In all cases where alterations to existing buildings are required it is necessary to commence operations by making accurate plans of them, unless such exist and are accessible. To the uninitiated this looks to be an easy thing to do, and so it is if the building is set out on regular lines; but this can never be assumed to be the case. The only safe assumption, in fact, is that there are imperceptible irregularities which can only be discovered by measuring and checking everything.

A plan is generally measured room by room in regular sequence, perferably starting with a central hall, if such a thing exists, out of which several rooms open, and, as it were, building up the plan round it. External measurements are generally taken after the interior has been finished - while sections follow plans, and elevations follow sections; but many circumstances may occur to modify this sequence, such as the temporary occupation of a room, or the fact that fine weather, with the possibility of rain to follow, may suggest the advisability of doing the external work first.

As each room is entered a sketch plan is made of it in a large sketch book, using pencil and set-square, and it is advisable for this to be at least approximately correct to scale, else difficulty may be experienced in making the various pieces fit together, before the whole plan is finished. For this purpose squared or "sectional" paper is useful, and a scale of 8 feet to an inch is about as small as can well be used even by a draughtsman who writes a small hand and uses small, neat figures.

When surveying large buildings, a key plan to a very small scale has to be got out, with various portions lettered A, B, C, and so on; and these portions are surveyed as if they were separate buildings, with a few connecting dimensions, each portion being sufficiently small to be sketched, to the scale adopted, on a sheet of the sketch book in use. In such cases the points of connection must be carefully noted and similarly numbered or lettered, preferably in colour, on each sheet of sketches.

To enable each room to be sketched, its leading dimensions are first taken, as shown in Fig. 24, round all four sides of the room, with a tape measure. It will be noticed that in this case neither the sides nor the ends are exactly alike, showing that the angles are not quite right angles, so that diagonals must also be measured (one is 16 ft. 8 in. and the other 16 ft. 6 in.) before the plan can be plotted with accuracy; but even if the opposite sides had appeared to be equal, diagonals should still have been taken, for the whole might have been slewed into a rhomboidal form without its being perceptible to the eye. It is difficult to exaggerate the necessity for taking diagonals whenever the opportunity for doing so presents itself, while important points should be connected by still further lines, unnecessary in themselves, to serve as checks upon the work.

Surveying Existing Buildings 36

Fig. 24.

The tape used should be tested now and then against a steel tape kept for the purpose, it being liable to stretch; but the steel tape itself is not flexible enough for employment in a room, as it cannot be got into a corner. An assistant holds the ring at the end of the tape at one extremity of the side or diagonal to be measured, while the person who is responsible for the survey, and who makes the entries in the sketch book, reads off the dimensions at the other end; and, if the assistant be new to the work, it must be impressed upon him that measurements start from the outer end of the ring, and not from the junction of ring and tape. If two assistants take the measurements while the surveyor enters them on his sketch the work is greatly facilitated; but the surveyor must then always repeat aloud every dimension as it is dictated to him, as a precaution against mis-entry.

After the main dimensions and diagonals have been taken, and the main plan sketched in, the details are measured, starting at one corner and working round. Thus the upper end of the room in Fig. 24, when fully detailed, appears as in Fig. 25. These measurements are taken either with a 5-foot rod or a 2-foot rule, as may be most convenient, and very rarely with the less accurate tape.

When a string of detailed dimensions has been taken, they should be added up to ascertain if the total agrees with the general over-all dimension. In this instance 1 ft. 2 in. + 2 ft. 4 1/2 in. + 2 ft. 6 in. + 2 ft. 4 1/2 in. + 1 ft. 3 in. =9 ft. 8 in., and consequently all measurements are correct, it being most unlikely that a mistake of similar amount should have been made both in the detailed and the general dimensions.

It sometimes happens that, owing to temporary obstructions, direct diagonals cannot be obtained, while in other cases they are not procurable, the room itself being of irregular shape. In such instances the best has to be done that circumstances will allow, several cross measurements, for example, being taken from the centre of a newel post in a staircase hall, or from the projecting angle of a chimney breast, - or even from a chalk line drawn on a wall. Technically all these cross measurements are known as "diagonals," and it is an accepted axiom that it is impossible to take too many of them if accuracy is to be ensured.

Surveying Existing Buildings 37

Fig. 25.

The thicknesses of walls can generally be obtained at door or window openings. These are measured gross, including plaster, contrary to the general practice in preparing working drawings for new work, for the inside measurements of the rooms have, as may be noticed, already been taken nett along the faces of the walls as they exist. The unwary may, however, be warned against taking measurements so low down as to involve the skirting boards also.