GPS Accuracy in Northern Forests1 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 (1):5-11

GPS Accuracy in Northern Forests2 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 Forests3 Improving GPS accuracy Improvements in hardware and software Differential correction Altering data collection methods

GPS Accuracy in Northern Forests4 Differential correction types Post-processed. Real-time. Coast Guard or other beacons. WAAS

GPS Accuracy in Northern Forests5 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 Forests6 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 Forests7 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 Forests8 Measuring Sites Open positions in agricultural fields or along roadways. Forest points had more than 70% sky obstruction.

GPS Accuracy in Northern Forests9 Measuring Process Very detailed set of measuring procedures. Detailed set of repetitions. Detailed set of statistical tests applied.

GPS Accuracy in Northern Forests10 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 Forests11 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.