382 



HYDROGRAPHIC MANUAL 



Page 298 



cuts if the locations are advantageous. When the vessel is underway, each cut must 

 be marked simultaneously with its corresponding fix. Vertical angles can be measured 

 a few seconds later without affecting the elevations, especially when the course parallels 

 the coast. Heights of trees on hills, ridges, and mountains shall be estimated in order 

 that the elevations may be reduced to ground elevations. On the form-line sheets 

 the elevations of the trees shall be shown in parentheses alongside the ground elevations. 



Various points on the slopes and mountainsides should be cut in and vertical 

 angles measured to them in order to control the form lines. The interval between form 

 lines shall be governed by the scale of the form-line sheet and the type of topography; 

 it should show clearly the general formation of the relief and should naturally be smaller 

 for less rugged terrain. 



Three or more sextant cuts forming a good intersection must be obtained to locate 

 each object; vertical angles at two of these cuts will be sufficient if the computed eleva- 

 tions check within 10 feet. If the cuts are taken intermittently during an extended 

 period of time so that there might be doubt as to the identity of an object, a vertical 

 angle should be observed and recorded with each cut to serve as additional identification. 



Illustrated are tabular forms for abstracting the "observed data" and for the "computation of 

 elevations" which shall be included in the Descriptive Report to accompany a form-line sheet compiled 

 from sextant cuts observed from a vessel. 



In the "observed data" form it is to be noted that the vertical angle, column 5, is always measured 

 from the waterline at the shore. In column 7 is listed the correction for tide with reference to mean 

 high water, which is usually to be subtracted from the observer's height of eye above the water (column 

 6) to obtain the height of the eye above mean high water (cohunn 8). The required tide correction in 

 column 7 may be obtained from the Coast and Geodetic Survey Tide Tables; it is the difference 

 between the predicted tide for the time of observation and the "mean range of tide," except when the 

 predicted tides are minus, in which case they are added to the mean range of tide. 



In the "computation of elevations" form, the various steps are self-evident with the possible 

 exception of column 6, "correction for dip," and column 10, "curvature and refraction correction." 

 The correction for dip is always subtracted from the observed vertical angle. The combined curvature 

 and refraction correction is always added to the computed elevation of the object (column 9). 



Sheet No. T-4240 



Form Lines 

 [OBSERVED DATA] 



Localitv: Bumble Bav 



Day letter: A 

 Date: 7/14/40 



