Cee INSTRUMENTATICN FLAN ~ sne usefulness of anowledge of oottom depth is dependent on corresponding «nowledge cr the geograpnical location where the depth measurement is made. In Yact, tue problem of determining the location of tne snio is more complicated ana iifficult tnan tne determination of deptn. For this reason more effort was devoted to tne location procedures tnan ~o tne depth measurements, coth in the planning and preparation Dhase anc in the measurement phase. Colel deptn Measurement Wepth was measured witn a standard recording sonic echo fathometer designed for small ships, Model NK-G. This Pathometer cperates et 14.25 keps anc et a repetition rate of l/sec om the "foot" scale, which has a maximum of 200 Ft. Tne transducer, of tne dounle-unit magnetostriction type, was mounted outboard of tne LCU assigned to the project, and the recorder was mounted inside a trailer whicn also housed equipment for tracking and plotting. The fatnometer recording paper had a depth scale of 1 in. per 30 ft of deptn and a paper speed of 1 in. /min. Since the speed of tne voat during survey operat ions was about 6 knots or 600 ft/min, the chart represents a bottom pro“ile with the depth dimension expanded by a factor of approximately 2C. “ne calibration was accompiishned by two procedures. First it was determined py finding a uniform hard bottom and checking the fatnometer readings against a lead .ine. By this method a satisfactory calibration was accomplisned in avout 4 nr with all points grouped closely around a straignt line snowing a 2-ft zero error and a slope such that the was GO rt. fatnometer read & “+ when the actual (lead line) depth Tne second procedure for calibration made use of a corner reflector. Tris reflector was lowered directly below the fathometer head on @ cotton line which had been previously measured and marked. The calibration oy this method gave tne result that the fathometer read depth correctly except for a 2eft zero error (which is accounted for by tne fact tnat the transducers were approximately 2 ft below the water surface). Simce tne surveys were taken under varying tide heights, it was necessary tc reduce all depth readings to a common datum plane. The plane used was tnat on which tne tide tables are based, namely 1/2 ft below mean low-water springs. Recording tide-gages were operated by Holmes and Nerver, Inc., (HEN ) at several islands in the atoll. The gage readings were witnin 1/2 ft .f the published tabular values. The time interval spanned :y a survey was ordinarily no more than 4 rr and the tide change dur:ng such an interval was less than 2 ft. Consequently the tide correction for eacn survey has been made by plotting tne tabular values from tne tables, drawing a smooth curve, and noting tne nearest integral Z:ot of tide height at the mid-time of the survey. Tnis value of tide neight was subtracted from the depth values noted oy fathometer (after taxing account of its calibration).