From 1839 to 1855, lines of initial levelling extending all over England, Scotland and Ireland were run, and the observed altitudes of the bench marks were reduced by the method of least squares.
In England and Scotland, these levels are based on the Ordnance Datum at Liverpool, which is approximately the mean tide level of that place; in Ireland, they are based on the low water level at Dublin, which is about 8 feet below the mean level round the coast of Ireland.
The detail levelling is carried out contemporaneously with the progress of the cadastral survey. Starting from the marks on the initial series, lines are run along nearly all the turnpikes and parish roads, and bench marks cut at intervals of about a quarter of a mile.
The whole of the bench marks of the initial levelling are shown in position on the 25-inch manuscript plans, and their heights given to the nearest tenth of a foot. Surface heights, to the nearest foot are also marked on the plans, at frequent intervals between the bench marks.
CONTOURING.
Contrary to the custom in other countries, the contours of the English survey have all been surveyed and levelled on the ground, checked by the numerous bench marks, the standard of accuracy demanded in levelling being two-tenths of a foot.
Owing to the expense of the process, about $1.25 per lineal mile, only the 100 foot contours have been surveyed, except where greater detail is required for military purposes, which information is not furnished to the public.
HILL SHADING.
The hill features for the one inch maps are first sketched in the field by the military method of slopes and sketch contours or proof impressions of the contoured sheet.
Finished drawings from the field sketches are then made on cardboard impressions from the one inch outline plates, and finished as guides for the engraver to work by.