392 EXPLORATION GEOPHYSICS 



giving the base station its true gravity value, rather than some arbitrary 

 value. 



Station Elevations and Surveying. — Station elevations are run with 

 an accuracy of 0.2 to 0.5 of a foot. The accuracy with which the elevations 

 are determined is one of the chief factors governing the overall accuracy of 

 the gravity work. One foot of elevation represents 0.09406 milligals in 

 free air correction, which is a detectable quantity with modern gravimeters. 

 Where high precision of final gravity values is desired, accuracy of this 

 order is necessary. Accurate level instruments are preferred for deter- 

 mining elevations, especially where large areas are to be surveyed. For 

 some distances a transit may be used almost as successfully. Alidade and 

 plane table usually sacrifice some accuracy in running elevations, but are 

 more convenient for plotting station locations and field contouring. 



A survey unit usually comprises a field surveyor and his rodman, with 

 a field car and driver. The car should be equipped with all necessary sup- 

 plies (flagging, stakes, drinking water, etc.), 



A land man or party chief usually secures the necessary permits for 

 station settings from local land owners, and establishes land lines and 

 map locations for the stations. Establishing stations and running the 

 elevations is often a limiting factor of crew production. Two survey units 

 with conventional level equipment usually are required to keep up with 

 one gravity-meter crew. 



The surveyor selects the location of stations judiciously so that they 

 may be conveniently occupied by the field truck, preferably in places that 

 are more or less flat. In areas of moderate topography, terrain survey and 

 correction is not needed in gravimeter work as it is with the torsion balance. 



TERRESTRIAL PHOTOGRAMMETRIC MAPPING 



The mapping of the gravitational field in an area is as much dependent 

 upon precise elevation measurements as upon the accuracy of the gravity 

 measurements. Also the personnel and time required for the surveying 

 usually exceed that required for the gravity measurements. For these 

 reasons, surveying techniques are of special importance. One recent devel- 

 opment which is becoming more widely used is photogrammetric mapping. 



The science of terrestrial photogrammetry has been known for the past fifty years, 

 but until recently its use has been confined principally to European countries. The 

 fundamental work on this subject was done in Germany by S. Finsterwalder at the 

 beginning of this century and was further developed by his son.f 



The European method made use of an instrument known as the "photo-theodolite", 

 which permitted surveying and mapping from ground positions by means of the result- 

 ing photographs taken of rod stations or other points under investigation. The standard 

 field procedure for terrestrial photogrammetric mapping using the photo-theodolite 

 consisted of taking shots of photographs from either end of a carefully measured base 

 line. By identifying common points in the photographs taken from these two instrument 



t Personal communication to W. O. Bazhaw, April 25, 1949, from Friedrich Breyer regarding 

 completion of this development by R. Finsterwalder. 



