Combining different coordinate systems

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05-23-2013 02:09 PM
TomFinch1
New Contributor
Hi,

I'm having some cartography issues. I started off with some raster basemaps which have "User_Defined_Lambert_Conformal_Conic" Spatial Reference. These maps were created by a colleague, who tells me that they actually use WGS84. When I import WGS84 coordinates from my GPS they all match up fine. They also match up perfectly with the ESRI basemaps. No problem.

I've now downloaded some more recent raster basemaps from IGN (the French mapping folk) which use "European_Lambert_Conformal_Conic". When I add these maps to the ESRI basemaps, they show up in the Arctic Ocean, when they should be in the South of France. They also don't match up with the original ("User_Defined") rasters or the coordinates from my GPS. Note that it doesn't make a difference in what order I add the different data.

Where am I going wrong? I assume it's a transformation issue, but don't know how to fix it. Surely if ArcMap knows that one feature uses x coordinate system and another y, then it should be able to display them correctly? I think I'm fundamentally misunderstanding how it all works anyway.

Thanks in advance

PS I'm using 10.1 Desktop
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3 Replies
MelitaKennedy
Esri Notable Contributor
Hi Tom,

If they're showing up in the arctic, that's more than just a geographic/datum transformation issue. It sounds to more like we're misinterpreting the coordinate system. Can you post the full details from one of the French rasters? Coordinate system (projection and parameters), raster's extent (in the projected coordsys and in lat/lon if you have the latter), approximate area of coverage. Is the IGN data available publicly, or at least its metadata?

Melita
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TomFinch1
New Contributor
Hi, thanks for your reply. I've almost figured it out myself..

I lied when I said it was "Europe_Lambert_Conformal_Conic" - it's actually "RGF_1993_Lambert_93". When I set the projection to the latter (it is 'Undefined' on download) the rasters match up with the ESRI basemap and my GPS coordinates. However, the data I inherited with "User_Defined_Lambert_Conformal_Conic" are out by about 50m NNE.

So, is it possible to modify the inherited data (a combination of point and polygon shapefiles) so it all matches up?

Thanks again,

Tom
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MelitaKennedy
Esri Notable Contributor
Hi Tom,

Even with the "User_Defined" rasters not having a transformation to RGF93 set, if they are in WGS84, they shouldn't be 50m offset. The RGF_1993_To_WGS_1984_1 transformation that we have uses zeroes for the parameters.

Is the User_Defined_LCC data the same as that in the first post which you said lined up fine with other data? What's behind the "User_defined" (it means Esri software doesn't quite understand what's there)? If you add it first to ArcMap, can you post what shows up in the data frame properties, coordinate system tab in the text box for the current coordinate system?

Thanks,
Melita
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