THE ASSESSMENT OF ORTHOPHOTO QUALITY WITH RESPECT TO THE STRUCTURE OF DIGITAL ELEVATION MODEL

Abstract. Orthophoto is an image which is being corrected geometrically so each object has to be situated on the corrected place consequently. Choosing the best DEM structure with respect to the area topographic is the most challenge which has more important role when dealing with rough surfaces displacements in duration of orthophoto procedures. The Lower DEM resolution makes points density lower and makes the procedure faster but cause to decreasing the product precision in compare to choosing the other one. However if a fine resolution DEM cause to very delicate displacement corrections aside of the other benefits but it makes to appear some undesired visualized errors like as elongation error especially in an areas which are hidden with some obstacles and there are lacks of data in an imaging. For preventing of such error in DEM structure calculation and earning the most benefits, we found and execute some solutions. In other word we answered to this question that what DEM resolution is the best for orthophoto production. In the following we have done some tests. First a dense DEM of a topographic area calculated and edited accurately then its density was reduced in some steps gradually. At each stage the root mean square error (RMSE) of interpolated heights of points which were laid in the distance between the corresponding DEMs pixels has been calculated respectively. Two interpolation methods (Nearest neighbour and Bilinear interpolation) have been used in this test. Decreasing the DEMs density or increasing the pixel size made the amounts of errors high and the rate of this changing dependent on the kind of topography directly. So we divided the area into some reasonable topographic classes then calculated our results for each class separately. The result of each strategy compared with each other and presented in both numerical tables and some illustrated images.

Because of the relation between horizontal precision of orthophotos which are existed in the standard producing instruction and the accuracy of the DEM which are mostly related to its density, the suitable resolution for producing different scale orthophotos at each kind of topographic class have been calculated from mentioned methods consequences and shown as a final result.