Digital orthophoto using different DEM approaches

Abstract

The generation of orthophoto from remotely sensed images, such as aerial photos, satellite images and airborne scanning images is an important task for various applications such as its use as a base map in GIS, map revision and update ,or it's use as a planimetric map.which guide us to think about Study the effect of relief on the planimetry .In order to obtain the best resulted Digital orthophoto.This research concerned only with the study of the orthophoto from a scanned aerial photo. Orthophoto is a photo, which have been transformed from a perspective projection to orthogonal projection through the process, which called . differential rectification. The transformed photo is free from tilt and relief displacements, and has the pictorial quality of the photo, further more the geometric quality of map thus it allows for accurate Measurements from photograph because ground features are represented in their true planimetric positions. Orthophoto were produced in the past by optical or electronic means. It was a cumbersome process. The Popular approaches of rectification of analogue photo include fixed line, rotating line, curved area, and plane area. In present, the movement towards the digital photo has revolutionized the rule of Orthophto in Photogrammetric mapping and GIS. So that almost all orthophotos are produced by digital systems to be in a digital form which so called "digital Orthophoto". There are different methodologies for digital photo rectification such as Projective, polynomial and the digital differential rectification, the later is the most prominent one. It requires a Digital Elevation Model (DEM) which is a representation of earth's surface by a set of X, Y, Z) data points, covering the area for which an Orthophoto is produced. As w•ell, orientation elements (Exterior) of original photo. Which means that the digital photo rectification is done by processing each pixel through Photogrammetric equations derived from photo identifiable ground Control points, camera calibration , orientation parameters and the Digital elevation model . A study area is chosen at" Koum Alhager "village at Kafr Alshikh Government DEM data acqusition were done with the following technique:

• Field measurements DGPS associated with extension of ground control points with aerial triangulation • Analytical technique • Digital photogrammetry This was followed by some experiments about the quality of the different DEM to get the best results concerning the digital Orthophoto. The quality of DEM was investigated by using 21 check points coming from a seprate aerial triangulation after that orthophoto was produced using the three different DEM approaches and without using any DEM(DEM=O) this was done with different resampling methods bilinear,bicubic,nearest neighbour. Resulting in twelve cases. The planimetric accuracy of digital orthophoto was investigated using a total 21 check points that were manually collected from an orthophoto and compared. as a first check:The root mean square error generalized for each orthophoto in E and N was calculated(i.e,mx and my ). The residual error calculated by taking the square root ofthe sum ofthe squares ofmx and my. A Fortran program has been introduced to faciliate and speed the calculations steps. a second check, The accuracy of digital orthophoto was evaluated by superimposing with digitized digital vector data of the study area.The mismatch in the location of