GPS-Based Stereo Photogrammetry in VORTEX2

Traditional photogrammetry techniques in severe storms research involve taking a picture at a known location and using time-intensive surveying to identify landmarks in the image to map pixels to space. For VORTEX2 we developed a new approach to stereo photogrammetry based off of highly accurate differential GPS technology commonly used in professional surveying. The essence of the approach is for a Digital SLR (DSLR) camera and a “base” GPS to be fixed to a specialized mount attached to a tripod. A second “rover” GPS is deployed in the field of view of the camera. The GPS observations are post processed using GPS software to calculate highly accurate positions to within millimeters. These positions are used to calculate a highly accurate azimuth (to within ~ 0.1 degree) between the base and rover GPSs. Because the base GPS antenna is positioned over the center of the camera's CCD chip, the location of the rover GPS antenna in the image can be used to calculate a reference azimuth in the image. In addition to the GPSs, two digital protractors attached to the camera and base GPS mount are used to calculate pitch and roll. A hose filled with water is also used to identify horizon level in the image for redundancy in calculating pitch and roll. After correcting the image for lens distortion, the pitch, roll, camera location, and camera focal length are used to map the azimuths for all the pixels in each image. In VORTEX2 we used two teams to collect stereo observations of cloud features in the rear-flank downdraft area of the storm at regular five second intervals using 12MP cameras capable of interval shooting. The stereo observations of cloud features allows two sets of images to be combined to map cloud features into 3D space using locally developed image processing software. These images are then combined with the high resolution radar datasets collected in VORTEX2 to understand how the precipitation and wind observed by radar relates to the edges of the clouds and other visual features. Some of the data are also being compared to experimental short wave infrared observations taken by the stereo photogrammetry teams. This study provides an overview of the technology, data collection in VORTEX2, and preliminary results of combining stereo photogrammetry with the other high-resolution datasets collected in VORTEX2.