Datum Conflict ArcGIS online and local data

[CortCagle]

Will arcgis online maps correctly re-project into a different datum? When i open a online map service and then try and re-project it to our Local projection (say UTM13 North Feet NAD 27) the online map looks like it re-project...but when i overlay local data in our local projection things seem to be off on the East west about 200ft. I know this is usually a datum issue. the question is should the online maps datum correctly re-project? or is there a best practice or way to get it to correctly re-project? 

thanks in advance-cort

[MollyZurn]

I assume you are using ArcMap? ArcMap only applies one transformation automatically: NAD_1927_To_NAD_1983_NADCON, which is useful for the contiguous US (lower 48 states). Because the ArcGIS Online data is in WGS84 or possibly WGS 1984 Major Auxiliary Sphere, a geographic (datum) transformation must be applied. For WGS 1984 / NAD 1927, try using NAD_1927_To_WGS_1984_79_CONUS.

Molly

[KristofVydt]

I had a similar problem, adding a map service (published by myself in a regional coordinate system, Belge_Lambert_72) on top of one of the ArcGIS Online basemaps in the webmap application on ArcGIS.com. The map service data had a fixed shift, due to missing datum transformation. I knew the issue in ArcMap, where I have to select a specific datum transformation, in order to make the layers match. How to make the layers match in the webmap application?

The only think I had to do is add the datum transformation of my choice with ArcMap in the MXD. Even when you don't have to select a datum transformation in ArcMap, because all layers are in the same, local coordinate system, you can still explicitly specify datum transformations the usual way. After saving this to my MXD and restarting my map service, the layers matched and the shift was gone.

Kristof Vydt