Geographic coordinate system issues

02-19-2012 05:13 PM
New Contributor II
Hello, I am attempting to use some DEM's of Spain, in ArcGIS 10, that I downloaded from the CGIAR website.  I get a warning saying that the geographic coordinate system differs from that of the data frame. 

The data frame properties coordinate system is:

Projection: Transverse_Mercator

Datum: D_European_1950

The DEM's coordinate system is listed as:

Spatial Reference - GCS_WGS_1984
Datum - D_WGS_1984

I'm not really sure how to rectify the problem.  I tried going into Data frame properties > Coordinate system > Transformations and convert from GCS_WGS_1984 to GCS_European_1950 using ED_1950 to WGS_1984, which is the datum transformation specified for Spain and Portugal, but that didn't seem to do anything.  I also read somewhere that you could use the Define Projection tool, but for that you need to know what projection the data layer was created in, and I don't have that information.  I can tell that the DEMs are using a geographic coordinate system, because the extent is defined in lat., long. coordinates, but beyond that I don't really know how to solve this problem.  I know that this may be a relatively basic problem, and I did search the forums for an answer, but didn't really find anything.  Any help would be greatly appreciated.

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4 Replies
New Contributor II
Please, somebody help me with this!  I have a presentation next week and cannot figure this out.....

Thank you, thank you, thank you!
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Esri Notable Contributor
I'm not quite sure what the problem is. Do the two datasets not overlay? As long as both have the correct coordinate systems, you should be able to overlay them within a hundred meters or so. If they do not overlay in ArcMap, what is the offset? Adding a ED50/WGS84 transformation should take care of most of any offset between ED50 and WGS84.

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New Contributor II
Hi Melita, Thanks for your reply.  The two data layers do not seem to overlay correctly, there is a visible offset between the boundary of Spain layer and the DEM layer.  I am not sure how to quantify exactly what the offset is, but it is visible at least.  If there is a way to make them line up a little better, that would be great. 

Thanks again for your help,

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New Contributor
Hi Graham,
You should expect an offset of several meters between those GRS (there is a button that looks like a ruler in your toolbar that you could use to measure the distance between redundant features in both layers).  You should always consider the scale/resolution of your original data when you assess the match in a transformation of GRS (do not expect a great match from low-scale cartography).  If you need a better match the only idea I can suggest is that you perform a geometric transformation based on some common features that you could identify in both layers.
I hope it helps.
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