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If my DEM data's spatial reference is the British National Grid, when I use the xy tool in ArcGIS 10.2, are the lat/lon coordinates in OSGB36 or WGS84?

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    a small trial and error will solve that. Greenwich is located at (0, 51) in WGS84 and (53841,177840) in UKGrid
    – nickves
    Commented Jun 3, 2017 at 12:28
  • I'm actually looking at sites in Scotland, so I don't have DEM data for Greenwhich. Is there another way to do it? thanks
    – user19622
    Commented Jun 3, 2017 at 16:39
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    @user19622 - it should still be obvious from the rough lat/long or X/Y numbers you are getting. If only one or two digits before a decimal place, its probably some kind of degrees based system, rather than in metres. Your local configuration will determine which specific result you are seeing though (e.g. UTM vs OS).
    – BradHards
    Commented Jun 4, 2017 at 1:19
  • Go to gridreferencefinder.com and find your scottish location where your dem is and right click on the map you get both sets of coordinates (lat/lng) and (x,y eastings/northings).
    – Mapperz
    Commented Jun 4, 2017 at 1:55
  • Hi, thanks for that. I went to the website and searched my site using the NGR number. I took the values it gave for Lat/Lon as well as the x,y (easting/northings). Then I used the xy tool in ArcGIS to add those points. The lat/lon (which is in WGS84) values created a point west of the point of the x,y (easting/northing coordinates). But how do I know which one is the correct one? I mean where should the point really be - should I choose the easting/northing or the lat/lon? would be extremely grateful for any insight into the matter.
    – user19622
    Commented Jun 4, 2017 at 6:34

2 Answers 2

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Lat/lon coordinates CAN be OSGB36 OR WGS84. The first is EPSG:4277, and has the same datum shift against WGS84 as the British National Grid EPSG:27700 which uses meters as units.

The difference between the meridians is about 122 m at Greenwich. Aparently, neither of them hits the Meridian Building. OSGB36 is nine meters west, probably due to tectonic moves between the errection of the building and settlement of the projection in 1936. The WGS84 meridian is 103 meters east of it. For other parts of Great Britain, see https://en.wikipedia.org/wiki/Ordnance_Survey_National_Grid#Datum_shift_between_OSGB_36_and_WGS_84

So you might be correct if you treat your lat/lon coordinates as EPSG:4277.

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When you set the DataFrame coordinate system (i.e. what your map is going to be in) and you are adding datasets with different coordinate systems then you MUST set the transformation. For transforming WGS84 into BNG (and vice versa) you would typically select the transformation ending with the word petroleum and this will apply the correct shift for your lat/long data. There are now better transformations but this should be sufficient.

To apply the transformation click on the Transformation button!

Transformation

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  • thanks for that! just a quick question though: right now in the transformations box it says in the 'convert from' section: 'gsc_osgb_1936' and in the 'into' section: 'gsc_osgb_1936'. if I want to keep the coordinates in the osgb36 format, shall I just leave that as it is? or do I need to choose the GSC_WGS_1984 in the 'into' section and for the 'Using' box select the 'osgb_1936_to_wgs_1984_petroleum'? thanks
    – user19622
    Commented Jun 5, 2017 at 22:32
  • If any of your datasets are WGS84 and you want your map to be displaying in BNG then you need to set the transformation. If you had other datasets in other coordinate systems you would see multiple transformation options being offered up and you set the appropriate one for each coordinate system. Remember all this is about aligning the datasets to a common coordinate system so they can all be overlaid correctly on the same map. It does not actually alter the underlying dataset, just something to be aware of.
    – Hornbydd
    Commented Jun 5, 2017 at 23:17

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