Ifi A MANUAL OF TOPOGRAPHIC METHODS. 



With these instruments nine-tenths of the work is done, and these 

 instruments will be described in their proper places with such fullness of 

 detail as seems necessary. 



Other instruments, such as transits, surveyors' theodolities, compasses, 

 wye levels, hand and Abne} T levels, telemeters, chains, tapes, and mercurial 

 barometers, are occasionally used. Most of these instruments, which are 

 commonly figured and described in all works on surveying, are assumed to 

 be well known to the readers of this manual and will therefore receive no 

 special attention. 



ASTRONOMIC DETERMINATIONS OF POSITION. 



The object of astronomic determinations of position is to locate the 

 map upon the earth's surface. They are made also for the purpose of 

 checking - and correcting positions determined by primary triangulation and 

 primary traverse. 



With regard to the checking of the primary triangulation by astronomic 

 determinations, it should be understood that in the case of a single determi- 

 nation the work by triangulation is far more accurate than by astronomic 

 determinations, even when made under the best of circumstances. It is, 

 therefore, desirable to introduce checks of this kind upon primary triangu- 

 lation only when the latter has been carried for a long distance, 200 or 300 

 miles, for instance, in the course of which it may have accumulated errors 

 greater than those incident to astronomic work. 



The case is different with primary traverse. The great number of 

 courses required in this work affords an opportunity for the accumulation 

 of error much greater than is the case with triangulation, and consequently 

 it is desirable to introduce more frequent checks in this work. It may be 

 said that, in general, such work should be checked at every 100 miles. 



As was suggested above, the best astronomic determinations are none 

 loo <n>o<l for the control of maps. Indeed, certain errors incident to this 

 work, some of which as yet can not be corrected, may be of magnitude 

 sufficient to show upon the scale of the map. It is necessary, therefore, in 

 these determinations to use the best instruments and the most refined 



