CHAPTER I, SECTION 1A HORIZONTAL CONTROL PRELIMINARY TRAVERSE. Preliminarytra- verses, of third order accuracy, were established for mapping purposes. These were generally on the perimeter of an is:and and were not referenced, since it was not intended to use them later for construction lavout. Due to the work load of the department it was not possible to establish more accurate controls in someof the areas before the start of construction operations. Where the accuracy of these traverses was considered consistent with accuracy requirements, they were utilized for construction layout. PRIMARY TRIANGULATION. Thebasic horizontal control for all surveys was a triangulation network established to second order specifications and procedure of the U.S. Coast and Geodetic Survey. This survey is outlined elsewhere in this report and consisted of a network of triangles expanding from a measured hase linand from which the interrelations of all areas and project features could be determined. SECONDARY TRIANGULATION. Requirements for determination of the precise interrelation of structures in some areas resulted in establishing local triangulation nets by breakdown from the primary network. In the Able-Charlie and Dog-George areas, local nets were established which expanded from base lines. whose lengths were determined byfirst order traversing and which were related to and oriented from the primary control network. PERMANENT LOCAL CONTROL.In addition to a station of the primary network on an island, additional permanent control was established where practical. On Fred and Elmer, at Eniwetok Atoll, traverses were established and referenced to monuments as controls for all surveys in these areas. ‘Lhese traverses are of approximately second order traverse accuracy. On smaller islands it was the intention to maintain at least two points whose interrelation and relation to the primary network were known. In some locations these points were destroved during test operations and were replaced before subsequent operations. PERMANENCY OF PRIMARY CONTROL MARKERS. At each Atoll, practical methods were used to assure the permanency of the primary network. In addition to a concrete marker at each station, the general procedure was to establish three reference markers and determine their relation to the primary marker. From any two of the reference markers it is possible to re-establish the primary marker. In the biast areas, stations and reference markers were destroyed and could be replaced only by expansion from the primary control stations existing at other locations. At each Page 2-42 RiAiletshphate anemem Atoll a reconnaissance will be made and recommendations submitted covering the additional surveys required to maintain an adequate primary control network. VERTICAL CONTROL. No records were available of vertical control established at either of the Atolls by previous surveys. There had been no scientific requirement for an over-all vertical control network, and such a network would involve extensive observations over a considerable period of time. A preliminary datum was established independently on each of the project islands as surveys were made and the accuracy was considered consistant with pro- ject requirements. This datum may be described as “A datum approximating mean low water springs was arrived at by applying corrections from the USC & GS purlications ‘Tide Tables of the Pacific Ocean’ to a series of tidal observations. This is a temporary datum but should he significant to less than a foot.” The procedure was to erect a tide staff or other means of measuring the height of water at each island and take periodical observations as surveys were made in the areas. After applying corrections, a mean of these corrected observations was adopted as the temporary datum. Thi: datum was transferred to permanent monuments in the vicinity and occame the origin of all vertical control for the area. Subsequent to establishing the preliminary datum, automatic tide gages were operated at three locations. At site Fred, at Eniwetok Atoll, a differential of 0.14 foot was determined between the preliminary and the automatic gage datum which was of no consequencein the tidal relations to project structures. The adjusted results of the observations at Bikini Atoll are nct available at this time. A comparison of the unadjusted data indicates a small diffential at site ‘are with approximately one [oot differential at site Charlie. This was believed to be accounted for by the difficulty of finding a location fer the preliminary observations which was free from effects of local currents causing abnormal tide level. CONSTRUCTION LAYOUT GENERAL. The layout for construction was the largest survey activity. This included preliminary staking for clearing and rough grading, followed by accurate layout of the particilar item. Due to the isolaticn of many of these items and the time required for transportation to and from the locations, the man-hours required for construction surveying were high. Where justified by the work load, survey parties were assigned to an area during the construction period. Where this was not practical, a party was made available to go to an area on short notice in order that there would be no delay to construction operations,