ArGIS Online Projections

[bryankrouse]

I have a question about the projections used in ArcGIS Online (AGO). For example, if I geocode a CSV file of four points what projection is this layer using in AGO? Then, for example, if I create a map note to calculate the area within these four points, what projection is being used? If it is web Mercator, how accurate is this measurement since area is not preserved in Mercator?
Thanks,
Bryan

[SimonLiu]

CSV files must specify point locations using WGS 1984 lat/longs. They are then projected into whatever coordinate system the basemap uses (Web Mercator for the ones in the standard basemap gallery). At the moment, points from a CSV layer do not have any datum transformation applied to them when they are being projected, so if the basemap uses a projected coordinate system not based on WGS 1984 (e.g. British National Grid), then the points could be several hundred metres out.

Map notes captured in the ArcGIS.com web viewer use the coordinate system of the basemap.

The measure widget in the ArcGIS.com web viewer uses geodesic area calculations on a server-side geometry service, so the calculation should be accurate. More details here:

http://resources.arcgis.com/en/help/rest/apiref/areasandlengths.html (see "calculationType" on that page)

[bryankrouse]

This help greatly. One more quick question along the same lines...if I upload a shapefile (with a .prj) different than WGS84 to ArcGIS Online (AGO), does AGO do a transformation to Web Mercator?

[HeathBrackett1]

Essentially, yes.  If you are using that to create a feature service then the feature service will be projected to WGS 1984 Web Mercator (Auxiliary Sphere).  A zipped shapefile that is added to a web map or added to ArcMap from AGOL will also be projected into WGS 1984 Web Mercator (Auxiliary Sphere).  But if the shapefile is an item in AGOL and the user downloads the zipped shapefile and adds that to ArcMap, it will maintain its original projection.

Heath

[bryankrouse]

Thank you.