2.2 INSTRUMENTATION PLAN The usefulness of knowledge of bottom depth is dependent on cor- responding knowledge of the geographical location where the depth measurement is made. In fact, the problem of determining the location of the ship is more complicated and difficult than the determination of depths: For this reason more effort was devoted to the location proce- dures than to the depth measurements, both in the planning and prepara- tion.Phase and in the measurement phase. 2.2.1 Depth Measurement Depth was measured with a standard recording sonic echo fathometer designed for small ships, Model NK-6. This fathometer operates at 14.25 keps and at 4 repetition rate of 1/sec on the "foot" scale, which has a maximum of 200ft. The transducer, of the double-unit magnetostriction type, was mounted outboard of the LCU assigned to the project, and the recorder was mounted inside.a trailer which also housed equipment for tracking and plotting. The-fathometer recording paper had a depth scale of 1 in. per 30 ft of depth and a paper speed of 1 in. /min. Since the speed of the boat during survey: Operations was about 6 knots or 600 ft/min, the chart represents a bottom profile with the depth dimension expanded by a factor of approximately 20. The calibration was accomplished by two procedures. First it was determined by finding a uniform hard bottom and checking the fathometer readings against a lead;line. By this method a satisfactory calibration was accomplished in” about 4 hr with all points grouped closely around a straight line showing a a-ft zero error and a slope such that the fathometer read 80 was 90 ft. When the actual (lead line) depth ions The second procedure for calibretijon made use of a corner re~ flector. This reflector was lowered directly below on a cotton line which had been previously measured calibration by this method gave the result that..the depth correctly except for a 2eft zero error(eiieh by the fact that the transducers were epproxinately water surface). the fathometer head and marked. The fathometer read is accounted for 2 ft below the Since the surveys were taken under varying tide heights, it was necessary to reduce all depth readings to a common datum plane. The plane used was that on which the tide tables are Dabed, namely 1/2 ft below mean low-water springs. Recording tide-gages' were-operated by Holmes and Nerver, Inc., (H&N) at several islands in the: atoll. gage readings were within 1/2 ft of the published tabularvalues. The The time interval spanned by a survey was ordinarily no more than 4 hr and the tide change during such an interval was lessthan 2 ft. Consequently the tide correction for each survey has been made by plotting the tabular values from the tables, drawing a smooth .curve, and noting the nearest integral foot of tide height at the mid-time of the survey. This value of tide height was subtracted from the depth values noted by fathometer (after taking account of its calibration). 19