In the areas of GIS data acquisition, visualization and general mapping, digital satellite imagery and aerial photographs have a significant place.
Photographs obviously provide a solid visual effect. Imperceptible spatial concepts are more clearly understood by viewing the photographs. These are not photographs taken by ordinary cameras. These are very professional high-end cameras with higher zoom and clarity.
Another important role of these photos is to provide a foundation for collecting the spatial information needed. Data in the form of the satellite images or aerial photographs must be taken without any distortions, if you need to gather information useful for a mapping or GIS system in the case of roads, marine forms or vegetation.
This process of correcting the distortions of a satellite image of aerial photograph is called orthorectification. This process allows you to make direct and precise measurements of areas, distances, angles, positions etc.
Why an aerial photograph needs correction?
The main challenge of an aerial photograph compared to a non-aerial photograph is that an aerial photograph needs perspective correction. An aerial photo is usually captured at an angle to the object being photographed. Here, the perspective of the photograph will be incorrect with near objects compared to distant objects. By perspective correction, the objects in the image and the real world will have equal size.
Process of Orthorectification
As topographical variations in earth’s surface and the tilt of the satellite or the aerial sensors can affect the display of the features on the satellite or aerial image with regard to their distance. The image distortion will be more as the topographical diversity of the landscape is more.
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.
There can be hundreds of meters of terrain displacement. Image data over an area with at least a kilometer of vertical relief, with the sensor having an elevation angle of 60° (30° from Nadir), the image output will have nearly 600 meters of terrain displacement.
Errors in setting the reference elevation can cause further terrain displacement. Other than this, low elevation angles of images, imperfect terrain models, and inconsistency of sensor azimuth and elevation angles within an image alters the accuracy potential if image orthorectification is attempted. To overcome this defect, high elevation angles of the sensor is used with new high resolution satellite image of irregular terrain.
Creation of Digital Elevation Model (DEM)
For the accurate removal of the image distortions, a digital elevation model (DEM) is used to make image orthorectification. The requisite DEM can be produced by semi-automatic DEM extraction software from stereo satellite scenes obtained by the QuickBird, IKONOS, SPOT-5, or ASTER satellite sensors, and stereo aerial photography.
In situations where higher mapping accuracy standards are required, the DEM is extracted from the already existing topographic maps with reference to a standard scale or collected using stereo satellite image data.
To obtain this accuracy standard, adequate GPS-derived ground control points (GCPs) are necessary. Other remote sensing techniques can also be used such as radar interferometry or LIDAR.
When a particular vector data needs to be extracted from satellite or aerial image data by raster-to-vector translation, the process of orthorectification of the remotely sensed image data can rectify all digital images of geological, environmental, topographic or any other source map which will be used in the GIS mapping environment.
Regards
SBL GEOMATICS
Article by : RARIMA N S
