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Digital satellite images and aerial photographs play an important role in general mapping, as well as GIS data acquisition and visualization. First, they help provide a solid visual effect. Many people are more able to put spatial concepts into perspective when they see photos. In addition, the secondary and perhaps more vital role is to provide a basis for gathering spatial information. Examples of this are features such as roads, vegetation, and water. Before this information can be gathered in a manner that is useful for a mapping or GIS system, the satellite image data or aerial photographs must be prepared in a way that removes distortion from the image. This process is called orthorectification. Without this process, you wouldn't be able to do such functions as make direct and accurate measurements of distances, angles, positions, and areas.
What is Orthorectification?
The topographical variations in the surface of the earth and the tilt of the satellite or aerial sensor affect the distance with which features on the satellite or aerial image are displayed. The more topographically diverse the landscape, the more distortion inherent in the photograph.
Image data acquired by airborne and satellite image sensors are affected by systematic sensor and platform-induced geometry errors, which introduce terrain distortions when the image sensor is not pointing directly at the Nadir location of the sensor.
A indicates Nadir, while B indicates the Nadir Line
Terrain displacement can be hundreds of meters. For example, if the IKONOS satellite sensor acquires image data over an area with a kilometer of vertical relief, with the sensor having an elevation angle of 60° (30° from Nadir), the image product will have nearly 600 meters of terrain displacement. Additional terrain displacement can result from errors in setting the reference elevation. Low elevation angles of images, imperfect terrain models, and variability of sensor azimuth and elevation angles within an image limit accuracy potential if image orthorectification is attempted. For this reason, when new high resolution satellite image data is acquired over rough terrain, high elevation angles of the sensor is required.
Orthorectification (animated): Please click on the image to view.
Digital Elevation Model (DEM)
In order to accurately remove the image distortions, a digital elevation model (DEM) is used to perform image orthorectification. The required DEM is generated by feature extraction from high resolution stereo satellite scenes acquired by the GeoEye-1, Worldview-2, Worldview-1, Pleiades-1A, IKONOS, Pleiades-1B, or Aster satellite sensors, and stereo aerial photography.
For many international products where DEMs are not available with a posting interval of 90m, Satellite Imaging Corporation utilizes the Shuttle Radar Topography Mission (SRTM) 90m DEM data set for the orthorectification of satellite image data. When higher mapping accuracy standards are required, the DEM is extracted from existing topographic maps at an acceptable scale, or acquired by stereo satellite image data, providing a DEM posting and accuracies standard at the 5-6m level when acquired with high resolution stereo satellite sensors. At this accuracy standard, sufficient GPS derived ground control points (GCPs) are required. Other remote sensing techniques are also utilized, such as radar interferometry or LiDAR.
When vector data is to be extracted from satellite or aerial image data by raster-to-vector translation, Satellite Imaging Corporation performs the orthorectification of the remotely sensed image data and rectifies all digital images of environmental, geological, topographic, or any source maps that will be used in the GIS mapping environment.
For orthorectified sample images, please see our image gallery.