REPEAT PHOTOGRAPHY, VIRTUAL REPEAT PHOTOGRAPHY, AND EARTH-SURFACE CHANGE IN THE PHOTOGRAPHIC ERA

Repeat photography refers to two or more pictures taken from the same place with the same field of view, ideally with cameras possessing the same focal characteristics, but at different times. In the earth sciences, such multiple sets of photographs are commonly used to document ecologic, geologic, geomorphic, and/or hydrologic change on time scales of a century or less, in some cases just decades where rapid change occurs. Virtual repeat photography, as we have come to use the phrase, is the use of GIS software to display and manipulate digital elevation data (USGS DEM) as well ortho-rectified digital aerial photographic data (USGS DOQ) to construct synthetic images. Using ESRI ArcInfo(1) software, we first aggregate 10-m-resolution, USGS 7.5-minute topographic quadrangles and then use ArcScene(1), a three-dimensional viewer extension of ArcInfo, to view this topographic domain, or any part of it, at any position, elevation, azimuth, and inclination we choose. We then drape the DOQ over the DEM to construct a synthetic photograph at that position, elevation, azimuth, and inclination. In areas of even modest relief, we have found that virtual repeat photography can locate otherwise poorly known camera stations with considerable accuracy, ~ 100 m or less, saving considerable time in the field and quite possibly providing locations of camera stations that might otherwise not be found. A more general and more important (but more time-consuming) use of virtual repeat photography lies in its capabilities to generate synthetic pictures from DEM + DOQ, thereby providing intermediate timelines between the time of the original photograph and the present. We illustrate these capabilities with photographs taken at unknown locations during the summers of 1927 and 1928 in the Colorado-San Juan River country of northern Arizona and southern Utah, even today a remote area of difficult terrain, and compare them to those available from the more easily usable Google Earth(1). (1) Use of trade names does not imply endorsement by the U.S. Geological Survey.