Projection change shifts point locations in ArcMap?

Question

I'm having an issue with points shifting their locations when projected differently.

I have several files with undefined coordinate systems (from a Trimble GPS). If I bring these shapefiles into a dataframe that's in WGS84, they appear in what I assume is the correct location based on the aerial. If I then define the coordinate system of the shapefile as WGS84, they continue to appear in the same location. If I then move that shapefile into an MXD with a dataframe that's in state plane, the shapefile projects on-the-fly to the same location. I then export that shapefile with the data frame's coordinate system, the data stays in that location. So far so good.

However--if I define the coordinate system of the shapefile as WGS84, then try to project the file into state plane using the Project tool (no matter which coordinate system the dataframe is in), the points shift by about three feet.

Does anyone know why this is happening? More importantly, which location is in the correct location--the original points or the shifted ones? How do you tell?

Answer 1

If you are trying to reproject the data from X to Y (in your case from WGS84 to a state plane system) the easiest most error proof way is to open a fresh map load the WGS84 file (x file) in Arcmap will automatically configure itself to project data in the map based on the first import. Then use the project tool and reproject to go to your desired projection. This will leave you with your original shapefile in WGS84 and a second in the state projection.

There is no order of precedence when showing data together they should all be displayed using the same datum. So one mxd would be used to display wgs84 and the other would be used to display your state plane data. If you are mixing and matching data with different projections in the same map you are going to run into all sorts of problems.