266 



HYDROGRAPHICAL SURVEYING [chap. xi. 



Eleva- 

 tions from 

 Sea-level 

 first 

 meaned. 



Absolute 

 Heights. 



Depen- 

 dent 

 Heights. 



Seztant 

 Eleva- 

 tions. 



Aneroids. 



Only for 

 Approxi- 

 mate 

 Heights. 



We must commence by collecting results of elevations from 

 stations at the sea-level, or from stations whose height above 

 the sea-level has been measured, which will give us the heights 

 of objects observed ; and also with depressions from stations 

 to the sea-level, or to stations whose height above sea-level 

 has been measured, which will give us the height of the 

 observing-stations . 



Heights so obtained are termed " absolute," as being calcu- 

 lated directly from the sea-level. 



All such heights must be obtained first ; then, meaning the 

 heights of one station which has the most observations, or 

 of which the results agree best, we can work out all other 

 observations from that station to other objects. We then 

 mean another, and so on, using our observations either to 

 obtain height of observer or of observed object, as is most 

 convenient, as we proceed. 



These heights will be " dependent," as resting on the ascer- 

 tained height of other stations. 



No height can be considered as exact that is not the result 

 of both elevations and depressions, as no matter how nicely 

 a set of depressions, say, comes out, they will all include the 

 refraction error, for the refraction correction is only approxi- 

 mate. This is with reference to detailed surveys only. 



Sextant angles of elevation must be corrected for the height 

 of eye before being entered in the Height Book as angle 

 observed. They are then treated in precisely the same manner 

 as the theodolite elevations. 



The pocket aneroids should be tested up to a known height, 

 to get the value of each tenth, which will be from 92 to 100 feet 

 for a tenth, each instrument varying slightly. As before 

 mentioned, they are useless in getting accurate heights, but 

 wiVL give very good approximations up to about 4,000 feet, if 

 in good order and constantly worked ; but their delicate chain- 

 work is so liable to rust slightly at sea, that the links will 

 frequently stick if the instrument is not carried up heights 

 continually to work it. Placing under an air-pump will serve 

 the same purpose. See "Barometer," p. 35. 



It is useless to enter into intricate calculations of data 

 obtained by so small a scaled instrument as a pocket aneroid ; 

 the impossibility of reading it exactly precludes any but 



