[Updated] I needed to programatically convert a series of Ordnance Survey easting and northings (e.g. 325940, 673060) to “six-figure” grid references (e.g. NT259731) for a project I’m currently working on. It’s a pretty straightforward conversion – no reprojection, just a different way of expressing the same position on the British National Grid.
The specific use is that the 1km x 1km tiles from NPEMap are coding according to eastings & northings, but the calibration files required by TimSC‘s warp-gbos require grid references for the corners and calibration points.
Such a procedure is already well catered for in Perl, PHP and Javascript. Here is a straightforward conversion of some Javascript I found here into Python:
# Derived from # http://www.movable-type.co.uk/scripts/latlong-gridref.html def getOSGridReference(e, n): import math # Note no I gridChars = "ABCDEFGHJKLMNOPQRSTUVWXYZ" # get the 100km-grid indices e100k = math.floor(e/100000) n100k = math.floor(n/100000) if e100k6 or n100k12: return '' # translate those into numeric equivalents # of the grid letters l1 = (19-n100k)-(19-n100k)%5+math.floor((e100k+10)/5) l2 = (19-n100k)*5%25 + e100k%5 letPair = gridChars[int(l1)] + gridChars[int(l2)] # strip 100km-grid indices from easting & northing, # round to 100m e100m = math.trunc(round(float(e)/100)) egr = str(e100m).rjust(4, "0")[1:] if n >= 1000000: n = n - 1000000 # Fix Shetland northings n100m = math.trunc(round(float(n)/100)) ngr = str(n100m).rjust(4, "0")[1:] return letPair + egr + ngr # test print getOSGridReference(96000,906000) # NA960060 print getOSGridReference(465149, 1214051) # HP651141
[Update - fixed a bug that gave the wrong grid references for the Shetland Islands - their northings have seven figures, and similarly places in the far west or south that only have five figure easting or northings respectively 
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