ArcServer best practices coordinate system

[BillChappell]

I have a question that's come up at work and I can't find a definitive answer.

If I'm going to create an ArcServer REST service, what is the ideal projection? I know a service has the option of re-projection data but this should have a server side performance hit on each callback. I'm working with stream/river data and it may have high demand for a while.. 

I know ArcGIS on-line uses 3857 Web Mercator and so do most base maps. However many of the data REST Services I use require WGS-84 with Decimal Degrees. (GeoJSON for example)

I'm in NY and it appears many services are in UTM zone 18, but Buffalo isn't even in zone 18, so this isn't ideal if I have it return Meter coordinates. 

I'm looking for the best solution, that would return the data in my web map (wgs-84) the fastest with the least demand on the server. 

It's a dynamic map service that is constantly updated so caching the service is not feasible. 

Your thoughts?

[BillChappell]

I did find this which was interesting.  

https://www.w3.org/TR/sdw-bp/#bp-crs-choice

[GerardEddy]

Hi Bill,

You are correct, services run faster when they do not need to project (re-project) on the fly, so the datum and projection are important.  I will caution you on the WGS84 with respect to accuracy.  If your use involves picking a coordinate value you may find that the WGS84 projection can be off in both the X,Y (not sure about Z) with respect to actual ground coordinates.

If you do need to transform your data, ultimately avoid going back and forth, try single one way transformation to reduce potential errors.  Each time you run a re-project transformation, it is like your data being stretched over new graph paper, and there are always distortions, especially as you move away from the centre either east or west to the margins of your zone.

Hope this helps in your situation.