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I was given a bunch of ASCII files that I needed to convert to raster and then create maps from. For some maps I had to do a simple difference calculation between two rasters (did this in ArcMap).

Well, after I had created dozens of these maps I realized the rasters didn't have a spatial reference. The ASCII files all contain the same spatial extent in lat/lon and same cell size in degrees. I thought to avoid any potential problems that may be caused by the lack of spatial reference, I would define the projection as WGS 84.

Would the lack of spatial reference have caused any problems when I did the difference calculations? I tried calculating the difference for a couple of sample files with and without a spatial reference and found no difference so I assume I can just add the spatial reference to my difference rasters that don't have a spatial reference instead of redoing them all.

Is this correct?

  • ASCII files (Esri ASCII grid) don't contain projection information only world translation. If you have only done differences between unreferenced rasters then there should be no problem - only if you have performed calculations against referenced data and their correct spatial reference differs from the assumed spatial reference (NAD I think, which is fairly much WGS84 for geographic). – Michael Stimson Nov 19 '14 at 2:14
  • Yes, I only calculated differences between rasters with no spatial reference (but they were all using the same coordinate system with the same extent and cell size). I didn't do any calculations between referenced and non-referenced data. Well, actually I did on one to just test if there was difference between the calculations using non-referenced vs referenced data and there wasn't. Thanks so much for your response! – Jenni Nov 19 '14 at 14:07
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You should be fine if it was consistently undefined in them all but they all really had the same coordinate system (just not defined).

  • Yes, all calculations were done on rasters that had the same coordinate system, same extent, and same cell size. Thank you so much for your response! – Jenni Nov 19 '14 at 14:04

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