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I have a dataset with an unknown and possibly custom projection. However, some of the features have lat/long coordinates in the attribute table. How might I figure out the projection and coordinate system of this data based on this info?

What I've tried:

  1. Asking the folks who provided me with the data--so far this isn't helpful.
  2. Assuming a datum and projecting the data in ArcMap to common systems, then checking if it looks right.
  3. Georeferencing an image of the data based on known coordinates. Looks good, but isn't precise enough.
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  • daan Strebe used ProMapper to find a coordinate system once but the map included a graticule (laittude and longitude lines). Link goes to a CartoTalk forum thread. – mkennedy May 4 '17 at 23:14
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If it is a custom projection, and the people providing the data can't tell you what it is, you can try to recreate it with your data points that have lat/longs but there's no way to ever be sure if you've got it 100% exactly right. It sounds like you've already done everything I normally do to figure this out, although for your step 3 I would use the Spatial Adjustment toolbar to move your vector data so those known points match their lat longs, rather than converting it to an image and georeferencing it. The results will probably be the same though (close but not exact enough).

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