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We have a Trimble unit we use to collect equipment.

We collect the data in WGS84 and differentially correct it and then reproject it to a state plan coordinate system.

The points always shift the same distance in the same direction after the reprojection.

What is the reason for this?

Thanks.

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closed as unclear what you're asking by PolyGeo, BradHards, Simbamangu, Chad Cooper, Paul Nov 24 '13 at 4:59

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2  
Are you applying the appropriate transformation? –  johns May 2 '13 at 14:38
1  
The default transformations are not the best one. Select the one meant for your area. Here is how you find which is the best one: gis.stackexchange.com/questions/57026/… –  Devdatta Tengshe May 2 '13 at 16:17
2  
Which state plane coord system? –  Kirk Kuykendall May 2 '13 at 21:05
    
Can you perhaps specify which State Plane Coordinate System you are using and include a screenshot of the Transformation that you chose to use with it? –  PolyGeo Nov 23 '13 at 21:09

1 Answer 1

up vote 2 down vote accepted

It may be a lack of datum transformation in your projection operation.

Your symptom sounds familiar. It is a common problem here in Wisconsin - GPS uses the WGS84 datum, but our state system uses NAD83HARN.

The symptom of doing a projection without datum transformation is that coordinates are systematically offset a few meters in X and ~meter in Y.

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