Using photogrammetry to study subsidence caused by gypsum dissolution around Ripon

This report describes the work undertaken in the Remote Sensing photogrammetry section of BGS to produce digital stereo models, digital elevation models (DEMs), orthophotos and associated products covering an area around Ripon. This report details how photogrammetry techniques were used to remotely identify known, unknown and potential gypsum subsidence features around Ripon from two dates of aerial photographs. Initially it was hoped that the DEMs for 1986 and 2003 could be compared to create a difference map. When the two data sets were compared there were errors and it is believed the reasons for this included one or more of the following: 1. The 1986 camera calibration certificate did not have the full details required to set up the ImageStation project. 2. The 2003 photos were scanned from prints. Stretch can occur in prints and cause errors. 3. It was hoped to use the same control points to create fixed points in both projects. Control positions from the 1986 project were either obscured or the ground had been altered in the 2003 photos. The 2003 project also covered a larger area and extra control points were required that fell outside the 1986 project area. 4. The bulk orient process was run on the final models. This creates a DEM for each stereo pair and does not use the overall solution to create the DEM. Errors can occur where models overlap. 5. There was insufficient sidelap on the two 2003 runs. Tie points were created manually. 6. The smoothing in the elevation points creation process was different in each project as some models did not run at the lower setting.