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ArcGIS suite of software categorises coordinate systems as either geographic or projected. From what I understand about coordinate systems (and I may be wrong), geographic co-ordinate systems are used to display features on a globe and projected co-ordinate systems are used to display features on a 2D plane.

How is it that when I use a GCS like WGS_1984 to display a layer of the entire earth, ArcMap shows it to me on a 2D plane? Doesn't that mean that the feature was projected.

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Longitude is simply being treated as an X value and Latitude as a Y value, there is no projection being used. –  Dan Patterson Aug 21 '11 at 14:11
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@Dan see second line in the wiki to Map Projection. Any mathematical function transforming coordinates from a curved surface to a plane is a projection. Because coordinates from a curved surface is transferred to a plane, doesn't that mean that any geographic coordinate system I use to display the earth is projected on ArcMap because I see it in a plane. –  Thomas Aug 21 '11 at 14:18
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from this link help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//… midway down... "It may be helpful to equate longitude values with X and latitude values with Y. Data defined on a geographic coordinate system is displayed as if a degree is a linear unit of measure. This method is basically the same as the Plate Carrée projection." –  Dan Patterson Aug 21 '11 at 15:15
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If you had a monitor in the shape of a sphere, using spherical coordinates to address its pixels, the ArcMap display would be perfect :-). –  whuber Aug 21 '11 at 19:30
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@whuber Microsoft made one. I think zooming might present problems though. –  Kirk Kuykendall Aug 25 '11 at 14:07
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Upon request, a comment is being posted as an answer, but do read all the comments...

From this link http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//003r00000006000000.htm midway down... "It may be helpful to equate longitude values with X and latitude values with Y. Data defined on a geographic coordinate system is displayed as if a degree is a linear unit of measure. This method is basically the same as the Plate Carrée projection."

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When I explain this, I call what we (Esri) do a pseudo-Plate Carrée projection. The angular units are treated as if they are linear. A true Plate Carrée projection scales the decimal degree values into linear units. –  mkennedy Aug 25 '11 at 16:08
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I think you need to connect arcmap to the appropriate display device.

enter image description here

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+1 Cool! Now advance a few years into the future when display devices will be printed onto thin sheets and be rolled up or stuck to your refrigerator. Separate the pixels slightly by material whose length can change under programmatic control, and voila, you can modify the curvature, thereby achieving a true physical zooming capability. Anybody want to work on the patent? :-) –  whuber Aug 25 '11 at 19:05
    
@Kirk ... why would you need a 3D display device (other that it being cool) ... Look at ArcGIS Explorer ... its got a 2D and 3D view. The 3D view only displays GCS (and 2D support both, like ArcGIS Desktop). –  Thomas Sep 7 '11 at 5:12
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