I have a polygon feature class in NAD_1983_UTM_Zone_13N (Green in map). I projected it to NAD_1927_StatePlane_New_Mexico_East_FIPS_3001 (Pink in map). Why are these not aligning? Does this mean the original definition was wrong? Since ArcMap projects different CRSs "on the fly", I can't figure out why they aren't perfectly aligned.

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    You must specify the correct geographic transformation when performing NADCON "on the fly". – Vince May 18 '16 at 14:45
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    Could you provide more information about the data? Are these two different shapefiles or one shapefile in two different projections? – GISKid May 18 '16 at 15:40
  • @Vince This is the the correct answer. I didn't realize ArcMap wasn't picking the transformation automatically for this type of conversion. – GeoJohn May 18 '16 at 15:40
  • @Vince Any idea why this has to be specified manually? It seems like AGS should know to do this automatically, maybe with the possibility to override some default behavior. (The preceding sentence is a polite way of saying, "It seems really stupid that you have to do this instead of ArcMap just doing it for you," so worded because I'm trying to leave room for me missing something.) – jpmc26 May 19 '16 at 1:16
  • My understanding is that there are multiple potential "correct" values, and that a human is required to select between them – Vince May 19 '16 at 1:30

Solved: My map's data frame is in NAD_1927_StatePlane_New_Mexico_East_FIPS_3001. When I added the NAD83 UTM data I had assumed it was spatially accurate "on the fly" not knowing that, as @Vince, pointed out, when using "on the fly" projection for NADCON the transformation must be specified manually. After re-adding the NAD83 UTM data and manually specifying the transformation everything came together nicely.

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Do they both have a defined coordinate system? I think this occurs because although Arc can project data on the fly, this does not apply when the data is in two different coordinate systems or datums, such as NAD 1927 vs NAD 1983. You need to apply a transformation. See related. I would recommend choosing a coordinate system and projection, and re-project as necessary. See also for an explanation of how to properly project when your data isn't lining up.

  • The datasets originated from the same FC. I projected the orginal data in NAD_1983_UTM_Z_13N(green) to NAD_1927_StatePlane_New_Mexico_East_FIPS_3001(pink). So yes, both have a defined coordinate system. ArcMap can definitely project "on the fly" even if the datasets have a different Datum/GCS. It applies the correct transformation internally. The only thing I can come up with is that the original definition was incorrect, but unfortunately I have no way to confirm this. How would going from NAD27 SP to NAD83 UTM be the problem if assuming the original definition is correct? – GeoJohn May 18 '16 at 15:32
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    Sorry, I think my answer was unclear. I am referring to Geographic coordinate systems or datums not Projected coordinate systems (which is what you are referring to). Since your data is in NAD 1927 and NAD 1983 you need to convert them into the same geographic coordinate system. – GISKid May 18 '16 at 15:47
  • ...In most cases I've seen it applies the transformation automatically. – GeoJohn May 18 '16 at 15:48
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    Edited my answer for clarity. I've found Arc to be finicky as well in that regard, so my usual workflow is to check that datum and dataframe are the same then project, and then open a new ArcMap and add everything back in. – GISKid May 18 '16 at 15:53
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    @ziggy simplistically: think of a datum as a representation of points on earth's surface ie. lat/long. A projection is when you want to represent a 3D surface (earth) onto a 2D surface (your screen). So yes, you need a datum/CS underlying a PCS. Good discussion here: geonet.esri.com/thread/23160 – GISKid May 18 '16 at 16:12

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