32 LANDS AND SURVEYS 



the map must be drawn in minute detail. It is advanta- 

 geous where there are terraces, trees, etc. Mistakes in 

 plotting are thus avoided, since the features are within 

 sight of the draftsman. A plane table is a useful instru- 

 ment for this kind of work; but it can be done with a 

 transit with vertical circle, the draftsman using a light 

 table 2 feet square mounted on a light tripod. 



THE HYDROGRAPfflCAL SURVEY 



Ponds and Lakes. The topography of the beds of ponds 

 and lakes is obtained by the gridiron method of survey. 

 The depths may be had in shallow ponds by use of a rod 

 lowered from a boat, the readings being referred in measure- 

 ment to the water level. When the depths are more than 

 6 feet, a sounding line is employed. A good sounding 

 line can be made of sash-cord with a colored string tied at 

 each foot-mark. Different colors are used to good advan- 

 tage, the number of feet being thus readily determined. 

 A sinker will be required to hold the lower end of the line 

 to the bottom. Alignment of the sounding rod or rope 

 may be had by means of rods on the shore or else by transit. 

 The distance is obtained by stadia or else by alignment 

 rods on a line at right angles to the other. 



Brooks and Creeks. The flow of streams is determined 

 by use of a weir unless the flow is large. In the latter case, 

 which is not common, a current meter or rod float must be 

 employed. Fig. 9 shows clearly the customary method of 

 making weir measurements. 



Depths over the crest are measured either by a car- 

 penter's square or a footrule. For measuring depths 

 the hook-gauge is never necessary except when litigation 

 is probable. Measurements of flow may be made twice a 

 day, or oftener when the water is rising or falling rapidly. 



