4468 HYDROGRAPHIC MANUAL PaGE 360 



the distance of the ship from the front range when the range was crossed. From these data the 

 distance run by the ship between the two range crossings may be computed for the true azimuth of 

 the course traversed. 



(d) The wire may be laid along a range, where there are two lighthouses or landmarlvs within a 

 half-mile of the range and at a suitable distance apart. The positions of the two range marks and 

 the other two objects must have been determined by triangulation. The distance along the range 

 between the two objects, when each is normal to the azimuth of the range, may be computed. On the 

 calibration run the revolution counter is read when each object bears 90° from the range, this being 

 determined by an observer with a sextant set at 90°, who marks the instant when each object is 

 exactly abeam. 



For best results the taut-wire apparatus should be calibrated iii a locality with a 

 type of bottom similar to that over which it is to be used to measure distances, but 

 this is seldom possible. A truly horizontal distance should be used for calibration, 

 so an area should be selected where the bottom is comparatively level and free from 

 shoals and depressions, and preferably the depth should not exceed 20 fathoms. 



4468. Accuracy of Taut-Wire Measurements 



The closure errors of loop traverses in the past indicate that an accuracy of about 

 1 meter per statute mile or, roughly, 1 part in 1,600 may be expected from careful 

 taut-wire measurements. To obtain this accuracy the apparatus must be operated 

 and the data observed and recorded with extreme care. There are sources of errors 

 not in control of the ship personnel, however, which may cause inaccuracies. 



The angle with which the wire leads over the stern can, of course, be observed, but 

 nothing is known of the trend that the wire takes below the surface of the water nor at 

 what distances astern of the ship the wire will rest on the bottom in various depths 

 of water and under different tensions. It is apparent that a true horizontal distance 

 will not be measured if the angle of the wire leading over the stern is appreciably differ- 

 ent at the two ends of a taut-wire measurement, but if this angle is the same at both 

 ends it is probable that, within measurable limits, a true horizontal distance will be 

 measured. In depths to 20 or 30 fathoms a preliminary run of % mile, or even less, 

 with the recommended tension applied and stabilized before the first mark is reached 

 should ensure the same lead angle throughout the measurement, provided a uniform 

 tension is maintained and provided the depths are fairly uniform. In depths greater 

 than 100 fathoms a preliminary ri;n of from 1 to ] }^ miles is generally required. 



If the fixed end of the wire is secured by some means on the surface of the water 

 but the wire is not buoyed at intervals as it is payed out, a true horizontal measure- 

 ment will be obtained for only a very short distance; as the wire is payed out its weight 

 will cause it to hang in a catenary curve until the center of the curve touches the bottom. 

 The measured distance will not be a true horizontal one, as an error will be introduced 

 wiiich will vary with the depth of water. A taut-wire measurement started with 

 the wire anchored on the bottom, where the depth of the water differs considerably 

 at the two ends of the course, will likewise be in error; but a more accurate result 

 will be obtained if the measurement is started in the deeper water. 



Irregularities in the depth of water between the tw^o ends of a distance measured 

 by taut wire prevent the measurement of an accurate horizontal distance. If the 

 distance is sufficiently long, the wire will lie on the bottom for most of the length, and 

 the distance measured, instead of being horizontal, will be along the bottom profile. 

 Prominent shoals and deep submarine valleys will introduce appreciable errors in the 

 measurement. 



