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.
The geometric quality of the image is very important in the process image processing and map making. The geometric precision of the image will not affect the difference in the shape of objects in images with real objects in the field, impact further is the distortion in the measurement of distance, area, or angle on the image.
Orthorectification is one of the techniques in digital image processing aims to reduce the geometric distortion, the technique is necessary because
Satellite images have geometric distortion (planimetrik) ranges from tens to hundreds meter. This is caused by the satellite image recording process is performed in space hundreds of kilometers from Earth, the satellite vehicle is moving at a position that is not always perpendicular to the surface of the earth. Besides the geometric error also occurred from earth topography effects.
Orthorectification process necessary reference tool geometric data and data DEM (Digital Elevation Model). Reference is often used field measurements, directly using a GPS (Global Potitioning System), or use the image been corrected as Landsat ETM + version 1. The use of DEM data is needed The main areas with high relief or in the recording with an oblique angle. DEM used in this project is the SRTM DEM (Shuttle Radar Topography Mission) 90m spatial resolution of NASA and USGS production. The method used to orthorectification is to use the Rational Function (RPC) with using PCI software, this method is considered the most suitable with Quickbird data caused by Quickbird data already contained in the RPC model consisting of 20 GCP points. The results are orthorectification process satellite images that contain positioning information (expressed in coordinates) are close to the actual position in the field