Each page of the field-book is then divided, as below, into three columns by two parallel lines drawn down the page: and beginning at the bottom of the column, the several bearings of objects, the lengths of the station-lines, and the several offsets from those lines are inserted, in order, ascending towards the top of the page, the offsets being placed on the right or left hand of the middle column, conformably to their positions with respect to the station-line to which they belong. And it is on this account that the several entries are made in succession from the bottom upwards. The distances in the middle column between the stations a and Q are reckoned from a; those between Q and n are reckoned from Q, and so on, each number in that column expressing the distance up to the place in the station-line where the offset whose length is given immediately on the right or left baud of the number was taken. When it Is required to determine by observed bearings the position of any object, as x, at a distance from the road, those bearings are also inserted in the field-book at the stations, as Q and R, where they were observed, and immediately under the bearing of the next forward station. The mark 0 is usually put to signify the word "station." The term " plotting " is applied to the process of laying down on paper the plan of the ground which has been surveyed. If the survey has been performed by the chain only, the several station-lines consti tute the sides of triangles extending over the whole of the ground ; the lengths of those sides are determined by admcasurement, end in places where they do not coincide with the roads, hedges, &c., offsets arc measured from the sides to the principal bends in those boundaries. In order to " plot " the survey in this case, a proper scale of gradua tions, usually representing " chains" and " links," is chosen, and the length of one of the station-lines taken from the scale being laid down on the paper as a base, from the two extremities of it as centres, with distances (taken from the scale) equal to the measured lengths of the two sides, which with the base form the first triangle, arcs are de scribed to intersect one another ; this intersection being joined to the extremities of the base by lines, the first triangle is constructed. Each side of this triangle is then used as a base on which another triangle is constructed with lines taken from the scale equal to the measured lengths of the aides, and so on. After the triangulation is thus formed, the offsets are laid down from them. This part of the process is accom plished by setting out with compasses upon each station-line, from one of its extremities, the several distances (taken from the scale) of the points at which the offsets were measured, drawing lines perpendicu larly to the station-line at these points, and on them placing, from the scale, the measured lengths of the offsets : lines joining the extremities of these offset lines, either drawn by hand or with a ruler, will repre sent the lines of roads, the boundaries of fields, and the like. In order to facilitate the operation of laying down the offsets from the station lines, the surveyor is usually provided with ivory scales graduated to represent chains and links on the edges; by laying an edge of such scale alt ng the station-line, with the zero of the graduations at one end, the several distances of the offset•lines from that extremity can be marked on the line in succession ; the scale may then be applied to each offset-line, and the measured extent marked by means of the graduations.
tut plotting scales are frequently made with graduations along the edges, and with a abort scale, also graduated on an edge, which is dis posed at right angles to the length of the principal scale, and is capable of being moved to any part of that scale by having one of its extremi ties out so as to slide in a groove formed in the direction of the length of the scale. The perpendicular scale is moved along the principal scale to the graduation which denotes the place of the offset, and the length of the latter is then marked by the graduations nu the perpen dicular scale. Since the offsets frequently occur on both sides of the
station-Iine, the zero of the graduations on the perpendicular scale may be at some distance from the edge of the principal scale, which is then placed, not in coincidence with the station-line, but parallel to it at such a distance that the zero may always be in that line. By this contrivance, which was first proposed by liajor Robe, the offsets from the line may be marked, whether they be above or below it, without displacing the principal scale. To find a convenient scale for plotting a survey, the length and breadth of the whole may be computed approxi matively in order to ascertain the number of chains in such length or breadth, and then the dimensions of the paper in inches being known, the number of chains in each inch may be formed by proportion. Plans of estates are usually made from scales of '2, 3, or 4 chains in an inch, and the linear dimensions, on a plan made from a scale of 3 chains in an inch, are equal to s of the actual dimensions on the ground.
In important surveys, where the process consists in measuring a base-line, and observing with a theodolite the three angles of every triangle, the base is laid down on the paper from some scale as before; and at each of its extremities all the angles contained between the base-line and visual rays from different objects to that extremity are set out by means of a protractor. The intersections of the several lines from the opposite extremities of the base determine the positions of the objects, and form with the base the first triangles. The sides of these triangles become then the bases of other triangles, and the angles observed at the extremities of their sides must be set out by the pro tractor. If any of these lines should have been measured by the chain on the ground, the construction of the triangles by means of the angles may be verified by measuring the lengths of such lines on the scale by which the baseline was laid down. Offsets may also be laid down as already described.
But the practice in ordinary surveys is to observe by the " back angle," as before described, the bearings which the different station lines make with the meridian-line passing through one of the stations, and to measure with the chain the length of each station-line. These bearings and lengths, together with the offsets, are registered in the field-book, and they are generally transferred to the paper in the fol lowing manner : A line, as N s, is drawn in any convenient part of the paper, gene rally in a direction parallel to the right and left sides, to represent the magnetic meridian ; and any point z is chosen in it, at which the centre of the protractor is placed. Then the bearings, or angles made with the magnetic meridian by the different station-lines, n is, x c, c &c., are set out by the graduations of the protractor about the point so chosen, and lines, as z 1, z 2, z 3, &c., are drawn from this point through the mark made on the paper on setting out each angle. These lines are so numbered in order to indicate the particular station at which each angle was observed. Then if the assumed point z on the meridian line should be the place of the first station, the first line so drawn is in the direction of the first station•line ; but if the assumed point is not the first station, the place of this first station must be chosen on the paper, as at rt ; and a line drawn through it, parallel to z 1, will be the direction of the first station-line. Its extremity is, found by setting out its length from the plotting scale, will be the place of the second station. Through K a line is to bo drawn parallel to z 2, and this will be the direction of the second station-line, whose length x c must then be set out as before. This process is to be continued till all the station-lines have been laid down ; when, if the survey should have been carried round the boundaries of a tract of ground, the second extremity of the last station-line will, provided the operations have been accurately performed, coincide with la, the place of the first station. From these lines the offsets must be set out as before described.