Reproduced from .the holdings of the National Archives Pacific Southwest Region #239 DLNUrevs 8-28-53 air across the line, not broken by obstructions such as buildings and clumps of trees. As the pipe hangers will be under construction before the control surveys can be completed it will be necessary to establish these controls on an offset line and it is desirable, because of refraction problems, that the line be on the windward side of the structures and reveral feet away from them. After completion of the offset line, markers can be set on the true centerline of the pipe array of ora closer offset where they will be protected by the hangers. Six points should be established on the control line. One opposite the working point at each end of the line and four approximately equally spaced along the line. Before the precise work starts these points should be established as accurately as is possible with stand- ard equipment working under daylight conditions, They should be monumented with a plate or cap set in concrete which is large enough for the final alignment nark to be made on it after the adjustment is completed, From these six points the longest distance to an interes mediate alignment point on the pipe lines will not exceed 750 feet and additional points can be easily established between any pair, if required, It should be recognized that as the line of sight is lengthened the probability of error in reading an offset to a pipe line increases and therefore the maximum distance between control stations should be well within the limitations of the equipment used for the survey. Wild T#2 theodolites with diaphragms containing double vertical hairs are available. It has been proven that there is less obse1-+ vational error in centering a target between two parallel hairs then by use of the single hair. Target and alignment lights, with narrow slits and illuminated by diffused light, are on requisition. The alignment lights will be mounted on a support with a lateral micro- ’ meter adjustment similar to those used for the site Gene alignment : (See Sketch No. 1). Portable radios will be required for communication. The alignment procedure is to set up a target light at one end of the offset line, the observing instrument at the other end and establish the nearest intermediate line point to the instrument. Then, moving to each new point successively establish the remaining line points. Always moving ahead in the direction of the target light. In establishing the intermediate points, an alignment light is used which is mounted on a base having lateral movement by means of a tangent screw and the amount of this movement can be read on 4 micrometer scale, The observer brings the vertical hairs of the theodolite in alignment with the illuminated slit in the target light then directs the movement of the alignment light, which has been set up over the immediate point, until the illuminated slit in this light bisects the vertical hairs in the instrument. The reading of the 5 a ey Ve ay i Lobo