Photos:
GPS survey of the author's test location. 
David L. Wilson's GPS Accuracy Web
Page
e-mail: dlwilson@erols.com
Pages linked below present mathematical modeling and measured data pertaining to GPS accuracy. Short visits to a survey marker or short GPS observations will not provide a useful appraisal of GPS accuracy. The author continuously collects and analyzes data from a precisely surveyed (independently by a licensed surveyor and the author) point near 38 degrees latitude.
Position accuracy will vary with GPS receiver configuration (receiver and
antenna), location (geographic latitude, as it influences HDOP, and surrounding
objects possibly blocking reception or causing multi-path reception), satellite
constellation status, and ionosphere conditions. Presented in these pages are results thought to be typical of
consumer-grade and a few higher-grade GPS receivers. Expensive survey-grade GPS receivers would generally be more
accurate than the consumer-grade receivers. The only way to obtain a very
precise measurement of the accuracy of a particular GPS receiver/antenna at a
location and a particular time is to measure it. Except when otherwise explicitly stated, theses pages are for
SA (Selective Availability) off.
http://www.erols.com/dlwilson/gpsacc.htm - Horizontal GPS accuracy.
http://www.erols.com/dlwilson/gpswaypt.htm - Horizontal error when returning to a position (earlier fix).
http://www.erols.com/dlwilson/gpsvert.htm - Vertical GPS accuracy.
http://www.erols.com/dlwilson/gpshdop.htm - Using HDOP to more precisely estimate horizontal error.
http://www.erols.com/dlwilson/gpsavg.htm - Error when position is averaged to improve accuracy.
http://www.erols.com/dlwilson/gpswaas.htm - GPS accuracy with WAAS
http://www.erols.com/dlwilson/gpscorr.htm - A look at correlations of errors.
http://www.erols.com/dlwilson/gpswtavg.htm - A discussion of weighting by HDOP when attempting to improve the horizontal accuracy by averaging.
http://www.erols.com/dlwilson/gpscomp.htm - A look at whether DGPS (differential GPS) improves accuracy now that SA has been removed. Note that some of the tests used a long baseline and the benefits of DGPS should be more noticeable on short baselines.
http://www.erols.com/dlwilson/gpscmpsa.htm - A comparison of horizontal accuracy with when SA was on.
http://www.erols.com/dlwilson/gpsfaq.htm - Answers to questions often asked of the author.
http://www.erols.com/dlwilson/gpseqtns.htm - Assorted mathematical equations for those desiring more details.
http://www.erols.com/dlwilson/gpsmisc.htm - Assorted plots, data and other items are presented here that were referenced in postings to the newsgroup sci.satellite-nav.
The author's home page can be found at http://www.erols.com/dlwilson/index.htm
The above pages will be updated and additional pages added as additional data is collected and analyzed. The reader is encouraged to keep reading these pages even if something is not understood. An attempt has been made to explain the needed concepts in a variety of ways - words, equations and graphs. If one such attempt fails, another might succeed. Although most of these pages are at the algebra level, the separate section on equations for the more mathematically experienced reader requires calculus. Probability and statistical concepts are explained only at as deep of a level as thought necessary.
Those seeking more information on GPS and links to other GPS related sites are recommended to visit:
http://www.cnde.iastate.edu/gps.html
http://www.tenlinks.com/mapgis/data/gps_data.htm
