You’ve probably been there. You used your trusty GPS unit to collect some data, processed it, then added the data to a map with reference data and your GPS data don’t line up. They are off by anywhere from a few feet to a few meters. What gives?
There are 3 basic issues that can cause this:
- Your GPS unit isn’t hitting the accuracy it is designed for.
- Your workflow in processing the data is flawed.
- Your field technique is flawed.
If your data are severely misaligned (like they are floating out in the ocean when they are supposed to be in Phoenix) then you have a coordinate system problem and this technique I’m going to show you will not help.
But if your data are misaligned by up to a few meters and your GPS unit’s accuracy is supposed to be better than what you are seeing, then this technique can help immensely.Find a Published Benchmark and Log a Point
The National Geodetic Survey (NGS) manages a huge network of benchmark monuments throughout the country. If you can find one in your area, all you have to do is get its published XY coordinates and datum, then head out into the field to find it.
Once you locate a benchmark, use your GPS unit to log a point directly on the benchmark. Process the data as you normally do and then compare the resulting point’s XY coordinates to those of the benchmark.
I’ll show you how in this video.