1:100,000 scale maps and booklets.

 THE 1:100,000 BEDROCK MAP SERIES



This map is replicated as a clickable image linking to details of each map in the series in our Publications Section - please click here.


The 1:100,000 Bedrock Map Series comprises 21 map sheets covering the whole of the land area of the Republic of Ireland and adjacent areas across the border into Northern Ireland. The last map in the series was published in 2003 and the last accompanying explanatory booklet was published in 2006.

The 1:100,000 series started in 1984 following a decision made the previous year to use the Ordnance Survey of Ireland (OSI) ½” topographic base on which to depict the geology. GSI had been promised 1:50,000 topographic base maps by OSI in the mid 1970s, but these had failed to materialise, so GSI had no other viable base map option on which to show the results of the detailed 6” field mapping programme that had been started by GSI's then Director, Dr. Cyril Williams, in 1967. By the early 1980s it had become crucial for the reputation of GSI to produce Bedrock maps, whatever the scale, as soon as possible.

Initially, it was decided that the geology would be published at ½” to the mile scale, but subsequently a decision was made to enlarge the OSI ½” base photographically to the nearest metric equivalent of 1:100,000.

Compilation of four maps was underway in the second half of the 1980s, namely Sheets 6 (North Mayo), 16 (Kildare – Wicklow), 19 (Carlow - Wexford), and 23 (South Wexford). It was intended that these would be published as full colour maps, produced by traditional cartographic methods within the GSI Cartographic Unit (along the same lines as maps published in the British Geological Survey) and printed at the Ordnance Survey.

Following an independent 1991 Review of the Geological Survey, speed of compilation and map production was emphasised, for the same reasons as in 1983, and a decision was made to produce the maps as black and white dyelines (similar to the 1:25,000 Bedrock Series which was first published in 1988), to hasten the completion of nationwide coverage. It soon became evident that it would be difficult to read the black and white linework maps with the amount of detail that it was intended to show. The decision to abandon ‘quick & dirty’ black and white maps in favour of digitally produced colour maps (using Autocad & ArcInfo output on a large Calcomp plotter) was made following a cost-benefit analysis and presentation in 1993 to the then Minister, Mr. Bobby Malloy T.D., of the advantages of digitally prepared colour maps. An extra sum of IR£60,000 was allocated at the time to purchase equipment towards a digital map production system (DMPS) in 1993.

The DMPS involved Bedrock geologists digitising the compilation maps in Autocad and transferring Autocad .dxf files to Cartography where they were imported into ArcInfo. Cartography then produced ArcInfo coverages so that the maps could be published by output to a Calcomp plotter in colour, with legends generated semi-automatically from Dataflex databases supplied by Bedrock Section, and imported into ArcInfo.

Part of the DMPS involved the development of a specific customisation of the software by Paradigm Technology Ltd. for GSI. This enabled the geologist to enter data from different sources onto different “layers” in the customisation. Each linetype and symbol type to be used in the series was defined and customisation supplied so that the linetype or symbol could be replicated automatically from a tablet menu and added to the correct layer.

At about the same time it was decided that each map should have an explanatory booklet published simultaneously. This was to be aimed at a wide market, and so the geology was to be described not only for geologists but also for other specialists, or the interested public, who needed or wanted geological maps. The introduction of the DMPS was not without teething problems that led to initial delays. Staff shortages also and the introduction of booklets further extended the eventual completion of the series. Latterly, maps were published ahead of the accompanying booklets.

The first map in the series to be published was Sheet 6, North Mayo, in 1993. This was finished in Cartography before the DMPS was introduced and so was produced by traditional cartographic methods, scribing the linework, and producing colour separations photographically for printing by conventional methods.

All subsequent maps were produced using the Digital Map Production System (DMPS) output on the Calcomp plotter, and latterly on its replacement. The last map in the series to be published was Sheet 15 (Galway - Offaly), which appeared in 2003.

In 2003 it was decided to join all the maps together into a seamless GIS in ArcView. This was done as a multi section project involving Bedrock, Cartography, Information Technology and Groundwater. As the compilation of the series had progressed over more than a decade, some of the sheet boundaries inevitably did not match due to more recent interpretations as well as minor problems with the raster bases not quite matching. The geological interpretation was “fudged” in order to ensure that boundaries and polygons crossed sheet boundaries where appropriate and joined up. No new geological work was done, however, to resolve any mismatches.

The principal sources of information for the new map series came from:

GSI Field Surveys:- The map series is based on GSI field surveys conducted mainly at the six inches to 1mile scale using OSi topographic base maps. The original surveys were started in 1845, when GSI was set up, and completed by 1890, when the last of the one-inch to 1 mile compilation map series was published. Some map revisions were made in the first half of the 20th century (principally in the coalfields) but no systematic mapping was undertaken again until the revitalisation of GSI after the appointment of Dr. Cyril Williams as Director in 1967. Since 1967 detailed re-mapping at six inches to the mile scale continued into the 1980s, by which time more than 1/3rd of the country had been remapped. This included contract mapping on behalf of GSI by PhD students in UCC and TCD.

Some reconnaissance mapping was undertaken by GSI from 1974 onwards, primarily in the midland Carboniferous areas, and field checking and drilling (to try and solve known problems) was undertaken to a limited extent during the compilation programme from 1992 until completion in 2003.

Published Maps:- Some of the maps were compiled largely from non GSI maps published at the one inch to one mile scale by W.S. Pitcher of Liverpool University (Donegal, Sheets 1 / 2 and 3 / 4) and B. Leake and J. Tanner of Glasgow University (Connemara Sheets 10 / 11); also P.M. Bruck of GSI and later UCC (North Co. Tipperary – parts of Sheet 15). These university maps were compiled from field work based on larger scale six inch to one mile mapping from MSc/PhD studies and GSI mapping (Bruck).

A compilation map, published in 1988, at one inch to one mile by G. Stanley, GSI, (The Limerick Basin) was also used in Sheet 17.

Compilations made by M. Hitzman of Chevron Mineral Corporation of Ireland and Ivernia West plc from Open File data, and published in 1992 by GSI at the 1:100,000 scale, were also incorporated into sheets covering the midlands Carboniferous (Sheets 12, 13, 15, 16, 18, and 19).

The geology for corners of the maps which cross the border into Northern Ireland has been supplied by the Geological Survey of Northern Ireland from their 1:50,000 Map Series and/or the GSNI 1:250,000 map of Northern Ireland.

Open File Data:- Data in the form of six inch and one inch maps deposited by exploration companies along with borehole logs and locations were also used in the 1:100,000 compilations. These sources proved most useful in the midlands Carboniferous, where GSI and other data were scarce, apart from the original 19th century mapping.

Other mapping:- The 1:100,000 series also used field surveys (mostly six inch to the mile) carried out in MSc/PhD mapping made available to GSI by Universities in Ireland and further afield. Unpublished compilations at 1:50,000 scale offered by I.A.J. MacCarthy (UCC) were also incorporated into parts of Sheet 24.

Other exploration company compilation maps, not available in the Open File, were made available to Bedrock Section. These covered areas on Sheet 15 principally.

The Future:- Now that all the maps in the 1:100,000 series have been combined as a seamless product in ArcGIS, the geological data can be used as a starting point for the new 1:50,000 GIS Map Series. Much remains to be done, however, to rectify the 1:100,000 data to fit the 1:50,000 base. Methodology has been developed to rectify the data but there is unfortunately no quick fix. The new map series will attempt to address those areas where there is little or no detailed modern mapping with new field studies and drilling to improve our knowledge of the geology. The new series will also add other point datasets not published with the 1:100,000 series.