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:

Map of OS squares

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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Acknowledgement: Screen captures of the ArcGIS GUI, pull down menus, buttons and icons  are reproduced with permission as per the UK CHEST agreement and both these and trademarks (ArcGIS, ArcMap, ArcCatalog, ArcToolbox, ArcScene, ESRI) are the property of and Copyright © 2010 ESRI. This website: Copyright © 2010 Dr Christopher Green. The views expressed in this document are those of the document holder.