Regional Control Supplied by Photogrammetry

Where second-order geodetic control has a spacing of 3 miles to 5 miles, the positions and elevations of a virtually unlimited number of premarked ground points within a control figure can be determined with a rms error of 0.15 ft horizontally and 0.25 ft vertically for a total cost of about $5,000 for an area of 25 sq miles. A block of 25 photographs is a practical unit taken from an altitude of 5,000 ft to 7,500 ft with a glass-plate camera and an overlap of 60% in both directions. Corrections are made for comparator errors, film deformation (if film is used), lens distortion and atmospheric refraction, and a rectangular geocentric coordinate system is applied to recognize the effect of earth curvature. The premarked ground points become recoverable supplemental control. Thereafter lower-altitude film photographs can be used for mapping in three dimensions with conventional plotting instruments without any further field surveys.