Effect of Densification and Distribution of ground Control points On the Accuracy of Geometric Correction

Abstract

Geometric distortions are inevitable in aerial photographs 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 equations, which depend on the 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 georeferenced 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 evaluated by using the total Root Mean Square Error (RMSE). In this study two tests were carried out: the first, the effect of the distribution of three, six and ten control points was investigated , The second test examined the effect of densification of control points on the accuracy of the geometric correction. The most important result obtain from this study was that 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, when selecting the GCPs selected in one line no image was obtained. The results obtained from the densified GCPs used to adjust the aerial photograph showed approximately the same results as when using 3 point. Therefore, the use of the three GCPs is quite enough for the adjustment of the aerial photograph. The process that preserves time and money.