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OS Terrain 50 ascii header info

A_man
    • 11 August 2017 10:22AM
    • 204 Views
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    • Registered: 11 August 2017 10:00AM
    • Last active: 14 August 2017 05:57PM

    Hi, I downloaded the terrain 50 ascii grid data for use in a mapping project. I just wondered if anyone can tell me what coordinate system the header on each of these files refers to. For example:

    The first 5 lines of the NY00.asc file read:

    ncols 200
    nrows 200
    xllcorner 300000
    yllcorner 500000
    cellsize 50

    The ncols and nrows describe the number of measurements in the grid and the cell size (50m?) gives the interval between points. But what do the xll and yll numbers (300000/500000) mean?

    Thanks

    2 responses

    neilmcfadyen
      • 14 August 2017 03:45PM
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      • Registered: 29 August 2013 11:54AM
      • Last active: 20 October 2017 10:53AM

      The Coordinate system is defined in the associated .prj file (e.g NY00.prj, British_National_Grid). The xllcorner/yllcorner are the Grid coordinates for the LowerLeft Corner of the tile.

      A_man
        • 14 August 2017 06:18PM
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        • Registered: 11 August 2017 10:00AM
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        Thanks Neil the .prj file is useful and I will use it in my script.

        I can see now that I was just confused about the interchangeable use of grid letters + numbers and all-numeric references (I thought that it was placing the NY grid square in the context of some other coordinate system but actually it is just using the same system in a different format (using the furthest SW grid square as the origin and giving the xll/yll starting distances from there).

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