Elevation Effects on GPS Positional Accuracy

Abstract

Data from a Coarse Acquisition (C/A) Global Positing System (GPS) map-grade receiver were evaluated to assess the accuracy of differentially corrected points. Many studies have focused on the accuracy of GPS units under ideal data collection conditions. Ideal conditions allow the collection of data with four satellites (3D mode), yet field data conditions are often less than ideal. Four satellites may not always be in view because of mountainous topography, heavy forest cover, or other obstructions which block satellite signals from the receiver. This study examines GPS accuracy when four satellites are not available, instead collecting data with only three satellites (2D mode).

3D GPS points compute four unknowns: x, y , z, and clock error. In comparison, 2D GPS points are less accurate as only three unknowns are calculated: x, y, and clock error. Elevation (or z) is not computed for 2D points, causing increased error in the horizontal (x, y) measurement. The effect of elevation was evaluated on 234 2D GPS data points. These points were collected and corrected at elevation intervals of true elevation, +-25 meters, +- 50 meters, and +-75 meters. These 2D points were then compared to surveyed points to measure the effect vertical error has on horizontal accuracy. In general, the more error in the vertical estimate during correction, the greater the horizontal error.