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

danan

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?

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Anonymous User

First off, dynamic map service can always mashup with other services. They can be projected on the fly to match up with the coordinate system of the web map. This projection happens on the server before an image is delivered to the client. Cached map services need to have the same coordinate system in order to overlay in a web client.

Answer to Q1: All of the Web API's treat WM and WMAS as the same coordinate system so they will overlay just fine. The Web ADF is another matter. There you need to use WMAS to overlay with ArcGIS Online.

Answer to Q2: It depends on the API. If the API treats both as the same coordinate system than you will be fine. or if it doesn't check the coordinate systems at all, you will be fine.

There is much more information on Web Mercator Auxiliary Sphere is available here: http://help.arcgis.com/en/arcgisonline/content/index.html#//011q00000005000000

Basically WMAS makes it easier to align your data with the WMAS basemap. So if you goal is to align your data with public basemaps then you likely want to go with WMAS. But if your goal is to publish services that will be used by non-esri API's that have more stringent coordinate system restrictions then you might want to go with WM. Just keep in mind that your data might not align all the well with WM.

JoshuaBixby

If you are interested in a bit of background on WM and WMAS at ESRI, you might want to check out the following older forum thread:
[INDENT]ArcGIS Desktop - Projections, Projection Engine forum - Why are there 2 Web Mercator PCS's?
[/INDENT]

CarolynWhite

The ArcGIS Server Help for 10.1 (Data Storage Considerations)
says ...

A common example is caching a web map in the WGS 1984 Web Mercator (Auxiliary Sphere) projection used by ArcGIS Online, Bing Maps, and Google Maps.

So do they all use the same WMAS or do Google and Bing use WM?

Mike Minami

Yes, I believe they do.

Here's more interesting background, if you're so inclined...

http://alastaira.wordpress.com/2011/01/23/the-google-maps-bing-maps-spherical-mercator-projection/

thanks,

Mike

AravinthrajRangasamy

Hi

In arcmap I used WGS 1984 Web Mercator (auxiliary sphere) but it will not placed exact location whatever we creating shapefile in arcgisonline and google maps.Please anyone tell which coordinates to use for exact location.

Thanks Advance
Raj

JasonSchwartz

Hi

In arcmap I used WGS 1984 Web Mercator (auxiliary sphere) but it will not placed exact location whatever we creating shapefile in arcgisonline and google maps.Please anyone tell which coordinates to use for exact location.

Thanks Advance
Raj

Raj,

Please clarify what problem(s) you see when you create a shapefile (You say the data are not placed in the "exact location" but that says you expected something different than you saw...A)what did you see, B)what did you expect? Perhaps two screenshots will support your answer.

I have a lot of experience with both WM and WMAS, and I do not experience the problem as you explained it. -- Specifically, my data appear where I expect it:

My data do not shift relative to a basemap when I change the map projection between WM and WMAS., nor when I substitute one (WM) basemap for another (WMAS) basemap.
Jason

Mike Minami

Raj,

It sounds like you need to transform your shapefile data. Here's a similar post.
http://forums.arcgis.com/threads/89781-bad-display-of-data?p=318456&viewfull=1#post318456

Thanks,

Mike

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