Planning.
In making a plan of any large area, such as a town, it is best to start with a rough key-plan divided into a few dozen squares, each row of squares lettered, each column of squares numbered, so that every square is designated, as B 5, etc. ([Fig. 32]). Then the detailed plan of each square is to be made on one opening of a note-book of squared paper, the openings running A 1, A 2, A 3; B 1, B 2, B 3, etc. Thus any connection from one page to another can be found at once by looking for the next letter or number: the whole plan is in the pocket, and can be added to, chamber by chamber, as the clearing progresses. It need hardly be said that every plan or detail should be drawn north upwards in the note-book. Main lines are of course to be connected together by long lines of measurement.
As a general principle it is best to measure positions of as many points as possible along one single line of measurement, rather than take many piecemeal short distances and add them together. Thus ([Fig. 29]) a series of walls should be stated as, 66, 76, 201, 220, 257, 269, 330, 353, 434, 446 inches, rather than as lengths of 66, 10, 25, 19, 37, 12, 61, 23, 81, and 12 inches; for the total is more accurate when measured all in one, the positions are plotted quicker, and the comparison with any symmetric lengths of the building are easier made on the spot, so as to detect errors.
Fig. 29.—Example of a plan measured entirely from two bounding lines.
In the direct measurement of groups of walls, etc., it is the quicker and more accurate method to adopt two outside sighting lines, say one along the north, the other along the east, of the ground, marked out by high walls or large stones always visible, and then measure every point out to the two sighting lines at right angles. Spaces of over 100 feet across can be divided into separate groups.
The general use of instruments cannot be entered upon here. But amongst the means of work the divided rod is indispensable, and it is all that is wanted for most small buildings that are met with. The tape is the most practical for distances of 10 to 50 feet; and the steel tape for accurate measuring of base lines, or long distances. The box-sextant is for very broken ground, and isolated details, or if working alone; and the theodolite for accurate work anywhere between the accuracy of, say, 1 inch on 500 feet and the refinement of a ¼ of an inch on a mile. The plane table may be convenient for approximate plans, and is simple and rapid to use. The prismatic compass is of use for the directions of single blocks or fragments of wall, and is handy for rough topography (generally with paced distances), or for underground passages.
In considering the accuracy required, if dimensions in figures are to be given, then minute measurement is wanted, somewhat more accurate than the original workmanship. But where only a plan is to be produced, it is seldom practicable to show more accuracy than 1/100th inch on a book page 10 inches high, or 1/1000th of the whole, and therefore it is of no use to measure closer than 1 inch on a space of 200 feet or so across.
Plotting.
It need hardly be said that the barbarous irregular fractions, such as ⅜ of an inch to a foot or to a mile, should never be used for plotting. Simple decimal scales should alone be used, and generally 1/100th is the most suitable and easy for all plans of ordinary buildings, towns, etc.; this is further reduced by photolithography to whatever scale will best fit the size of publication.