Evaluation of photographic properties for area estimation

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Virginia Tech


From the known image positional errors on aerial photographs, this thesis computes and evaluates acreage estimation errors. Four hypothetical tracts were used in simulating aerial photographs with 104 different camera orientation combinations. Flying heights of 4000 and 6000 feet, focal lengths of 24 and 50 millimeters with and without lens distortion, and tilts of 0, 3, 6, and 12 degrees were simulated. The 416 photographs were all simulated with the camera exposure station centered above the midpoint of the respective tract's bounding rectangle. The topographic relief of the tracts ranged from 19 feet in the Coastal Plain to 105 feet in the Piedmont.

It was found that lens focal length did not have an independent effect on the acreage estimates. Relief error, the lowest, averaged -0.080%. In comparison, small errors in calculating scale were shown to be larger than relief errors. Tilt was recommended to be limited to six degrees, averaging +1.6% error at six degrees tilt. Because of its positive exponential nature when the tracts are centered, tilt can induce large biases. including tilts from zero to six degrees,the average was 0.634%. Lens distortion error averaged -0.686%. Overall, the average acreage error was 0.363% for simulations up to and including six degrees of tilt with and without lens distortion. This result is for centered tracts, and it was felt many of the errors were compensating given this situation. In conclusion, the photographic images can estimate areas to $1%, however, additional errors are imparted during actual measurement of the photographs.