Survey of Detail.

Survey of Detail.

  The survey of detail from a chain line is carried out mainly by right-angled offsets or by small offset triangles, the latter being small triangles of which two sides are measured from two points on the chain line, as ends of the base. The important point in this kind of work is to run the chain lines as close as possible to the detail to be surveyed, so that offsets are as short, and offset triangles as small, as possible. For accurate work, and when important detail has to be surveyed, the lengths of offsets, and the size of the offset triangles, should be strictly limited and care should be taken to see that offsets are set out as closely as possible at right angles to the chain line. When less important or ill-defined detail has to be surveyed, longer offsets and larger offset triangles may be used, and less care taken in setting out the offsets at right angles to the chain line. In addition, detail, such as walls and hedges, that is sloping fairly sharply away from the chain line requires more careful survey than detail that is running more or less parallel to the line. The first rule of chain survey being to work from the whole to the part, this leads to the second rule, which is to try and keep the chain lines as close to and as nearly parallel as possible to the general run of the detail.

  When a point of detail is on a line whose direction is otherwise established, the point can be fixed by noting the chainage of the point where the line, produced if necessary, cuts the chain line, and then measuring the distance along the line from this point to the point to be fixed.

  Up to very recently, the Ordnance Survey, in the revision of the large-scale plans, made extensive use of a system of graphical intersection from points already fixed. Thus, in fig. 30, P is the point of intersection of two fences which it is desired to fix, PE being an existing fence, which is shown on the plan. The surveyor determines a point E on the fence which he sees is in line with the church spire A and the corner of a building B, both of these points being easily identified both on the ground and on the plan which is being revised. The intersection of the line AB and the line of the fence fixes the point E, and a measurement of the length of EP along the fence on which these points lie fixes the point P. The same method can sometimes be used in a new survey when a number of points have been established by other means. In the particular case illustrated, the fixing of E could, if desired, be checked or strengthened if the two fence intersections D and C which are in line with E on the ground are also in line with it on the plan, as the intersection of the lines AB and CD would give an independent fixing for E.

As a preliminary example of the survey of detail from a chain line consider the building ABCDEFGHA in fig. 31 which is to be fixed from the chain line XY. If it is an important building, we are fairly safe in assuming that the corners will be right angles.

  The face ABCDEF could be fixed by right-angled offsets from the chain line to the corners A, B, C, E and F, and the corner D by the distances CD and ED, the plotting of B, C, E and F being checked, if necessary, by the lengths AB, BC, EF. The corners A and B are rather far from the chain line, so that, instead of fixing them by right-angled offsets, it would be better to fix them by the offset triangles Aab and Bbc. The corner G can also be fixed by the right-angled offset jG, but, as it is rather far away from the line and is an important point governing the survey of the back of the building, it would be better to fix it by the offset triangle Gjk. The corner H could most easily be fixed by noting the chainage of the point a, where the line HA produced meets the chain line, and then measuring the distance AH from A.

(Next post on “Booking of Detail”)