A PLANE-TABLE SURVEYOR MAPPING A SECTION OP SHORE-LINE 



In making a survey of a harbor or section of shore-line, it is necessary to prepare a 

 topographic map of the surrounding territory, showing all objects in the landscape that will 

 help the navigator to fix his position (see page 658). 



carried around by currents whose pres- 

 ence he does not suspect. Information 

 concerning' them is gathered by means 

 of current rods, as a rule. A current 

 rod is an instrument made to float verti- 

 cally beneath the water, with only its 

 tip showing above the surface, so that 

 it is not disturbed by the wind. Its 

 movement is observed, and the observa- 

 tions give definite information concern- 

 ing the currents. 



MAKING THE CHART 



When all the work in the field has 

 been finished, including observations 

 which show the deflection of the mag- 

 netic needle, the force goes back to head- 

 quarters and begins the task of "writing 

 up" its notes. And a bunch of notes 

 they are to be written up ! To make a 

 single chart of a single harbor may re- 

 quire as many as 12 sheets of field 

 hvdrographic drawings, 16 sheets of 

 field topographic drawings, 57 volumes 

 of soundings, 22 volumes of tidal rec- 

 ords, 20 volumes of trigonometric rec- 



ords, 7 volumes of altitude records, and 

 8 volumes of magnetic records. 



Obviously the proposed chart cannot 

 be as big as the area it is to represent, 

 so the first thing to be determined is the 

 scale to which it is to be drawn. In rep- 

 resenting a harbor the usual scale is ap- 

 proximately half a foot to the mile, or 

 exactly one foot to ten thousand feet. 

 For ordinary navigation along the shore, 

 where ships keep outside the 100-fathoni 

 line, a foot on the chart may represent 

 nearly 80 miles, or, to be exact, 400,000 

 feet. On charts used to approach the 

 coast the usual scale is about a foot to 

 16 miles, or one foot to 80,000, to be ex- 

 act again. In all cases a foot on the 

 chart represents just as great a distance 

 on the land or on the water as a clear 

 exposition of the facts to be placed on 

 the chart will permit. 



After the map is drawn it is repro- 

 duced either on copper plates or on alumi- 

 num plates. By photography a copy is 

 made from the drawing, and this is laid 

 down on the copper to fit the latitude and 



661 



