PHOTOGRAMMETRIC DETERMINATION OF ELEVATIONS FOR REGIONAL GRAVITY SURVEYS

Abstract

Precision photogrammetric methods were used to obtain elevations of gravity stations in an area in the southern Sierra Nevada and in west‐central Idaho. In both areas, relief is more than 3,000 ft and in the Idaho area there is also a dense cover of forest and brush. In addition to photogrammetric determinations of gravity station elevations in the Idaho area, 55 photogrammetric elevations were determined for ground points that had been established previously by topographic surveys. A statistical analysis of the differences between photogrammetric and known ground elevations indicated that the mean of the errors is −0.2 ft; 71 percent of the photogrammetric errors are included within one standard deviation (3.4 ft), and 96 percent of the errors are included within twice the standard deviation (6.8 ft). The photogrammetric combination used in both areas was the multiplex stereoscopic plotting instrument and 1:49,500 scale photography taken with a 6-inch focal length lens. It is probable that smaller standard deviations can be obtained by combining larger scale photography with more precise stereoscopic plotting instruments.