Planimetrically accurate images are a necessary pre requisite for any mapping projects. Accurate maps are a basic necessity of the developmental projects like, infrastructure development, mining, master planning, precision agriculture etc. Orthorectification is a process by which a raw image is transformed into a planimetrically accurate image. Often georeferencing is used in synonym with the orthorectification. But in true sense, through georeferencing only x, y corrections and transformation is dealt with while ortho rectification will do a correction on relief displacement caused due to elevations of the objects within the map. So orthorectification is considered as most accurate process by which an image is brought to the real world coordinates by considering correction in all three directions of x, y and z.
Orthorectification is the process of eliminating the relief displacement due to sensor mal functioning, height of the terrain, earth rotation, panoramic distortions etc. The outcome of the orthorectification is a planimetrically true image. It also brings uniformity in scale throughout the image. SBL has the capability to do the orthorectification to a wide variety of images such as satellite images, aerial photographs and UAV images.
The main input for the orthorectification process is as follows
- Near Nadir image
- Rational Polynomial Coefficient (RPC) file or camera calibration certificates
- Digital Elevation Model (DEM)
Orthorectification can be done in a variety of software. SBL uses ortho master module of Inpho for the aerial photographs and satellite images and PIX4D for UAV images. The process involves importing of the image, DEM, external and internal parameterization files in to the model. Parametric approach can be followed in the model where internal and external correction files in the form of GCP files and RPC files are involved. At this step desired projection can also applied in the model. Back ward projection can be applied in the process. The object space X, Y coordinates related to every pixel of the final ortho image is determined. The height Z at a specific X, Y point will be calculated from the DEM or the 3D model and then the X, Y, Z object space coordinates are projected in the original image in order to acquire the gray level value for the ortho image pixel. Interpolation or re-sampling process in the original image is also essential because of the fact that the projected coordinates will not fit accurately at the original image pixel centers. Output cell size and extent will be defined at this step. Output file type can also be specified at this step. Error calculation is generally derived in the form of RMSE report.
Quality assessment of the orthorectified images is one of the activities which SBL perform to ensure highest standard of the output images. Pan sharpening, mosaicking, color balancing, and tiling are the associated activities once the generalized orthorectified output images were generated. Quality assessment will be majorly done through visual inspection of the images. Major checks on the orthorectified images include spatial resolution, edge matching, double feature checking, tonal balance, and pixel dropouts.