Transformation from GRS_1980 to OGSB_1936

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05-04-2016 05:21 AM
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Hi, I'm attempting a transformation between these two coordinate systems. I've tried by adding the GRS_1980 raster then selecting Transformations > Using, however the only option is <custom> New Geographic Transformation. I used this method then tried to clip the GRS_1980 raster to the OGSB_1936 however I dont think it has worked correctly as the GRS raster doesnt align and overlay properly with the OGSB raster. Anyone have any advice, some details on the datasets below.

GRS_1980:

Spatial coverage: Europe

Pixel size: 500m

OGSB_1936:

Digital Elevation Model of area in southern England

Pixel size:50

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MVP Notable Contributor

Hi, are you using ArcMap? Which version? In any case, I am tagging Melita Kennedy​ on this to hopefully give you a good answer!

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Hi, yes I'm using ArcMap 10.3.1. Thanks

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MVP Esteemed Contributor

You are using the Project Raster—Help | ArcGIS for Desktop tool, I presume.  As the help topic suggests there is a transformation document (geographic_transformations.pdf) in you install directory ... C:\Program Files (x86)\ArcGIS\Desktop10.4\Documentation  ... in my case 10.4 ... where you can find the transformation parameters if they haven't been implemented specifically

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Hi Dan, I wasn't using that tool initially, I attempted it through the 'Geographic Coordinate Systems Warning' that opened when i added the layer. I will try what you suggested. Thanks

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Can't seem to find any mention of a GRS_1980_IUGG to OGSB_1936 transformation in the .pdf.

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MVP Esteemed Contributor

there may not be a transformation create, but you can at least compare the spheroids used to see if there are any matches...given that the latter one seems to be old

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The OSGB spheroid is Airy 1980 (EPSG 7001), not sure of the spheroid for the GRS_1980. The pdf doesnt contain mentions of spheroids used in the transformations either.

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MVP Esteemed Contributor
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Esri Notable Contributor

I would expect that the European data is on ETRS89 which uses GRS80. Can you redefine the (Define Projection or in "catalog") the geographic coordinate system for the raster to ETRS 1989? Then you should starting seeing OSGB_1936_To_ETRS_1989_1 as a possible transformation. It's file-based and will only affect data in the UK.

Edit: If you don't see that transformation, you might be on an earlier version that didn't have the grid file. Please post what software version you're using.

Melita

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