New: Help on choosing the right Magellan Receiver: [ ChoosingTheRightGPS ]

Measurement Resolution

Consumer GPS receivers don't accurately record the relative position of points within 30 to 100 feet of each other. This makes it impossible to record the spatial relationship of nearby points even when the points are recorded in rapid succession.

To compare the resolution of Consumer and Mapping grade receivers, we placed two receivers on a flat surface and record evenly spaced waypoints along a straight, level line.

These plots show the results of 13 waypoints, spaced 1 foot apart, along a level rail using a Garmin GPSmap 60C (red dots) and a Thales MobileMapper Pro receiver (blue dots). Both receivers enjoyed WAAS signals for this test.

a  bc

a. Raw data 1 foot spacing on bench.
b. Post processing moves MobileMapper points to a straight line.
c. Post processed MobileMapper Pro vs Map 60C.

You can see that the Map 60C coordinates are about three times worse than those for the autonomous mapping grade receiver. The second and third plots show the effects of post processing the mapping grade points using NGS CORS reference data.

The consumer GPS receiver performs so poorly that it is impossible to determine the actual relative horizontal position of any of the 13 points. In addition, the consumer receiver reports a 35.5 foot range in elevation for the 13 points so vertical resolution/accuracy is relatively useless.

Post-processing the Thales MobileMapper Pro coordinates results in coordinates that are consistent with their true relative locations. The average horizontal error, measured from the first point is 0.57 inches! The elevation range of post-processed waypoints is less than 0.65 inches, and every point is within 1 foot of the actual ellipsoid height!

Measurement Accuracy

Our tests comparing the accuracy of consumer vs. mapping grade receivers have found that mapping grade receivers are more than 4 times more accurate for raw, autonomous readings. Post processed measurements are more than 24 times more accurate.

If you are navigating to features with your GPS the increased accuracy is very significant. The area of a 29 foot uncertainty circle is over 2,500 square feet, while a 6.7 foot circle is only 141 square feet.

Post Processing

Mapping Grade GPS receivers store pseudo-range (the time from the receiver to the GPS satellite) information in addition to the computed geographic position.

Once you return to your office, you can quickly ‘Post Process’ the collected data with free correction data from the web. Post processing removes errors generated by ionospheric changes and imprecise satellite orbit data greatly improving the accuracy of the computed position.

The National Geodetic Survey [ www.ngs.noaa.gov ] maintains a network of CORS sites in the United States. Free correction data is available via the internet for post-processing collected GPS data.

The ‘MobileMapper Office’ program included with the ‘MobileMapper Pro’ GPS receiver completely automates the process of downloading correction data. With just four button clicks you can download the appropriate correction data and apply it to your data for dependable sub-meter accuracy.

If you are working in an area without a nearby CORS station, you can use a second MobilMapper Pro receiver as a local reference station. No additional software or license keys are required.

Job Based Data Collection

Mapping grade receivers store collected data in ‘Job Files’. These files contain point, polyline and polygon features with field collected attribute data for a project.

Libraries of features and acceptable codes can be predefined making it easy to enter attributes for collected data. We use the custom ‘Trail Survey’ library (shown to the right) to collect trail data for our trail mapping products.

Our library includes every feature we are likely to encounter on the trail, and most features include stock values to minimize the number of characters we need to enter in the field.

Best of all, the GPS receiver checks entered data for consistency in the field while it is collected which helps us deliver more accurate mapping products.

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