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GAZETTE
JANUARY/FEBRUARY 1979
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.
(c) No reference codes included, the despatcher still
having to check carefully to see if one is included.
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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