Geographic Information Systems Asked by LamberWBY on April 8, 2021
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:
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?
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.
Answered by Nicolas Cadieux on April 8, 2021
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.
Answered by mkennedy on April 8, 2021
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