Close range digital photogrammetry for the measurement of erosion to historic adobe structures

Unbaked earth structures, like adobe structures are found in abundance throughout the world. Like most structures, they are subject to deterioration through time. The preservation of historic adobe structures requires careful attention and evaluation of several factors, but material erosion is the main concern. In order to calculate the amount of erosion, accurate measurement of the structures is required. Any measurement method used to quantify erosion needs a level of accuracy to detect the small changes in the surface of the structures. Digital photograrnrnetry is the measurement of objects by digital image analysis using specialized software packages. Historic preservationists and architects use digital photogrammetry to measure historic structures. The accuracy of the measurement process is crucial, especially when an accuracy of less than 1/8th of an inch is required. Erosion measurements of historic adobe wall ruins at Fort Davis National Historic Site requires this level of accuracy in order to quantify volume differences that are used to calculate erosion rates. An experiment was conducted to investigate how; the combination of photographs taken from different positions; the resolution of digital images and the dimensional control withn the image effects the accuracy of the measurements. The experiment compared measurements from a model eroded adobe wall using digital photogrammetry with measurements obtained using a 3D digitizer, a method whose accuracy is assumed to be perfect. Multiple regression models identified factors affecting accuracy and precision, and magnitude of the effect. The results of the regression models were used to make recommendations for field measurements. An adobe wall ruin at Fort Davis National Historic Site was measured to test the recommendations.