Orthorectification

The production process of raster maps that are geometrically precise is called orthorectification. These type of maps are also called planimetric or orthographic maps. Orthorectification is the process of using a mathematical model and a digital elevation model (DEM) to correct distortions in raw images such as aerial photographs. The process can be termed as orthophoto production. The product, digital orthophoto is a raster photographic image that is combined with differential rectification to remove image displacements caused by camera tilt and terrain relief.

Aerial photographs are useful for providing spatial information, but they usually contain geometric distortion. But, an orthophoto or orthophotograph is an aerial photograph that has been geometrically corrected ("orthorectified") such that the scale of the photograph is uniform, meaning that the photo can be considered equivalent to a map. An orthographic map plots the position of objects after they have been projected onto a datum plane. Orthophotographs have the positive attributes of a photograph such as detail and timely coverage, and the positive attributes of a map including uniform scale and true geometry.

Digital photos are created by scanning an aerial photograph and converting it into a raster image file format. Digital cameras provide the product in raster format itself. Remotesencing products of various satelite sensors can also be used as inputs. A digital elevation model (DEM) is then added as a means of collecting ground points to indicate the changes in elevation. The DEM is a regularly spaced grid of masspoints, each containing an x, y, and z value. A more robust digital terrain model (DTM) can also be used because it includes strategically placed masspoints, dense breaklines, and ridgelines. When control points are identified on the aerial photos, stereomodels are developed and geometric distortions are estimated. The image is rectified and georeferenced using mathematical models of photogrammetry for the removal of tilt, terrain, atmospheric and other digital image distortions. The process is called differential rectification or orthorectification, which is a point-by-point correction of the scale and relief displacements normally resulting from variations in elevation between the aircraft and the topography over which the aircraft flies. Multiple photographs can be analyzed, corrected and mosaiced all at once by a process called “bundle block adjustment”, in which interrelated sets of equations are used to find a globally optimum set of corrections across all photographs. These processes collectively termed as Aerial Triangulation (AT) which serves to orient images in the project to one another and a ground coordinate system.The goal is to solve the orientation parameters (X, Y, Z, omega, phi, kappa) for each image. True ground coordinates for each measured point will also be established. The AT process can be a time-consuming and critical component of the digital photogrammetry workflow. Digital Orthophotos are usually the primary final product derived from the photogrammetric workflow. There are many different specifications for orthos, including accuracy, radiometric quality, output tile definitions, output projection, and output file format as per the requirements of the user. A mosaicking process is usually included in the ortho workflow to produce a smooth, seamless, and radiometrically appealing product for the entire project area. Mosaicking may be performed as part of the orthophoto process directly (orthomosaicking) or performed as a postprocess later.

The requirements of orthophoto generation are: a) Camera calibration report, b) A digital elevation model (DEM), c) A minimum of three ground control points that can be identified on the photograph and d) The digital image to be rectified.

A Digital Ortho image or mosaic can be integrated into any geographic information system (GIS) that can manipulate raster images. Orthophotos can function as a cartographic base for displaying, generating and modifying associated digital planimetric data. A orthophotograph can be used in other applications such as, vegetation, timber management, routing and habitat analysis, environmental impact assessment, emergency evacuation planning, flood analysis, soil erosion assessment, facility management, ground water, watershed analysis etc,. The accuracy and detail provided by orthophotos enable users to evaluate the data for accuracy, make real time modifications to data and generate new files. Digital orthophotography is a major resource being utilized by most of the GIS users. It has become a popular base layer in modern GIS. With the price of disk space dropping and the speed of computers increasing, digital orthophotos are a viable option for building a fully developed GIS and can be utilised for technically specific needs such as planimetric or cadastral mapping; utility data capture and quality control; and accurate project analysis and design implementations. Digital orthophotos can also be used to explain projects and issues to the public because real-world pictures are easier for the untrained eyes to percieve. Proposed changes can be overlaid as vector data. Digital orthophotography can help to monitor constantly changing world.

GIS@SBL is pioneers in orthophoto production, ortho mosaicing and tiling in virtually any raster format and have satisfied clients all over the world. Seamless mosaicing of orthophotos and subsequent tiling is a well apreciated service offered at SBL.