Introduction to ArcGIS for Archaeologists

Converting OS National Grid coordinates for GIS usage:

OS grid references usually take the following form: SK123456 or HZ12345678, etc., depending on their precision.  Here, the letters give the location of the large 100km grid square and the numbers give the easting and northing.  To enter these grid references into a GIS, we need to split out the easting and northing into separate attributes and convert the letter portion into numbers, using the system shown in the diagram and conversion table on the next page.  We also usually need to add some zeroes to the end of each number to make sure that the grid references are to the nearest metre.

Thus, the grid reference SK123456 would have an easting of 412300 and a northing of 345600; the 4... and 3... are read off the  diagram and the rest of the grid reference split in the middle.  We have to add two zeroes to each number as this was a six figure grid reference which is precise to the nearest 100 metres (essentially it refers to a 100m x 100m square on the ground).  As another example, HZ12345678 would have an easting of 412340 and a northing of 1056780, with the 4... and the 10... read off the diagram; here we add one zero to each number as an eight figure grid reference is precise to the nearest 10 metres.  We would not have to add any zeroes to a ten digit grid reference, as it is already precise to one metre.  We would need to add more zeroes to two and four figure grid references, but these are very imprecise and so would be better represented as grid squares in any event.

We usually do this conversion using spreadsheet software (such as Excel) prior to entering the data into the GIS. First we would create three fields that split the grid reference into the letter, easting and northing portions (either manually or by creating a macro). Then we convert those eastings and northings into full coordinates by multiplying by 10 (for an eight figure grid reference) or 100 (for a six figure) and then adding on the conversion figure, e.g. 400000 to the easting and 300000 to the northing for the SK square.  The formula would looks something like this (where A1 is the cell holding the unconverted easting value):
=(A1*10)+400000

 Grid square: Easting value: Northing value: Easting metres: Northing metres: HP 4 12 400000 1200000 HT 3 11 300000 1100000 HU 4 11 400000 1100000 HW 1 10 100000 1000000 HX 2 10 200000 1000000 HY 3 10 300000 1000000 HZ 4 10 400000 1000000 NA 0 9 0 900000 NB 1 9 100000 900000 NC 2 9 200000 900000 ND 3 9 300000 900000 NF 0 8 0 800000 NG 1 8 100000 800000 NH 2 8 200000 800000 NJ 3 8 300000 800000 NK 4 8 400000 800000 NL 0 7 0 700000 NM 1 7 100000 700000 NN 2 7 200000 700000 NO 3 7 300000 700000 NR 1 6 100000 600000 NS 2 6 200000 600000 NT 3 6 300000 600000 NU 4 6 400000 600000 NW 1 5 100000 500000 NX 2 5 200000 500000 NY 3 5 300000 500000 NZ 4 5 400000 500000 OV 5 5 500000 500000 SC 2 4 200000 400000 SD 3 4 300000 400000 SE 4 4 400000 400000 SH 2 3 200000 300000 SJ 3 3 300000 300000 SK 4 3 400000 300000 SM 1 2 100000 200000 SN 2 2 200000 200000 SO 3 2 300000 200000 SP 4 2 400000 200000 SR 1 1 100000 100000 SS 2 1 200000 100000 ST 3 1 300000 100000 SU 4 1 400000 100000 SV 0 0 0 0 SW 1 0 100000 0 SX 2 0 200000 0 SY 3 0 300000 0 SZ 4 0 400000 0 TA 5 4 500000 400000 TF 5 3 500000 300000 TG 6 3 600000 300000 TL 5 2 500000 200000 TM 6 2 600000 200000 TQ 5 1 500000 100000 TR 6 1 600000 100000 TV 5 0 500000 0

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