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Today I was trying to use ArcMap to find out what is the distance between two locations with same coordinate values in NAD27 and NAD83, but the points I drew appeared to be at the exact same location. There must be something wrong with my understanding in datum or my knowledge in ArcMap, but I can't figure it out. Below is what I did:

  1. Open a blank mxd
  2. Create a Feature Class A in a File Geodatabase in NAD27 coordinate system
  3. Create a point in Feature Class A at (40, -75), save edit
  4. Open a new mxd
  5. Create a Feature Class B in a File Geodatabase in NAD83 coordinate system
  6. Create a point in Feature Class B at (40, -75), save edit
  7. Load Feature Class B in the first mxd

The result was that the points in two feature class appeared at the exact same location. I believe there is something to do with projection on the fly, but I thought if the two points represented two different real world locations, they should appear in different locations on a map no matter what coordinate system the map uses. What am I missing here?

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    When you added your second Feature Class in your mxd, did you get prompted for a Geographic Coordinate System Transformation? Which one did you choose?
    – FSimardGIS
    Oct 30, 2020 at 1:54
  • Thanks for the hint. I see which step I was missing. When this warning came up I just closed it, so when Feature Class B was loaded in the NAD27 mxd, the mxd just plot the point treating its coordinate as if they are in NAD27, and thus the point is plotted at the same location.
    – LamberWBY
    Oct 30, 2020 at 3:08

2 Answers 2

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ArcMap doesn't automatically apply geographic (datum) transformations. Depending on how you add data, you may or may not be prompted to set a transformation. If you aren't, open the data frame properties, select the Coordinate System tab and click the Transformations button.

In the Transformations dialog, in the middle pulldown will be the geographic coordinate reference system (GeoCRS) of the map. In the top box, select the other GeoCRS. So if the map is using NAD83, select NAD27 AKA North American Datum 1927.

In the bottom pull-down, select NAD_1927_To_NAD_1983_NADCON. OK all dialogs. You should then see that the two points are offset. The offset isn't that much in latitude-longitude. You can see a large offset if you change the data frame's coordinate system to the local UTM zone. Then the north-south offset should be about 200 meters with a smaller east-west offset.

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Not a bid ESRI user but if you have projection on the fly activated, they should appear in the same spot. If you can disable it, then the point could shift.

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  • I would expect it to be the other way around in this case, on the fly means it will perform a transformation so the points should appear in different locations. If it's disabled, then no transformation is executed and they would appear on top of each other.
    – FSimardGIS
    Oct 30, 2020 at 2:17

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