Does ArcGIS Online re-project ArcGIS Server services on-the-fly in the web map?

Answer

When a dynamic map service or feature service is in a different coordinate system and projection than the ArcGIS Online Esri basemaps (Web Mercator, SR 102100), they are re-projected on the fly in the web map. The way this works for map services and feature services is slightly different, due to the nature in which the services are rendered in the web map.

The following explanation goes through how this re-projection works and how tools can be used to check if the service is being re-projected on the fly in the web map.

• Dynamic map services: If a dynamic map service is added to a web map in ArcGIS Online, ArcGIS Online sends an ExportMap request to the service. If the map service is in a different projection than the basemap, the ExportMap request is issued to the projection of the basemap. Using Fiddler or the browser's developer tools, the ExportMap request can be viewed. The following example uses the Fiddler app.
  1. Add this sample service (which uses the NAD 1983 HARN State Plane Illinois projection) to an ArcGIS Online web map.
     http://sampleserver6.arcgisonline.com/arcgis/rest/services/CommercialDamageAssessment/MapServer
      
  2. Using Fiddler, observe the ExportMap request sent to this map service. This request sends a number of parameters, including the output spatial reference of the image rendered in the map.
     
     
• Feature services: If a feature service is added to a web map, a query request is sent to the map service. After the query is sent, ArcGIS Online's built-in GeometryServer projects the data into the projection of the web map (Web Mercator, SR 102100 for Esri basemaps). Follow the example process below.
  1. Add the feature service listed below (which uses the NAD 1983 HARN State Plane Illinois projection) to ArcGIS Online:
     
     http://sampleserver6.arcgisonline.com/arcgis/rest/services/CommercialDamageAssessment/FeatureServer
      
  2. In Fiddler, observe the request made to the Esri GeometryServer, requesting the data to be projected in SR 102100 (the coordinate system and projection of the Esri basemap).