I have a point feature class in a geodatabase that uses WGS84 as its SRS. A colleage of mine created points in this feature class using an external database containing lat and long values. However, the lat and long values in the external database were using NAD83 (my fault -- I knew they were, I just forgot). I thought it wouldn't be a problem -- I just created a copy of the geodatabase and changed the coordinate system used by the feature class to NAD83. Should be good, right?
I added this feature class into ArcMap. I saw that the points in the copied feature class (now using NAD83) exactly line up with the original feature class (using WGS84). That doesn't seem right. If two points, having the same lat / long values, but referenced in different coordinate systems are overlaid on the same map, one of them should be projected on the fly to the coordinate system of the data frame, no?
Now, here's the weird part -- if I project the original point feature class (using WGS84) to NAD83, and add it to the map, then the points don't line up. See, this isn't what i expected, either. I expected that they would be projected to the other coordinate system (i.e., their lat / long values would be different by virtue of being projected, but they would be aligned with the original point because they were simply projected to the new coordinate system (then projected on the fly back to the original coordinate system).
Can anyone explain the behaviour that I'm seeing? And, how do I properly ensure that the NAD83 lat and long values collected by my colleague are projected to WGS84 in my target geodatabase properly?