TRACKING DINOSAURS USING LOW-ALTITUDE AERIAL PHOTOGRAPHY AT THE TWENTYMILE WASH DINOSAUR TRACKSITE, GRAND STAIRCASE ESCALANTE NATIONAL MONUMENT, UTAH

The Twentymile Wash Dinosaur Tracksite (TWDT) is located in Grand Staircase-Escalante N.M. approximately 25 km southeast of the town of Escalante, Utah. At this site over 300 dinosaur footprints are preserved in the Middle Jurassic Entrada Sandstone. The main track-bearing horizon crops out at the top of a 400 m-long east/west trending bench and is nearly 2 m thick. Erosion has smoothed this bench so that it gradually slopes to the south, exposing tracks and trackways from multiple levels. Stratigraphically this site is approximately the same as the megatracksite at Moab, Utah. The tridactyl tracks range in length from 15 to 45 cm and are assigned to theropod dinosaurs. In addition, unique sauropod tracks and traces are also known from this site. Track preservation occurs as darker sediment infillings and as light and dark underprinted sand laminations. At least 30 theropod trackways are present that contain from 2 to 30 steps. Due to the number and extent of tracks and trackways, the variable mode of preservation, and the exposure of multiple levels of tracks, the TWDT has a very complex and intriguing story waiting to be unraveled.

To aid in unraveling this intriguing story and to preserve the value of this unique paleontological resource, the BLM is utilizing photogrammetry. Aerial photography at a scale of 1:3,000 and 70 mm-format images taken from a low-altitude Camera Blimp System (CBS) are being used to document the site. The CBS consists of a 6 m-long, helium-filled blimp, which is capable of lifting a camera to 76 m above the ground. The onboard camera is positioned from the ground and can be oriented to acquire photographs that are near vertical or oblique to the surface. The aerial photos will be combined with ground control coordinates collected utilizing very precise GPS locations to produce an orthorectified digital photomosaic of the track-bearing surface. This photomosaic will be spatially correct and can be used in a GIS environment to digitize the locations of individual dinosaur tracks at the site. The mosaic can also be used in the field as a base map to collect other information about the tracks and the surface in which they are preserved. This technology will aid the understanding of interactions between dinosaurs and their environment during the Middle Jurassic.