CHAPTERIl, SECTION 1A For second order triangulation surveys, the allowable discrepancy in the length of any line of the scheme cannot exceed 1:10,000 and for third order 1:5,000. The base line, from which a second order survey expands, can have a probable error of not to exceed 1:500,000. PREVIOUS SURVEYS. The available records of earlier surveys of Bikini Atoll consist of Chart No. 6032 published in 1944 by the U.S. Hydrographic Office, and the adjusted data of third order surveys completed in 1946 by the USS Sumner and USS Bowditch. A geodetic report on the preparation of the chart and adjustment of the 1946 surveys was obtained from the U.S. Hydrographic Office. U.8. Hydrographic Office chart, No. 6032, as prepared in 1944 and with only minor changes in the later editions, is stated to be a photo tranfer of Japanese Chart No. 458. Although records of hydrographic and geodetic surveys, made by the Japanese in 1919, were obtained by the U. S. Hydrographic Office, none of these surveys were recovered. The datum of all charting in the area was based on a scaled value from the Japanese Secondary stations were established at locations of lesser importance by expansion from the primary net and by local triangulation at project areas. The permanency of the station marks was considered in locating the stations, and all stations were referenced with the exception of a station located on 4 coral pinnacle in the lagoon. References independent of the station structure could not be established for this station. Standard procedure and specifications of the U.S. Coast and Geodetic Survey for second order triangulation were carefully followed in executing the survey. The geometry of the scheme was strengthened by construction of a station in the lagoon which became the hub of the network and resulted in all figures being well above minimum strength. DISTRIBUTION OF CONTROL POINTS. The scheme consisted of ten primary stations es-tablished to second order specifications, nine of which were on the perimeter of the lagoon and the tenth was the lagoon station. Figure 2-4 Chart and this value was adopted for the origin of surveys in the area. The 1946 survey by the USS Sumner and the USS Bowditch consisted of two surveys expanding from independent base lines and covering the eastern portion of the atoll. Three triangulation stations were common to both surveys, permitting consolidation of the surveys by a least square adjustment. The apparent purpose of the surveys was to establish the in- terrelation of Scientific Stations which were used in OPERATION CROSSROADS. As the scheme was stated to be of third order accuracy and the geometry of the scheme was not consistant with requirements of CASTLE, it was not considered practical to expand from these surveys. Most of the stations of these surveys were recovered and their relation to the new second order network determined. They were also used for preliminary location and orientation of island traverses furnishing basic information for design of project features. SUMMARY OF GENERAL FEATURES OUTLINE OF SCHEME. A triangulation network encompassing the Atoll was established by expanding througha series of check figures from a base line on the south perimeter of the Atoll. The base line extended from Oboe to Sugar. The survey was for purposes of coordinating local surveys on the project islands and determining the interrelation of the various project areas. The scheme was designed to provide horizontal control throughout the Atoll, with primary stations established where necessary to meet project requirements and consistent with the geometrical specifications for strength of figures. Figure 2-4. Survey Tower, Site Able shows a typical survey tower, located on site Able. Seven additional stations were established to third order specifications. Six stations of the earlier USN Surveys were incorporated into Page 2-21