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Thread: Coordinate systems and Transformations in Explorer on-line.

  1. #1
    Jonathan Virtual Nature Systems
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    Default Coordinate systems and Transformations in Explorer on-line.

    I wanted to know whether ArCGIS Explorer on-line supports on the fly transformations for data that is in different coordinate systems.

    So if I start a new map, and load in a basemap (Bing Hybrid) does this basemap set the default coordinate system for the map ?
    Are any subsequent layers automatically transformed to match this ?
    Is this true if they are WMS layers ?
    Is this true if they are imported shapefiles ?
    Does this apply if they are different geographic coordinate systems ?

    thanks !

    Jonathan

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    Shelly Gill
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    Default Re: Coordinate systems and Transformations in Explorer on-line.

    ArcGIS Explorer does support on the fly projections using the default transformation, for vector data or dynamic services where data can be requested in a specific coordinate system; however it cannot transform tiled data services.

    Quote Originally Posted by virtualjon View Post
    So if I start a new map, and load in a basemap (Bing Hybrid) does this basemap set the default coordinate system for the map ?
    Yes, picking the basemap sets the coordinate system of the map, which for the Esri basemaps is Web Mercator Auxiliary Sphere - you might find the Map Projections and Coordinate Systems help topic useful.
    If you create a map using a specific service as a basemap (in the New Map -> Advanced options) then the map will use the coordinate system of that service, and therefore the coordinate system for that map might be different to the default Web Mercator (note that switching basemaps is no longer possible at this point).

    Quote Originally Posted by virtualjon View Post
    Are any subsequent layers automatically transformed to match this ?
    For dynamic map services, or for a feature service, where the service can return the map tiles in a specific coordinate system, yes the data will be reprojected. For a tiled map service the coodinate system of the data is predefined, and if it doesnt match that of the existing map, you will get a message indicating that you cannot add the layer to the map (see the Troubleshooting guide for this info, as there may be other reasons a map service cannot be added).

    Quote Originally Posted by virtualjon View Post
    Is this true if they are WMS layers ?
    WMS are tiled services, and can be published so that they support a range of coordinate systems, or just a single coordinate system. If the service supports a matching coordinate system to that of the map, then you'll be able to add it.

    Quote Originally Posted by virtualjon View Post
    Is this true if they are imported shapefiles ?
    Does this apply if they are different geographic coordinate systems ?
    Shapefiles are vector data so yes, we can reproject them to match the map, whether they are geographic or projected coordinate systems. However, note that the default transformaion is used, there's no facility to pick a specific geographic transformation to account for datum shifts, so if you want the most control and accuracy over importing your shapefiles, it would be best to make sure they're in the coordinate system you want to work in before you import them into a map. You might find that the accuracy is fine however, it just depends on the exact data and reprojection required.

    Hope this helps.
    Shelly Gill
    Product Engineer at ESRI

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    Jonathan Virtual Nature Systems
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    Default Re: Coordinate systems and Transformations in Explorer on-line.

    OK thanks for this !

    So one final question related to importing shapefiles that follows from your last point....

    I have a shapefile that uses the following projection details (taken from the .prj file):

    PROJCS["ED_1950_UTM_Zone_30N",GEOGCS["GCS_European_1950",DATUM["D_European_1950",SPHEROID["International_1924",6378388.0,297.0]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",500000.0],PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",-3.0],PARAMETER["Scale_Factor",0.9996],PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0]]

    If I import this shapefile into an existing Explorer on-line file with the Bing Basemap pre-loaded do you know which "default transformation" will be used ? I want to know basically whether this will produce an accurate result or not. The file I am working on covers southern France and northern Spain.

    thanks again for your help.

    Jonathan

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    Shelly Gill
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    Default Re: Coordinate systems and Transformations in Explorer on-line.

    To the best of my knowledge, there will be no datum transformation used, the data just will be reprojected - I'll try and confirm that for use with colleagues though. You can create a web map with a different coordinate system by using a service as a basemap that has the coordinate system that you want - perhaps this would be a better way to display your european data (note the limitations here though):
    http://help.arcgis.com/en/arcgisexpl...000003t000000/ - see the section 'To create a new map based on a known URL of a map or image service'.
    Shelly Gill
    Product Engineer at ESRI

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    Shelly Gill
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    Default Re: Coordinate systems and Transformations in Explorer on-line.

    Quote Originally Posted by sgill View Post
    To the best of my knowledge, there will be no datum transformation used, the data just will be reprojected - I'll try and confirm that for use with colleagues though.
    I have updated information on this bit from a colleague - there is a default transformation picked for many common datum shifts. In your case given that you're moving from the Eurpoean 1950 GCS to the web mercator auxiliary sphere PCS used by the map, the ED_1950_To_WGS_1984_1 transformtion will be used, WKID 1133. This is a transformation with a best average for Austria, Belgium, Denmark, Finland, France, Germany (West), Gibraltar, Greece, Italy, Luxembourg, Netherlands, Norway, Spain, Sweden, Switzerland, and Portugal.

    Hope this info helps,
    Shelly Gill
    Product Engineer at ESRI

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    Jonathan Virtual Nature Systems
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    Default Re: Coordinate systems and Transformations in Explorer on-line.

    Ok - this is great to know. Thanks for taking the time to chase it down. At least I know what is happening in the background now :-)

    best wishes

    Jon

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    Default Re: Coordinate systems and Transformations in Explorer on-line.

    I am trying to check and specify the coordinate system for lat, long DD location data. According to the online help

    http://help.arcgis.com/en/arcgisexpl...0000003p000000

    one should click the "Service Details" button in the "Layer Details" view. Unfortunately I don't think the "Service Details" button exists. Any suggestions would be much appreciated.

    Anders

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