1st. The selection and measurement of a base.—The base is measured with an ordinary chain and a five-inch theodolite, and this measurement having been reduced to its horizontal value at the level of the sea, the section of the base is laid down on paper.
2d. Triangulation.—The measured base is extended by a triangulation over 10 or 16 square miles of country, and the relative altitudes of, and the distances between, the stations selected are determined from observations. The computed horizontal distances are laid down, and the azimuth of one of them is determined.
3d. Traversing.—The positions of the roads, streams, boundaries of woods, and other marked features, surrounding and intersecting an area of six or eight square miles of the country triangulated, are then determined by running traverses with a theodolite from one station to another, so as to cut up this area into spaces which will admit of being filled in by a less accurate method, without introducing an error in the plan.
4th. Plotting of detail and completion of the work.—The protracted lines are now transferred to another sheet of paper, and the detail, obtained as the traverses proceed, is plotted from the field-book. From this plot sketch sheets are prepared, and the remainder of the work is sketched in with the aid of a prismatic compass, the form of the ground being represented by pencil strokes, assisted by contours put in with the aid of a portable level.
The sketch sheets are etched in with a pen, and a finished brush-work plan of the complete survey, embracing all the information collected, is prepared from them, with the original plotted detail, as a basis.
Special Survey.
A piece of ground, about half a mile in area, is surveyed with minute accuracy as for some special purpose, and is laid down on a scale sufficiently large to admit of the calculation of the areas of the enclosures from the paper. The method followed is the same as that pursued on the Ordnance Survey, and with the Tithe Commutation Surveys, etc.
Contouring.—On the ground thus specially surveyed contours are traced instrumentally at given vertical distances apart and are plotted on the plan.
Military Reconnaissance.
This is conducted on principles similar to those which govern the operations of the general survey; the instruments employed, however, are all portable. The measurement of a base is made by such means as readily offer themselves (generally by pacing), and the trigonometrical points are fixed simply by protracting angles observed with a box sextant or compass. The whole of the remaining features and details considered necessary in a military point of view are sketched in with the aid of bearings and pacing. The reconnaissance embraces about six square miles.