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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


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