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OSI Half Inch

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Half Inch
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Publisher:  Ordnance Survey Ireland
Scale:  1:126,720
First Published:  1911
Final Publication:  1991
SABRE Maps layer:  Yes
1969 example of Sheet 21
See Also: SABRE Maps/Half Inch coverage project

The OSI Half Inch maps were an extremely long-lived series of mapping covering the whole island of Ireland, first produced prior to independence, with the final maps being produced in the 1990s. Northern Ireland was also covered in this series (though produced by OSNI) until the late 1960s, when the OSNI Half Inch Second Series was produced covering the north.

Unlike in Great Britain, the Half Inch series rather than the One Inch was the primary series covering the island in the twentieth century, and so the sheets are valuable for SABRE Maps use. However, due to the poorer quality paper and the smaller population of Ireland; rather less mapping has survived than that of OSGB.

The island was covered by 25 distinct sheets; but whilst the sheet numbers and sheet areas remained consistent throughout the life of the series, the names of those sheets did not; with three different names for each sheet being common. For example, Sheet 1 was variously known as Londonderry, Letterkenny and North Donegal.

The series used the Bonne (Ireland) projection for all sheets, although the Irish Grid started to appear on sheets from the 1950s, but always skew to the map.

SABRE Maps coverage

As the primary mapping series in Ireland, there are a number of layers available on SABRE Maps, which are shared with the far less common OSGB Half Inch sheets.

Sheetline co-ordinates

As the Half Inch series pre-dates any form of the Irish Grid, each sheet prior to the 1950s does not have co-ordinates printed on them, but simply an alphanumeric grid with letters on the vertical axis and numbers across the top. Later sheets do have the Irish Grid printed on them, but these values are skew to the maps.

As the sheets were built on the Bonne (Ireland) projection, it is possible to calculate the locations of each side of every sheet as a distance from the origin. Knowing the locations of the corners of each sheet, whilst interesting in its own right, is essential to georeference each map and allow them to appear on SABRE Maps.

The below tables have therefore been produced so that the relevant values for the map series can simply be referenced for these purposes rather than having to calculate via the SABRE Maps Co-ordinate Finder.

In these tables, the values of each corner location are in miles from the origin point in Lough Ree, with positive numbers being north and east of that point, and negative numbers being south and west of the origin.

Sheet Top left corner Bottom right corner
01 -23.121:132.765 30.878:96.765
02 30.878:132.765 84.878:96.765
03 -41.121:102.765 12.878:66.765
04 12.878:102.765 66.878:66.765
05 57.878:102.765 111.878:66.765
06 -95.121:60.765 -41.121:24.765
07 -41.121:66.765 12.879:30.765
08 12.879:66.765 66.879:30.765
09 57.878:66.765 111.878:30.765
10 -95.121:30.765 -59.121:-19.260
11 -77.121:30.765 -23.121:-5.235
12 -23.121:30.765 30.879:-5.235
13 30.879:30.765 84.879:-5.235
14 -77.121:-5.235 -23.121:-41.235
15 -23.121:-5.235 30.879:-41.235
16 30.879:-5.235 84.879:-41.235
17 -77.121:-41.235 -23.121:-77.235
18 -23.121:-41.235 30.879:-77.235
19 30.879:-41.235 84.879:-77.235
20 -113.121:-77.235 -59.121:-113.235
21 -77.121:-77.235 -23.121:-113.235
22 -23.121:-77.235 30.879:-113.235
23 30.879:-65.235 84.879:-101.235
24 -95.121:-113.235 -41.121:-149.235
25 -41.121:-101.235 12.879:-137.235

OSI Half Inch
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