Data Shifted Against Basemap

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04-15-2019 02:44 PM
SarahTalvy
New Contributor

Hi there, I have been doing work in Southern California using StatePlane V (SPV).  I've noticed "shifts" in data (sewers, property lines, manholes, etc.).  All of the shapefiles are in SPV and I am using World Imagery Basemap.  I then go to Data Frame Properties, Coordinate System, and choose "NAD 1983 StatePlane California V FIPS 0406 (US Feet)".  I have also had this problem with StatePlane VI.  Is there something I am missing?  Is it just the basemap that is off?  What I am doing requires precision and some of these shifts are 30-60 feet off.  Please help!

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3 Replies
DylanHarwell
Occasional Contributor

Is the task you are doing relying on the accuracy of only the vector data? Or are you attempting to make precise measurements from your (assumed to be survey grade vector data) to the basemap imagery? 

If your basemap is for visual purposes only and your analysis is using your vector data, you've got nothing to worry about. If you need imagery at the same level of precision you will most likely need to buy/fly your own as the provided world-scale basemaps do not provide this level of precision.

We hire a company to fly every other year at our city with a spatial resolution of 3 inches. This still results in a shift of our survey grade data (sewer/storm MHs, water valves, etc.) sometimes up to several feet when compared to the basemap. You can only do so much with georeferenced imagery over wide areas, splicing tiles together. We also have a UAV program and can fly very small areas with very high spatial accuracy.

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SarahTalvy
New Contributor

I am trying to choose a location for a new well based on shapefiles in SPV.  I add these shapefiles and add buffers around them indicating where I can't site the well.  But you can clearly see the data is shifted, like I said, sometimes 30-60 feet.  Sometimes it's only a couple feet, other times it's a lot more.  I'm not a GIS expert, so I'm not sure if it's some kind of projection or transformation issue.  I do know that imagery gets tricky though and isn't exact.  The more help, the merrier!  I am stressing out.

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MelitaKennedy
Esri Notable Contributor

Are the original shapefiles offset? Or the buffered data or both? How accurate are the vector point data? Can you compare the data against a different basemap (or two) like the topo or streets basemaps? Do they show the same offsets?

Melita

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