Last updated Tuesday May 09, 2006
10 things you can do to increase the accuracy of GPS locations measured by your consumer grade GPS.
Wake / Warm Up - For the GPS unit to calculate locations correctly it must receive ephemeris data (precise orbital data) from each of the satellites it detects. Most GPS units have a screen that shows which satellites it has detected and the bar graph showing the strength of the signal. On most GPS units, these bars are gray or hollow while the ephemeris data is being downloaded and then turns solid back once the ephemeris data has been downloaded. All this said, turn on the GPS unit at least 5 to 10 minutes before you plan to use the unit and leave it in an area which it can pickup satellite signals.
Keep it on - Leave the unit on and outside, so it will continue to collect data and updating the ephemeris data which needs to be updated about every hour. (avoid putting the unit inside your jacket or pack).
Differential Correction - Use some form of differential corrections (DGPS) such as the Wide Area Augmentation System (WAAS) in the USA. Most differential corrections will allow accuracy of 3-5 meters and WAAS will allow accuracy to 1 to 3 meters. See my GPS Accuracy Testing page for test results of a WAAS unit vs. non-WAAS unit. Keep in mind that on some GPS units that have WAAS still must be turn on or enabled. I hear that Garmin is this way. My Magellan SporTrak came with it enabled.
Average - Use "Averaged" waypoint to locate the point. Most units will start Averaging once the unit sits still for a few seconds or there is a way to tell the unit you want to start averaging. Allow the unit to average for 30 to 60 seconds.
Record the Data - Use the unit to store the point vs. writing it down. This reduces the chance of a transcription error.
Know the Units - If you do record it by writing it down, make sure you record down to a unit that will give you the precision you require. To get down to around 5 feet or so you will need to record to:
00.00001° (Decimal Degrees)
00° 00.001' (Degrees, and decimal minutes)
00° 00' 00.01" (Degrees, minutes, seconds)
Avoid Blocks - Make sure the unit is not blocked from the satellites. If you are beside a building or cliff (especially if it is to the south) it will block the signal from the satellites. If you are under thick tree cover, you will have reduced coverage. You may want to invest in an external antenna. Some of these will increase the signal in areas of poor reception.
Avoid Bouncing - Avoid getting a multi-path error which is caused by being close to an object that is bouncing the signal. Again, avoid standing close to buildings. (Note: one surveyor I know said they had one guy that kept on having bad errors in his survey. When they called the manufacturer of the unit, the manufacturer asked if the gentleman was bald. He was... the signal was bouncing off his head. He now wears a hat when he uses the unit.)
Data Quality - Most GPS units will provide you with an indication of the quality of data. For example, Estimated Position Error (EPE) or Position dilution of precision (PDOP). The smaller these numbers are the better (for PDOP less than 6).
Plan - Due to the way satellites orbit the Earth, there are times when coverage is good and poor. You can download Mission Planning Software from Trimble or Thales . Once installed, you will need to download the most current Ephemeris Files aka Almanac (Click here for Trimble and here for Thales. The almanac is a file that contains the orbit, clock corrections and atmospheric delay parameters for all of the satellites. The software will then calculate the PDOP for the area you will be working in and for the date you need. Below, you can see a screen shot of the PDOP (red) and the Availability (green) graph. Notice that between 10:30 and 11:30 availability drops and the PDOP goes up from 3 to 10. So you would want to avoid this time.
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Keywords: Estimated Position Error, EPE, Position dilution of precision, PDOP, accuracy, mission planning.