Photogrammetric Processes

Photogrammetry by definition means measurement from photographs. Here photographs being taken from an aerial platform like an air craft. These photos taken as stereo images need lots of processing before putting into actual usage. These processes are collectively known as photogrammetric processes. Stereo aerial photographs, ground control points (GCP), GNSS/IMU data and camera calibration certificates are the main input for a photogrammetric project. Photogrammetric services include processing these inputs to derive ready to use geometrically correct images. All these processing will be done in a suitable photogrammetric software platform. Aerial triangulation, bundle block adjustments, and stereo compiling are the preliminary steps of a photogrammtric process. During this process the photographs will be corrected for external as well as internal distortions. External orientation correction can be done using GCPs and tie points. Internal orientation corrections can be done using GNSS/IMU values and camera calibration certificates. Stereo viewing is possible after this stage of the processing. Now the terrain can be visualized in a 3D environment in suitable software platforms. Once the stereo compilation has done, digital surface models (DSM) can be generation from stereo models. Digital surfaces are nothing but visualization of the terrain not only on an x,y place but also with the z dimension in the form of elevation. Digital surfaces derived from the software platforms are with failed areas in many locations. These failures are due to cloud cover in images, shadows, or due to poor automatic compilation. These areas need to be corrected by editing the digital surface models. Corrected digital surface models are devoid of any collapsed areas. Ortho photos and Digital Terrain Model (DTM) can be generated using the digital surface models and stereo models. DTMs are bare earth models depicting the surface of the earth if there is not vegetation and artifacts over it. It is the true representation of the primitive earth surface. Many of the geological and other developmental projects require this sort of 3D models to know the exact and accurate height of the terrain with respect to the mean sea level. Other form of topographical maps such as contour lines, slope map, aspect map etc can be derived using these kind of 3D models. Ortho photos are another product which can be derived from these processes. Ortho rectified photos are geometrically corrected photos. Relief displacement and related distortions will also be corrected during ortho rectification process. These photos were later corrected for collapsed areas and then mosacicked to get a complete view of the area. Seam lines are commonly used for mosaicking of the images. The images thus derived can be color balanced to remove viewing of the seam lines and remove the difference to color saturation from image to image. Later images can be cut into tiles of desired dimension for easy usage. Once these kind of pre and post processing of the images are over, these will be put into use to derive information in the form of 2D and 3D feature mapping. Base mapping, land use land cover mapping and other kind of thematic mapping can be then be possible from these images.