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The Rio de Janeiro State University - UERJ Laboratory of Photogrammetry and Remote SensingThe E-Foto projectThe Generation of a Digital Ortho-imageAuthors: Patricia Farias Reolon and Lia de Souza e Simões FigueiredoRevision: Jorge Luís Nunes e Silva Brito

Introduction

An ortho-image is an image in an orthogonal perspective, when the

projective rays are parallel and the viewpoint is at the infinite. More details about

this subject can be found in the technical literature about ortho-rectification and

ortho-image generation.

The e-foto software uses the method of differential rectification for

generating an ortho-image. Such a method requires a DSM model, from which it

is possible to generate its corresponding ortho-image. The differential

rectification algorithm starts from the ground-space domain back into the image-

space, what is so named a “bottom-up projection approach”. This bottom-up

procedure starts from each center of a grid cell interpolated from a DSM, which

ground coordinates are known, and projects it back into the image-space, by

means of the collinearity equations. Therefore both interior and exterior

orientation parameters of each image, along with the DSM, are a pre-

requirement for the generation of an ortho-image or an orthomosaic.

The Ortho-rectification Process

After making the Interior Orientation, the Exterior Orientation (by spatial

resection or by phototriangulation), the automatic extraction of a DSM and the

interpolation of a grid file, we can now run the Ortho-rectification. If the process

was done in different steps, just upload the previously saved files. Then, on the

main screen of the e-foto, in the menu Execute, select Ortho-rectification or

press the CTRL+R simultaneously.

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To run the Ortho-rectification, we must locate the previously saved DSM

file (extraction and interpolation), with the extension *.dsm. The ortho-image

resolution will be automatically filled on the Ortho-image resolution field, as

saved during the process of the DSM execution. In the Ortho Image File

Format you can choose in which format you want to generate your ortho-image

(E-FOTO ortho-image or GeoTIFF). To start the process, click on the button

(Run Ortho-rectification). This can be seen in Figure 1.

Figure 1 – The ortho-rectification screen

After running the Ortho-rectification it will open a window displaying the

superimposed images, as shown in figure 2.

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Figure 2 – Result of ortho-rectification displaying the resulting orthomosaic

After the orthorectification you can evaluate the quality of the ortho-image

using the Ortho-Image Quality Tool, to do that you can click on the

button. In Figure 3 we can see the window of the Ortho Image Quality Control

Tool.

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Figure 3 – The Ortho-Image Quality Control Tool

Now, we will load a file with a certain amount of reference points to be

measured and compared with the ortho-image. In Figure 4 we can see 12 points

plotted in the ortho-image to compare with their original location.

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Figure 4 – Points loaded in the ortho-image

We can test the quality of a specific point compared to the sketch. In

Figure 5 below, with the Zoom tool we approach the point here indexed as "5" in

the table. Now we need to seek its location in the sketch to check if is well

referenced in the ortho-image.

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Figure 5– Zoom in to Reference point 5 to checking the ortho-image positional quality

Now we will compare the location in the image with the actual location of

the point on the ground. To do that we should zoom in to the point and click in

the button and them in the correct position of the point in the Ortho-rectified

image. In the table which is depicted in the bottom of the ortho quality checking

will appear the X and Y coordinates in the image and its corresponding

coordinates in the ground reference coordinate system. (Figure 6).

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Figure 6 – Ground Coordinates of Quality checking point 5 taken from the ortho-image and

directly measured in the terrain

When you finish the measurements of all the points you want, you can

delete the remaining points by selecting the point to be eliminated and then

clicking in . To proceed with the calculation of the errors, click on the

button. In Figure 7 you can see the measured point, the errors and the

deviations.

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Figure 7 – Errors in Ortho-image quality checking, their statistics, highlighting the planimetric

error calculated for point 5

Any contribution for correcting and improving this tutorial is very

welcome. Please send your comments and/or suggestions to the e-foto

team at http://www.efoto.eng.uerj.br/forum

<END OF TUTORIAL>

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