348 SURVEY OP THE COAST 



Description of the Plane Table, and the Alhidade to the same. 



The best method of surveying the minute details which 

 are to fill up a triangulation, is undoubtedly by the plane ta- 

 ble and its alhidade, with a telescope revolving in the verti- 

 cal. This method will give to the detail surveyor the full 

 result of the triangulation with respect to the relative posi- 

 tion and distances of the points to be determined, in a me- 

 chanical form, appropiiated to the nature of liis work ; and 

 which will not only be a guide and reference, but also a 

 means of enabling him to determine his distances, and to ve- 

 rify his work constantly as he proceeds, and by reviewing 

 the fundamental points, to discover an error immediately, 

 before it may mislead him. The detail surveyor can there- 

 fore proceed with confidence and celerity, and his work will 

 be greatly diminished by this method, as well as by saving 

 all the work commonly called plotting, (necessary in all 

 Other methods,) which besides introduces new errors, while 

 those made in the field remain concealed until it is too late 

 to correct them properly. 



The plan of the triangulation being propeily adapted, will 

 besides be made at once sufficiently by the mere projection 

 of the triangulafion, as will be observed in its proper place. 

 The details lieing introduced in the field, immediately under 

 the eye, will be much more numerous, more accurate, and 

 natural ; so that to a man acquainted with tlie subject, it will 

 be easy to distinguish details and plans surveyed by this me- 

 thod fiom those taken by the theodolite or the needle. The 

 last of these instruments is the worst that can be employed 

 for the purpose, and has probably been transferred to land 

 only from its use at sea. As a historical proof of these asser- 

 fions, 1 shall only mention tliat the plane table has been used 

 in the surveys made in East India by the East India Company, 



