Title page for ETD etd-51198-162850


Type of Document Master's Thesis
Author Heselton, Robert Reid
Author's Email Address rheselto@vt.edu
URN etd-51198-162850
Title Elevation Effects on GPS Positional Accuracy
Degree Master of Science
Department Geography
Advisory Committee
Advisor Name Title
Carstensen, Laurence William Jr. Committee Chair
Campbell, James B. Jr. Committee Member
Oderwald, Richard G. Committee Member
Keywords
  • Global Positioning System
  • GPS
  • 2D
  • Accuracy
Date of Defense 1998-06-05
Availability unrestricted
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.

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