accuracy. • To measure how often WAAS corrections are available under a forest canopy. GPS Accuracy in Northern Forests
Methods • Collecting positions at three known open positions and three known forest locations. • Comparing GPS-measured positions to “true” positions. • Very detailed and specific conditions used to standardize measurement. GPS Accuracy in Northern Forests
Measuring Sites • Open positions in agricultural fields or along roadways. • Forest points had more than 70% sky obstruction. GPS Accuracy in Northern Forests
Measuring Process • Very detailed set of measuring procedures. • Detailed set of repetitions. • Detailed set of statistical tests applied. GPS Accuracy in Northern Forests
Some Results • No statistically significant differences under open locations. • Significant differences in the mean positional error due to receiver type under forest canopies. • Recreational accuracies much less consistent than GIS receivers with higher frequencies of large errors. GPS Accuracy in Northern Forests
Results continued • In subcanopy conditions WAAS signals available between 8 (moving) and 23 (stationary) % of the time for the recreational receivers. • And for GIS receivers 22 (moving) and 33 (stationary) % of the time. • Hand-held GPS did have the fewest interruptions in forest canopy conditions. GPS Accuracy in Northern Forests