The map, at first intended to be confined to the Highlands only, was at last extended to the Lowlands and thus made general in what related to the mainland of Scotland, the islands (except some lesser ones near the coast), not having been surveyed.
It is spoken of by Lieutenant-Colonel White, in his excellent book on the Ordnance Survey, as a "piece of work which appears to have been excellently carried out as far as it went, qualified by the remark of Roy that owing to the comparative inferiority of the instruments used and the inadequacy of the annual grants provided for the survey it is rather to be considered as a magnificent military sketch than a very accurate map of the country."
The survey of Scotland was interrupted by the breaking out in 1755 of another of England's intermittent wars with France, that which gained her Canada, and the work was never completed.
"On the conclusion of the peace of 1763," writes General Roy, "it came for the first time under the consideration of government to make a general survey of the whole island at the public cost." But, for reasons not assigned, the twelve years' interval of peace before the outbreak in 1775 of the American War of Independence was allowed to pass away without anything being done. There the matter remained in abeyance until, after renewed hostilities with France and Spain, peace was negotiated in 1783.
The trigonometrical survey of Great Britain may be said to have been begun one hundred and six years ago.
Astronomers of that day were desirous that the difference of longitude between the Greenwich and Paris observatories should be ascertained by trigonometrical measurement; and under the auspices of the king and of the Royal Society, General Roy, R. E., in April, 1784, began the task by the measurement of a base line on Hounslow Heath which was to serve as the starting point of a series of triangles to be extended to Dover and across the channel.
This work was carried out, a connection with the French triangulation being established in 1786.
Soon after this the government decided on having a general survey made of the entire kingdom, on the scale of one inch to one mile for military purposes, and General Roy's triangulation in the southeastern counties became the basis of the Great Triangulation, which was gradually extended over the whole of the British Isles and finished in 1853.
The one-inch survey was carried northward through England and Wales under the successive superintendence of artillery and engineer officers, and by 1824 had reached the southern borders of Yorkshire and Lincolnshire.
At this time it became necessary that a survey of Ireland should be made on a large scale as a basis for general land valuation. On the recommendation of Colonel Colby, then director, the scale of six inches to one mile was agreed upon; the work in England was suspended and the force transferred to Ireland.