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Why different results for NeighborFinder versus GeographicNeighborFinder transformers for LL84 data?

  • February 28, 2022
  • 2 replies
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ssmith_ftr
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I'm struggling to understand why I get different results for what should essentially be the same transformers. I created a set of sample data and workbench to demonstrate my question.

 

Using NeighborFinder I'm getting points about 6 miles away from my test point. Using the same test point and using GeographicNeighborFinder I find the correct nearest points only 5 miles away. Since all my data is in same LL84 I don't understand why NeighborFinder isn't giving identical results.

 

I'm asking because I eventually have to run millions of features and GeograpicNeighborFinder is nice but has a lot more overhead and will take much longer to complete. I don't need the extra features if NeighborFinder would give identical results.

Best answer by markatsafe

@ssmith_ftr​ Thanks for including the example workspace. Unfortunately the FFS file was not complete and I couldn't read it.

The issue you're seeing is usually becasue the the source data is in LL84. NeighbourFinder uses the current projection for the units of measure. So when the projection is geographic LL, the output 'distance' is not very meaningful as it's some function of latitude and longitude. GeographicNeighborFinder does a temporary reprojection to a local coordinate system (_AZMED_) and uses those for the units of measure for the output distance (meters) . To compare the two, try reprojecting to UTM or state plane before using the NeighborFinder.

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2 replies

M
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  • markatsafe
  • 1891 replies
  • Best Answer
  • March 1, 2022

@ssmith_ftr​ Thanks for including the example workspace. Unfortunately the FFS file was not complete and I couldn't read it.

The issue you're seeing is usually becasue the the source data is in LL84. NeighbourFinder uses the current projection for the units of measure. So when the projection is geographic LL, the output 'distance' is not very meaningful as it's some function of latitude and longitude. GeographicNeighborFinder does a temporary reprojection to a local coordinate system (_AZMED_) and uses those for the units of measure for the output distance (meters) . To compare the two, try reprojecting to UTM or state plane before using the NeighborFinder.

ssmith_ftr
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  • ssmith_ftrContributor
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  • 14 replies
  • March 1, 2022

@Mark Döring​ Thanks very much. That was solution.

I still don't fully understand all the differences in Coordinate Systems, but that was my problem. Using UTM for reprojection I now get correct answers.

 

Sorry about the FFS file. I'm not sure what wrong. I used it to read in data to that sample workbench so it worked for me before I zipped.

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