2381 HYDROGRAPHIC MANUAL PaGE 96 



2381. Wire Traverse 



Distance measurements by wire, instead of stadia, may be used to advantage in 

 planetable traverse in areas of low flat terrain and sparse vegetation, especially where 

 heat waves are prevalent and are likely to decrease the visibility and the accuracy of 

 stadia distances. Under such conditions wire traverse distance measurements increase 

 the progress of the planetable traverse considerably, not to mention the increased 

 accuracy obtained. This method is very efficient along broad, straight beaches, which 

 are low and flat, where the principal object is to locate hydrographic signals. 



Ordinary stranded sounding wire (see 4651) in a 100-meter length has been found 

 very satisfactory for this purpose. The wire is marked at 25-meter intervals by the 

 insertion of small pieces of cloth between the wire strands. Extra length is allowed 

 at each end of the measurmg wire for holding toggles. 



Pins about 20 inches long are used for marking the 100-meter lengths on the ground 

 when measurmg. A convenient way of accounting for the number of lengths measured 

 is to use six pins. The rear chamman retains one pua at the startmg point and the 

 head chainman starts out with the other five. Wlien the latter has used his five pins he 

 has advanced 500 meters. The rear chainman then comes forward and gives the 

 forward chainman five pins and the measurements are resumed. A plus or minus 

 distance at the end of a traverse, or at a signal, is measured with a steel tape and the 

 total distance plotted along the azimuth line drawn on the planetable sheet. The 

 topographer, sighting through the alidade, can keep the chainmen on line by signaling 

 to them with a white fiag on a long pole. 



Intermediate points may be located along the traverse where necessary for locating 

 detail, the positions of other objects being determined by intersection cuts as the 

 traverse progresses. 



A calibrated wire can be used in a similar manner over the surface of the w^ater, 

 to span narrow waterways and to measure from one rocky point to another, if it is 

 equipped at short intervals with fittings to which floats may be attached. It cannot 

 be used where there are crosscurrents. The azimuth can be carried forward with a 

 planetable or a transit, 



2382. Sextometer Method 



The sextometer method is a substitute for planetable traverse for short distances 

 in unimportant areas where the use of the planetable is impracticable. The survey 

 party operates in a launch, with skiffs for transportation of the rodmen. Azimuth is 

 carried forward by means of angles accurately measured with a navigating sextant. 

 Distance is determined by measuring, also with the navigating sextant, the small hori- 

 zontal angle between fixed targets on a special rod held horizontally; such angles should 

 be measured to the nearest 10 seconds. The angle may be converted into a distance 

 in meters by means of a hypsograph. (See Special Publication No. 144, pp. 20-23.) 



The method is not accurate and is resorted to only to locate the high-water line 

 and hydrographic signals for short distances up minor streams or tributaries, which 

 are too narrow and crooked and where the vegetation along the banks is too dense and 

 overhanging for the planetable to be used economically, and the water beyond is 

 too deep for water set-ups. It is useful in sloughs through swamps and in narrow 

 channels through mangrove growths. The use of the method is authorized only where 

 the waterway is unimportant, its width is less than 200 meters, and a more accurate 

 method would be uneconomic; and for the occasional location of an isolated hydro- 

 graphic signal. (See also 2462.) 



