When using local projection in a service, is it better to apply a transformation in ArcMap or let AGOL project on the fly?

[OliviaDeSimone]

I'm about to publish a very large service to ArcGIS Server 10.4.1 to be used in an AGOL web map and application.  Usually, my organization just leaves our data our local projected coordinate system, which is based on NAD 83, before publishing.  We have run into issues, particularly with our basemap layers, where we have had to apply a transformation in ArcMap to transform our data into WGS 1984 before publishing to be consumed in an AGOL web map.

Ultimately, I know both methods work if you are just consuming your service as an operational layer (rather than as a basemap layer), but which method would be considered the "best practice"?  Which method will make your services run most smoothly?

[RandallWilliams · Accepted solution]

In general, we'd recommend doing as much 'pre-processing' as possible on a web service (Pre-Process for Performance). On the fly transformations and such can be taxing on the GIS Server, which stacks up when you have a busy site. Same goes for GP - if you can reduce your script/model to only what's needed to get to the final product, you'll be better off.

Here's a few resources that speak to this topic:

https://desktop.arcgis.com/en/arcmap/latest/map/working-with-layers/optimizing-map-content-for-perfo...

https://desktop.arcgis.com/en/arcmap/latest/map/publish-map-services/map-authoring-considerations.ht...

[OliviaDeSimone]

Thanks, Randall.  That second link, "Map authoring considerations," said explicitly, "For optimal performance, use the same coordinate system for both your data and your map document. If this is not possible, make sure appropriate geographic transformations are applied to the layers that need to be projected on the fly."