When there is any great extent of measuring by chain to be done, a Field Book will be found useful to record the distances, and any important features to the right or to the left of the chain line. A useful size is one about 7 in. by 4 1/2 in., containing about twenty-four leaves. A column about 3/5 in. wide is ruled up the centre, and is intended to correspond with the straight line that is being chained. The entries in the field book commence at the bottom of the page, and from the end of the book. Certain leading points along the line to be chained are called "stations", and are marked in the field book with a circle, thus O. The stations may be called A, B, C, D, etc. The distances chained are entered in the central column. At certain distances, however, it may be necessary to note some feature to the right or left of the chain line. Thus, in the specimen page of the field book given here (fig. 401), it will be noticed that at 20 links there is an iron fence 26 links to the right of the chain line, and this iron fence gets as near as 12 links when the straight line has been chained 200 links. In the same way, on the left, the " farm road " is indicated as 8 links to the left, while the corners of a building are shown to be at various distances from the chain line. Although there is a column in the field book it must be regarded as a single line, and not as two distinct ones. Thus, when the farm road crosses from left to right at 228 and 246 links, the lines indicating it appear opposite the point at which they intersected the chain line. In surveying a piece of land, chain lines should be marked out so as to take in the most important features or to form the longest and best base lines. Afterwards the chain lines with their stations and distinctive features can be plotted to scale on paper, and will give a correct idea as to the shape of the land surveyed. The area is then computed by measuring up the triangles or rectangles made by the various chain lines.

Fig. 401. - Specimen of Field Book.

Offsets

The distances to the right or left of a chain line are marked off by means of "offset" staffs, each 10 links (792 in.) long. Each link is painted alternately red and white, or black, and numbered; and the bottom end is pointed to stick in the ground. Offset distances are always taken at right angles to the chain line; but those for buildings are taken obliquely for the corners, as shown in the sketch.