392 HYDROGRAPHIC MANUAL PaGE 308 



The construction and operation of the wire drag and sweep are described, and 

 instructions for wire-drag surveys are contained, in Special Publication No. 118. 



392. Survey in Advance of Control 



When, in exceptional cases, it is advantageous to carry on hydrographic or topo- 

 graphic surveys simultaneously with or in advance of the triangnlation, the work must 

 be planned with the view of ultimate compliance with the control specifications. lu 

 order to save time or to utilize the sei-vices of all of the party at the beginning of the 

 season, preliminary locations of the control stations may be determined by sextant cuts 

 or three-point fixes for use on the boat sheet. It is to be understood that the stations 

 will fmally be accurately located and these positions used on the smooth sheet. If 

 the preliminary locations are so weak as to cast doubt on the adequacy and coverage 

 of the survey, the work should be replotted in ample time to permit additional field 

 work before the party leaves the field. 



Likewise with a topographic survey in advance of the triangulation, the methods 

 used should be planned so that the ultimate result will comply with the control specifica- 

 tions. In an area where the coastline is regular, usual traverse methods may be used on 

 a sheet without projection, to which a projection is added after several of the stations 

 within the area have been accurately located by triangulation. This can be done only 

 where there is reasonable assurance that any discrepancies are the result of accumulated 

 systematic errors. A projection can be applied to such a sheet by one of the methods 

 described in 737. For topograpliic surveys of complicated areas, final acceptable results 

 can be obtained only by the subsequent location by triangulation of a comparatively 

 large number of the control points. A new projection must be made on which these 

 common control points are plotted and to these the details from the original sheet must 

 be transferred, area by area, and adjusted. 



393. Isolated Harbor Survey 



It is sometunes desirable to survey an anchorage or a harbor in an unknown region 

 in advance of the regular survey of the area and prior to the establishment of the 

 permanent control. 



A method of establishing local control by planetable triangulation is described on 

 pages 51 and 52 of Special Publication No. 144, Topographic Manual. Tliis may also 

 be done by sextant triangulation. The stations thus located may be used to control 

 a survey of the harbor and its approaches to any extent desired. The data so obtained 

 if based on local astronomic observations can be used for the construction of a prelim- 

 inary harbor chart. If it is expected that permanent control will be extended into 

 the region within a reasonable time, sojne of the stations used should be permanently 

 marked for subsequent connection with the general control scheme. 



If time does not permit a complete survey of an isolated harbor, a partial recon- 

 naissance survey can be made by less accurate methods. A few signals erected on 

 salient points or a few buoys anchored and located by cuts may serve as the control. 

 The survey should include a measured base, and in heu of one measured by tape, a 

 stadia or taut-wire distance will suffice. A meridian should be established, and in lieu of 

 more accurate methods, a sun azimuth may be measured by sextant. 



The shoreline and topographic details may be sketched with reference to the 

 located stations from the sounding boat during the hydrography. The part of the area 



