The survey of detail on reference lines traced and measured, as described above, gives its position with much greater accuracy than when the detail is surveyed on a traverse of lines along the streets. The superiority of the survey on the secondary reference 'block' lines may be readily seen, as in this mode the front and rear of buildings and backward detail is alike directly obtained on the reference lines, the length and position of which are independently verified. The superiority may be also judged on a consideration of the accuracy with which the lines may be laid down on paper, and the direct verification obtained from the field-book, as well as the close proximity of the detail to the reference line for its survey. In the traverse there is no absolute verifica *The town of Barajas de Madrid, in Spain, was surveyed by the author, in 1861, on secondary and primary reference lines. The survey was undertaken in this manner chiefly with a view to demonstrate to others the superiority of skilful over unskilful surveying, as a traverse survey was that approved by the field superintendent of detail surveyors. The town is in part skirted by olive (walled) gardens, and has an irregular circular outline. The streets are numerous, and it was found impracticable to trace a single unobstructed principal secondary reference line. The obstructions and inaccessible distances were comparatively numerous. The church steeple, near the middle of the town, was surveyed on a detail line, the survey of which was obstructed by ranges of buildings at three parts, the church being between two of these obstructions. The axis of the spire, as found by the detail survey, differed by 3 in., or rather less than half a link, from the position determined by the trigonometrical survey. The plan was laid down to a scale of 1 in 300, and subjected to a rigid examination. It passed through the ordeal without condemnation in any particular. [This is alluded to, to show the success attained, with inexperienced assistants, for a difficult town district, by the adoption of that system, or geometrical construction, in which the chain and scale measurements verify each other, and so the fidelity of the plan is assured before its detailed examination.] tion of the position or length of a particular reference line practicable; and besides, the remoteness of the reference line from the rear detail, generally indirectly surveyed on it, is a serious difficulty, not only on the ground, but also in the office-not only in the survey of the detail, but also in the laying down the measurements on paper. In the survey of towns and buildings generally the perpendiculars should not exceed 20 to 40 links. From this it may be seen that in streets of considerable width it is according to good practice to run reference lines for the survey of detail on each side. Each house with parallel sides should be surveyed by perpendiculars to three corners, and face measurements from these to the remaining angle or corner. The point of intersection, on reference lines, of the production of the lines of the face walls of chimneys, gables, faces of buildings, walls, &c., should be surveyed on different lines, and shown on the field-book, to verify the position given those faces by the detail measurements. The measurements of the internal details of buildings should be checked by diagonals, prolongations, &c., as it will be impracticable in some instances to obtain directly, on reference lines, measurements of such details. An inspection of fig. 34, illustrating a survey of buildings, will show the points requiring attention. SURVEYS FOR RAILWAY, CANAL, ROAD, &C., PURPOSES. Surveys for these, or like purposes, require to be conducted with much skill, and made with a high degree of accuracy. The chief features of the districts for survey in those services are great length, narrow width, and irregular outline. Having regard to the surface configuration, the special service in each particular case, the limitation of time, and the general financial interests of the projectors, the 'traverse survey,' with trigonometrical checks more or less perfect, may be in most cases advantageously adopted. In these, as in other surveys, the surveyor should make himself acquainted with the position of the leading features of the whole line of district, and the more particular features at certain points (which, if the district be extensive, should be obtained by a preliminary reconnoitering survey), before determining the particular locality and lateral extent of the district for detail survey. After the route of the detail survey operations is determined, the selected district should be 'poled,' so that the reference lines may traverse the triangles, of which the 'poled' shall be the angular points. The triangles so formed should be well conditioned. The sides of remote triangles should be accurately measured as base lines, and base lines for verification, so that the whole district may be simultaneously surveyed in sections, which, when applied to one another, shall make one continuous whole. The angles of the triangles should be measured with a suitable theodolite, and the sides computed before proceeding with the survey of detail. The traverse may be conducted with the theodolite as already described in Chap. III. The points in which the traverse reference line intersects the side of a triangle should be determined by tracing the intersected side at the point of intersection. From these points the angular or trig. point (poles) should be observed, and the angles made by the side of the triangle with the traverse reference line, and the lines to the observed trig. points, measured. From these data the position of the traverse reference point in the side of the triangle, the distance from this point to the observed trig. points and the traverse reference line traversing the triangle, may be computed. The points and lines may be laid down from the computed distances. An inspection of the illustration (fig. 35) will suffice to show the reference and trig. points, and the lines and distances for computation, |