XV. POSITIONING 



Engineering Section 

 Survey Branch 



A. ANGLE AND DISTANCE MEASURING INSTRUMENTS 



1, Introduction 



The geodetic surveys of the Hydrographic Office are made to 

 establish the control for various other types of surveys. The field 

 work varies according to the accuracy requirements of the work for 

 which the control is being established. The work also varies according 

 to whether it is: (1) The extension of an existing control by means of 

 triangulation, trilateration, traverse, or a combination of these; or 

 (2) the establishment of an independent datum, as is usually the case 

 in isolated areas or on remote islands, where an astronomic position 

 is determined independently and corrected for plumb line deflection by 

 a gravity survey, and where the local control nets are tied in. 



In both types of surveys, the general specifications given in Hydro- 

 graphic Office Publication No. 4 and USC&GS special publications are 

 followed but are modified by the specific requirements for each field 

 project. 



Until recently, standard survey practice consisted entirely of 

 measuring angles with a theodolite and measuring distances with a 

 tape, subtense bar, or stadia rod. Today, the picture is changing 

 rapidly. Although the theodolite is virtually unchanged, the older 

 instruments for measuring distance are being replaced with new 

 electronic devices that make the job easier, faster, and more econo- 

 mical. 



The instruments available in the Hydrographic Office for field 

 surveying consist primarily of theodolites, conventional measuring 

 devices, the geodimeter, and the tellurometer. The capabilities of 

 these instruments are presented below. 



2. Angle measuring instruments (theodolites) 



In afield survey, all angles, horizontal and vertical, are observed 

 with a theodolite, usually a Wild theodolite manufactured by the Henry 

 Wild Company, Heerbrugg, Switzerland. This is an optical micrometer- 

 reading instrument in which two diametrically opposed axes are brought 

 to coincidence and read through a microscopic eyepiece. The various 



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