Hi—

 

     I am the new GIS coordinator at the City of Laconia, NH. I was wondering if someone could shed some light on what’s going on with my GPS data. I have a good system, but I have been disappointed with the accuracy.

    I use a Trimble Pro XH with a Yuma Tablet. Typically, my uncorrected data yields a horizontal precision of 1.5 to 2 feet. I have been post processing using 3 reference stations: 1) CORS NHDOT CONCORD (NHCO) 2) CORS, U NEW HAMPSHIRE (NHUN) and 3) CORS, BARLETT(BARN) NEW HAMPSHIRE. My corrected data generally has a horizontal precision of 0.5 to 1 foot. When I compare my data points to those taken in the past, or to those on my CAI basemap, I am consistently 2 or 3 feet too far to the north. If this looked like a bell curve in the data, I would write it off as random variation. However, it looks very systematic. Here’s the kicker. My basemap uses NAD 1983 State Plane New Hampshire FIPS 2800 feet. I have experimented collecting/processing my GPS data points using the same datum as my basemap and using ‘96 corrected datum, [NAD 1983 (Conus) CORS 96 State Plane New Hampshire 2800 feet]. When I use the ’96 corrected datum, my GPS points line up with those on the basemap better.

     I guess that that I am I overlooking something in the datum/projection, but what?  Are my expectations for accuracy too high? Should I only use 1 reference station for the differential correction? Is there some hidden setting in Terrasync/Pathfinder that I should check?

     I thank you ahead of time for your help.

 

Sincerely,

Paul

 

Paul Elsholz, GIS Coordinator

City of Laconia

27 Bisson Ave.

Laconia, NH 03246

Phone: (603) 528-6379 x304

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