ANALYSIS AND THE SOLUTIONS FOR GENERATING A TRUE DIGITAL ORTHO 
PHOTO IN CLOSE RANGE PHOTOGRAMMETRY 
M. Shariat 3 *, A. Azizi b , M. Saadatseresht b 
a Dept. of Geomatic Engineering, Faculty of Engineering, University of Tehran, Iran - mashariat@ut.ac.ir 
b Centre of Excellence for Disaster Management, Dept, of Geomatic Engineering, College of Engineering, University of Tehran, Iran 
-aazizi@ut. ac.ir 
KEY WORDS: True Orthophotos, Occluded Areas, Grey Shade Interpolation, Histogram Matching, Mosaic, Close Range 
Photogrammetry 
ABSTRACT: 
Digital orthophoto is an image where the relief displacements and the camera's tilt have been removed. So it is an efficient, 
inexpensive and accurate approach for purposes such as evaluation, analysis or measurement of the objects presented in the images. 
For this reason digital orthophoto has occupied an important place in spatial database in GIS. The traditional orthophoto is based on 
Digital terrain model and it does not include the man-made objects. Therefore, the orthophotos of urban areas do not have 
satisfactory accuracy. True orthophoto generation by using the digital surface model in rectification process and detecting occluded 
areas through the visibility analysis is the only solution. True orthophoto can be produced from close range as well as aerial and 
satellite images. In this paper, to simplify the true orthophoto generation process several close range images of a cubic object are 
used with a regularly spaced grid DSM. In close range Photogrammetry the distance between camera and object is small and as a 
result the occluded areas are extensive. Therefore, the occluded areas detection method must be efficient. In this research work the 
height-based raytracing method is used to detect the occluded areas. To restore the information of the occluded areas the seamless 
automatic mosaicking considering histogram equalization of images is applied. 
1 INTRODUCTION 
Today with increasing importance of GIS, particularly in urban 
areas, the demands for digital orthophotos with high details and 
greater accuracy are growing. Digital orthophotos provide 
spatial and spectral information and have many applications 
such as measurable virtual reality. There is not any relief 
displacement in orthophoto and there are consistent scale in it 
that cause to be similar to the maps. Since in the traditional 
orthophoto production, DTM and aerial photographs of urban 
areas are used and DTM does not contain man-made objects 
such as buildings, bridges etc., and also because of the 
perspective geometry inherent in the images, these features do 
not locate in their true positions and hence a geometrical error 
occur in orthophoto. This problem shows itself clearly if the 
vector data of man-made objects is superimposed over the 
corresponding orthophoto. Moreover, the man-made objects in 
the image, produces occluded areas so that the quality of 
orthophoto deteriorates. Therefore an orthophoto that is 
geometrically complete and accurate is needed. This demand is 
fulfilled with the so called true orthophoto that is produced 
using the DSM. But when a DSM is used the double mappings 
occur. These occluded areas must be filled using the 
neighboring overlap images and a mosaicking procedure is 
required. 
In this paper, close range images of a cubic object are used. 
Four images are taken from different direction by using a non 
metric camera. One image is considered as a master image and 
others are slave images. For recovering the position and 
orientation of the camera during the exposure time some retro- 
reflective targets as ground control points are used. 
The final accuracy of the ortho-rectified image may be 
improved by incorporating self calibration strategy. After the 
computation of the exterior orientation and self calibration 
parameters, the occluded areas are detected using the raytracing 
approach. In the final stage, the image segments are stitched to 
form a uniform image. 
The sections that follow, after a brief review of classical method 
of ortho-rectification, the above mentioned procedures for 
generating true ortho are described and the final result is 
presented. 
2 TRADITIONAL ORTHOPHOTO 
Two points with the same location and different height in the 
object space will project at two different locations in image 
space because of using the perspective projection. Differential 
rectification is the process of removing these relief 
displacements using the DTM, inner and outer orientation 
parameters of image. The rectification can be done in two ways: 
Forward projection and backward projection: 
In forward projection for each pixel of master image, using 
DLT equations, the ground coordinate is calculated. If the 
ground coordinates of the comer points and the pixel size of the 
orthophoto is known, the pixel coordinate in the orthophoto is 
determined. In this way the three dimensional ground 
coordinates of each image pixel is calculated using two 
dimensional image coordinates and hence an iterative process is 
required (Bang, 2007). Moreover, the regularly spaced points in 
the image are projected to a set of irregular spaced points, so 
they must be interpolated into regular points. To avoid aliasing 
problem, the distribution of the regular points in object space 
should satisfy the sampling theorem. 
Figure 1 shows the basic geometry of the forward projection 
approach for ortho rectification.