The Effect of Densification and Distribution of Control Points in The Accuracy of Geometric Correction

Abstract

Geometric distortions are inevitable in aerial images due to many factors, such as the curvature and rotation of the earth and the motion of the scanning system. Ground control points (GCPs) are important features used in non-parametric approach for aerial image rectification. The most common approach for geometric corrections is the use of polynomials. . It depends on selection of several clearly discernible points, called Ground Control Points (GCPs), in the distorted image, and map them either to their true positions in ground coordinates (e.g. latitude, longitude) measured from a map, or to geo referenced image (corrected before), coordinates of corresponding points, through a mathematical transformation, that will convert the raw image coordinates into the desired coordinates. The rectification results are invested by using the total Root Mean Square Error (RMSE).

In this study two tests were carried out. Firstly, the distribution of three, six and ten control points were investigated .the second test examines the densification of control points and their effects in the accuracy of geometric correction. The bad location and bad distribution of the selected GCPs lead to an increase in the average RMS error value of correction of an image which should be taken into consideration; for example, the results obtain from the densified GCPs used to adjust the aerial   when GCPs had been selected in one line we did not achieved an image. The results obtained from the densified GCPs used to adjust the aerial photographs showed approximately the same results those in the first case.  Therefore, the use of the three GCPs is sufficiently enough to adjust the aerial image. The thing that preserve time and money.