GPS post processing - Differential Correction

Question

I have a large set of collected GPS points that I'm pulling into Arcgis from SQL and creating an xy event layer to then map them. The fields for the points are as follows: recorded, lat, long, altitude, speed, hAccuracy, vAccuracy.

Overall the points are fairly accurate, but of course there are a few cases where the accuracy is off by quite a bit. I have worked with differential correction before, but it was with the same software/hardware (Trimble/Pathfinder) used to collect the data.

Is it possible to correct the points after I have created a shapefile? If not, is it possible to correct with the Pathfinder software them before I bring them into GIS? I also realize that if in fact I can correct them I will have to pull use multiple base files as the GPS points collected are over multiple dates.

Any ideas or help is greatly appreciated!

Answer 1

If you have the ssf file that the points were in then you should be able to run them through the Pathfinder software for the correction. I've generally found that the differential correction by pathfinder was never more than a few inches. So the data collector may have been off instead of the fact that the satellites broadcast was mis-interpreted. If anyone has seen different I'd be happy to know.

To correct just the shapefiles (or the points themselves) I think you would need to know which satellites were broadcasting the positions and at exaclty what time. And possibly how many points were collected for each shot and how they averaged. Whew!

That would be very cool if someone has some insight on that.

Answer 2

Sounds like a job for geodatabases and Trimble GPS Analyst. GPS Analyst will store the ssf files in a geodatabase. It uses the same differential correction engine as Trimble Pathfinder Office to find all the necessary base files and correct the data. It will then build your features and store them in the same database.

The coolest part is that since it stores both the .ssf files and resulting feature classes in the same database, if your accuracy requirements change later, you can rebuild the features to meet the new standards. Some detailed resources at http://www.spatial-ed.com/datums/datums-tutorial.html

Answer 3

I can't answer the first question (whether it's possible to do differential correction after the GPS data is in a shapefile), but Trimble's Pathfinder Office does indeed have a differential correction utility, and it will take care of locating the base station nearest to your GPS points (of course it helps if your points are all in the same general area), downloading the appropriate files from that base station's FTP site, unzipping them, and correcting your data. Usually all you have to do is point it at the rover (.SSF) files and pick the base station (the software can download an updated base station list when you run it), the software does the rest.

If you don't have Pathfinder Office, try calling your local Trimble sales rep, where I work we rent Trimbles when we need them and the sales guy was willing to give me a free copy of an older version of Pathfinder Office so I could run differential correction on the files. It's not the most recent version by a long shot but all I really use it for is differential correction and export to shapefile (or whatever format).