The Gazette 1979

GAZETTE

JANUARY/FEBRUARY 1979

(c) No reference codes included, the despatcher still having to check carefully to see if one is included.

such as the North East (Louth/Westmeath) and the South East. Preparatory mapping is now taking place in these areas. What is accuracy? "How accurate is the map?" This question, so often asked bur rarely if ever answered, is a fundamental issue and of universal concern to most map users. The reason why the question is so difficult to answer is that it cannot be defined rigidly. Accuracy is relative rather than absolute as far a cartographers are concerned, maps being generalisations and copies of the detail shown on the ground. Accuracy will vary with scale but not necessarily in proportion to the scale and even over a single map sheet accuracy may vary for different types of detail. A map quite suitable for use by a solicitor, as a conveyancing document showing a parcel of land, may not have been accurately described in land use terms and as such may be of little value to a botanist or an agricul- turalist. The apparently simple question of how accurate is a map can then conjure up many, possibly unforseen, difficulties in providing a simple answer. Recognising that accuracy will vary, depending on what is being defined, it would be reasonable to expect a root mean accuracy of 0.5 metre on 1:1000 mapping and 2 metres on 1:2500 for the positional accuracy of firm detail. Supply of O.S. Maps The Ordnance Survey has approximately 700 1:1000, 18,000 25 ins and 1,600 6 inch maps to keep in stock. The demand varies greatly. A particular map sheet might only be issued at the rate of one or two per year. Then suddenly due to development, exploration etc., the demand could rise to a hundred or more. The fluctuation in demand for individual large scale maps makes stock control particularly difficult. However at any one time there are usually less than 50 map sheets out of print. If, due to unexpected demand, a particular map sheet goes out of print delays can and do occur in replacing it. Unfortunately most of the master documents from which the maps and printing plates are reproduced are still paper. To make a satisfactory printing from these documents is a time consuming task and can take up to four weeks and longer on occasions. A delay of six to eight weeks may occur if a map, when ordered, is out of print. This is very much the exception, maps being normally available ex-stock and sent by return post. The Ordnance Survey also endeavours to expedite the delivery of a map if it is urgently required, and to this end it will supply a sub-standard product (in terms of printing quality) if this will meet the immediate requirement of the customer. Obviously the Ordnance Survey does not like to supply such product but if it facilitates the customer it will do so. Representations have been made to include the customer's reference code on the dispatch voucher when fulfilling an order, and every effort is made to do so. Because of the time involved, however, it is not possible in every case, and the Ordnance Survey is not always at fault. Some difficulties the Office finds in attempting to meet the request are:- (a) Illegible reference codes. (b) Reference codes located in varying positions on order forms thus making it difficult to find them.

Map Reference Systems Ordnance Survey large scale maps are published in two series, the National Grid and the County Series. The County maps will gradually be replaced by National Grid sheets but the changeover will take many years to complete. In the meantime a dual system will operate. The sheet lines of the two series do not coincide and there is no direct relationship between both systems. Numbering for National Grid large scale maps is based on the 1:5000 series. They are numbered from 1 starting in the North-West corner, reading from left to right, and ending in the South-East of the country. The National Grid 1:2500 maps are distinguished by letters A, B, C and D. The sheet reference for that scale would first give the relevant 1:5000 sheet numbers, followed by the appropriate 1:2500 letters A, B, C andD. 4866-A would, for example, uniquely define a 1:2500 map. There are twenty five 1:1000 map sheets in a 1:5000 plan and these are numbered from 1 to 25. A unique reference for a 1:1000 map sheet would, for example, be 6384-6. These map sheets all have a 600 x 800mm format. The reference system for the County Series is based on the county. In this series each county was surveyed as an entity having its own 6 inch sheet numbering system. Depiction of topographical detail ceases at the county boundary. Map sheets are numbered serially for reading left to right from the North to the South of the county. There are sixteen 1:2500 maps in a 6 inch sheet which are numbered 1 to 16. There are four 1:1250 sheets in a 1:2500 urban map and they are identified by the letters A to D. Always quote the County name when giving reference numbers for county sheet. A reference for a 1:1250 map should be as follows:- Wexford 3 7 - 1 2 - B. 37 identifies the 6 inch sheet. 12 identifies the relevant 1:2500 map within the six inch map and B identifies the 1:1250 map within the 1:12500 map. If the customers expect reasonable service it is essential that they identify the map clearly and un- ambiguously, the most common fault being omission of the county name. The fact that the customer may have a Carlow address does not necessarily mean a Carlow map is wanted. Another major cause of delay is when the customer sends in a copy of a small portion of the map with no identifying detail and asks for the corresponding map sheet. This creates delay when it is appreciated that it might be any one of 17,000 sheets. Metrication and Revision All large scale national grid maps published since January 1st, 1969, are in metric form. Bench marks, spot heights and contours are given in metres, areas in hectacres and boundary mereings (the exact positioning of a boundary in relation to the adjacent physical feature) are also given in metres. It will be many years before the process is completed for all maps. The basic map scale for Ireland was already in decimal form (e.g. 1:2500). However the six inch (1:10560) is being replaced by the 1:5000 sclae with metric contours. One of the most important changes introduced in recent years has been the concept of continuous revision.

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