Geographic coordinate system issues

[grahamlangley-edwards]

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:

ED_1950_UTM_Zone_30N
Projection: Transverse_Mercator

GCS_European_1950
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.

Thanks

[grahamlangley-edwards]

Please, somebody help me with this!  I have a presentation next week and cannot figure this out.....

Thank you, thank you, thank you!

[MelitaKennedy]

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.

Melita

[grahamlangley-edwards]

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,

Graham

[SebastianSantibanez]

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.