7 Replies Latest reply on Aug 6, 2013 9:21 AM by mminami-esristaff

    Web Mercator (auxiliary sphere) vs. Web Mercator - overlay consequences mixing them?

    dananrg
      Bing/Google map services use a tiling scheme having a WGS 84 Web Mercator (WM) coordinate system.
      The new ArcGIS Online services use a tiling scheme having a WGS 84 Web Mercator Auxiliary Sphere (WMAS) coordinate system.

      As I understand it:
      * WM uses a perfect sphere for its earth model
      * WMAS uses a spheroid or ellipsoid (better for aligning local data to than a perfect sphere)

      As far as I know, these are the only differences; e.g. both tiling schemes share the same origin, map scales, and tile dimension size. I've also heard that the two coordinate systems are "mathematically identical". Not sure what's meant by that--but it seems to imply they are functionally identical but have different performance characteristics for overlaying dynamic data against. Are there other "differences that make a difference?"

      My questions:

      Q1) In an ESRI-based web app (e.g. one created with an ESRI web mapping API), what happens when two cached map services, one WM and one WMAS, are added? Are both displayed without a fuss? What about when one or both are dynamic web services?

      Q2) In a non-ESRI based web app, what happens when two cached map services, one WM and one WMAS, are added? What about when one or both are dynamic web services? Can non-ESRI map servers even generate WMAS map services?

      Something feels incomplete in the descriptions I've read about WM and WMAS. Isn't WM the defacto tiling scheme? And might serving WMAS map services to non-ESRI clients present a problem when others try to perform mashups?