The House of Commons passed an act defining its principal object, prescribing a legal mode for ascertaining the boundaries which were to be surveyed, granting the surveyors power to enter lands for the purposes of the survey, and preventing the removal of the objects used.
The earlier methods of military surveying were abandoned, and new instruments and a system were devised for its execution.
It is important to note that the organization of the Irish survey marked an important epoch in the history of the Ordnance Survey, viz: its change from a topographic to a cadastral survey.
In Ireland, subordinate to the parishes, there is an internal division of smaller denomination called townlands, which are very frequently, but not uniformly, conterminous with property.
The townland was the lowest unit of taxation for country purposes, of an average size of 200 or 300 acres, and originally the map was to be simply a topographic map, containing the boundaries of the townlands, the roads, the streams and the houses, with a view to the valuation of Ireland for the county assessment. The six inch was considered to be the smallest scale that could be available for that purpose.
There was no intention in the original Irish survey to insert the fields, but when the valuation began, it was found by the valuators that additional minuteness was necessary to enable them to subdivide the townlands into the qualities of lands of which they consisted, and more especially that the boundary between the cultivated and uncultivated portions ought to be inserted on the maps with great accuracy.
This rendered necessary a very extensive revision which was undertaken in 1830, and it became a survey by fields instead of townlands.
This was clearly a wide and most important departure from the original intention of the six inch survey in Ireland, and it is not to be doubted that General Colby, who would not trust to paper measurements for the areas of entire townlands, would have adopted at the very outset, for his manuscript plans of these minute subdivisions, a scale much larger than that of six inches to one mile.
The engraving of the six inch survey appears to have resulted from a demand for six copies of one sheet for valuation purposes when it was found that it would be as cheap to engrave it as to make that number of copies.
So valuable did the six inch map of Ireland prove for many purposes over and above that for which it had been originally designed, that, in 1840, when the Irish survey was completed, and that of England resumed, the Government gave their consent to the adoption of the same scale for the unsurveyed parts of Great Britain.