Ordnance Survey Maps - Six-inch 1st edition, Scotland, 1843-1882

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Triangulation, projection, meridians and sheet lines

Triangulation had been employed by European map-makers intermittently from the sixteenth century as a means of working out a network of distances from measuring a single base line and the related angles of set of triangles. Ordnance Survey's achievements in triangulation were to use the latest instruments and greater manpower to measure more accurate angles and base line distances than before in order between 1791 and 1822, largely under William Mudge, to position a network of triangulation stations over the whole of Britain other than North-West Scotland. This is often termed the Primary Triangulation.

In the following decades, work on geodesy progressed on several fronts, and much of the earlier work was checked and revised in an ongoing quest for greater accuracy. The British triangulation was extended under Thomas Colby to include Ireland and NW Scotland. The Greenwich-Paris connection by trigonometry was re-measured in 1821-1823, new chronometers for better determination of longitude were used under the direction of the Astronomer Royal George Airy in the 1840s, latitudes were more accurately determined with improved zenith sectors, and the shape of the earth or spheroid was extensively researched. In the 1840s the triangulation was improved and strengthened under William Yolland. His Diagram of the Principal Triangulation published about 1852 marks a significant transition. (For further details on this subject and related Scottish triangulation work, see David L. Walker's articles in Sheetlines).

In 1854 the formidable task of adjusting the triangulation to form a consistent national network was passed on to Alexander Ross Clarke (1828-1914). Unencumbered by other duties, Clarke compiled the exhaustive report attributed to Henry James in 1858. To reduce the number of calculations, he excluded about a quarter of the stations. Hence the diagram of the principal triangulation originally published in 1858 (and summarised for the Royal Society in 1856 and in Plate 11 of Seymour) differs significantly.

In the early 1860s, the triangulations of England, France and Belgium were connected, and further international collaboration with Russia and Prussia allowed the 52 degree North parallel, from the West of Ireland to the Urals, to be accurately measured. Clarke headed the Trigonometrical and Levelling Branch until the early 1880s, and the shape of his central network was retained for the Retriangulation in 1936.

Scottish agitation for Ordnance Survey work

Map showing county meridians in Scotland
County meridians in Scotland.
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Within this emerging national framework, practical surveying always focused on the more local detail of estates, parishes, or counties, and Ordnance Survey followed in these traditions at the larger scales. Early work in Ireland had progressed on a county-by-county basis, without a complete national triangulation. This tradition was followed from the 1840s when 'county series' mapping began in England and Scotland [see note 2]. Apart from a minority of 'unified' counties (Argyll and Bute, Fife and Kinross, Perth and Clackmannan, and Ross and Cromarty) all other Scottish counties were initially mapped on their own individual meridian and projection, with sheets numbered separately within each county. It was not until the 1880s that 'combined' sheets were produced for areas straddling county boundaries showing the mapping on both sides, and the opportunity taken of new revisions of counties from the 1890s onwards to transfer or unify some counties onto the same origin as other counties.

Cassini Projection

Ordnance Survey County Series maps were based on the Cassini Projection, a transverse cylindrical projection, where a central meridian of longitude forms the point of origin and the line of zero distortion (where all distances are true to scale). Distances along the great circles of the earth that meet this central meridian at right angles are also plotted true to scale, but all (North-South) distances parallel to the central meridian are increasingly too great on the map, the farther they are away from the central meridian. It follows that the Cassini Projection is best suited for areas with a relatively narrow extent in longitude, perhaps no more than three or four counties width in the case of the large-scale maps. Most Scottish counties had their own separate county point of origin (see map – Plate 8 in Winterbotham), usually the nearest triangulation station to the North-South centre line of the county, with sheets laid out from North-West to South East in separate county sequences. It also follows that due to the separate origins and the nature of the Cassini projection, areas at the edges of counties do not fit precisely with their neighbouring county sheets, especially at their East-West extents. For the first edition using 'full' double-elephant sized sheets, each six-inch map covered an area of four by six miles in extent, with a map extent of 24 inches in height by 36 inches in width.


Battle of the Scales

During the mid-19th century there was an extensive and at times heated debate over the most appropriate scale to choose for larger scale Ordnance Survey mapping. On the one hand, pressures to economise and to speed up the progress of mapping, as well as the vested interests of private surveyors fearing competition with their large-scale work meant that preference was given to the smaller one-inch to the mile scale. On the other hand, the growing need for detailed maps for land valuation, registration and conveyancing, agricultural improvement, mineral development, railways, together with the facts of urban expansion, lent advantage to the more detailed mapping at 24" to 26" to the mile scales. Although the initial mapping of England and Wales had been undertaken at the one-inch scale, the successful mapping of Ireland at the six-inch scale had lead to the decision to follow this in the 1840s in Lancashire, Yorkshire, and Scotland. Conflicting pressure groups and successive committees successively disputed the subject until the early 1860s.

Change to 25 inch scale from 1854

As a result, six Scottish counties (Edinburghshire, Fife, Haddingtonshire, Kinross-shire Kirkcudbrightshire, and Wigtownshire) and Lewis were surveyed at the six-inch scale prior to 1851, when a House of Commons Select Committee chaired by Francis Charteris (later Lord Elcho) recommended abandonment of this scale, and a reversion to one-inch publication. Amongst other arguments, it was stated as 'absurd' to map at the six-inch scale 'the barren wastes and rugged mountains of Scotland' that were said to comprise two-thirds of the country! Despite protestations from many, including the then Director General of the Ordnance Survey, Lewis A. Hall, it was not until 1854 that approval was given for surveying all cultivated rural areas at the 25.344" to the mile scale (1:2,500), and the publishing of six-inch and one-inch maps by reduction from this larger scale. Although survey work of Dumfries-shire, Ayrshire and Linlithgowshire that had been suspended in 1851-3 now recommenced and at the new 25" to the mile scale, survey work at the one-inch scale on Berwickshire, Haddingtonshire and Peebles-shire was all discarded. The new scales policy was again reconsidered in 1857, when a House of Commons vote ruled against the 25" to the mile scale, and it was not until a Royal Commission Report of the following year, that the decision was repealed.

In effect this meant that all Scottish counties surveyed after 1855 had their cultivated rural areas mapped at 25" to the mile scale, (which was reduced by pantographs for engraving the six-inch to the mile sheets) and that the six-inch should be the minimum scale covering all areas. For this reason, from 1856 all six-inch sheets carry the same date of survey as their larger-scale 25" to the mile sheets. The value of the larger scale for cultivated areas was indirectly proved by the infamous 'replotted counties' episode in the 1890s, where the six counties and Lewis originally surveyed at the six-inch scale only were replotted at the larger 25" with minimal resurvey, described in the official history as 'one of Ordnance Survey's worst errors', and not fully corrected until after the Second World War.


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Note 2:
This tradition was also followed at the larger 1:2,500 scale, mapping proceeded on a parish-by-parish basis until the 1870s, the so-called 'parish edition' maps, with map detail stopping at parish boundaries.
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