Rio de Janeiro State University | E-Foto ProjectDEM Extraction and Orthorectification Module Tutorial for E-Foto

Project Integrated VersionAuthor: Patrícia Farias Reolon Coordination: Jorge Luís Nunes e Silva Brito. Revision: Jorge Luís Nunes e Silva Brito and Marcelo Teixeira Silveira.

DEM Extraction and Orthorectification Module TutorialE-Foto Free Software

Introduction

The DEM acronym means, in the context of the E-Foto Project, “digital elevation model”.

It is a digital surface model based on altitudes.

An orthoimage is an image rectified under the orthogonal perspective. This means that

projective rays are paralel and the point of view is set up to infinity. Therefore, an

orthoimage does not possess distortions due to relief and parallax. However, imagery

acquired by conventional cameras follows a conical or central perspective and cannot be

taken as a source of secure photogrammetric data, since they possess errors caused by

camera rotation and terrain shifts that are inherent to a conical perspective system.

Therefore, such images need to be orthorectified. More detail on this subject can be found

in (Coelho Filho e Brito, 2007).

This module extracts digital elevation models from a set of imagery belonging to a

photogrammetric project. It uses as source a set of images pertaining to a photogrammetric

project. Then, it extracts a digital elevation model for a section of the imaged terrain. From

such model, it is possible to generate a corresponding orthoimage. e-foto uses the

differential rectification method.

The DEM module is ran after the phototriangulation one, from which it is possible to obtain

interior and exterior orientation parameters for each project. Such parameters must be used

for the DEM extraction and orthorectification.

The DEM extraction process is divided in two parts:

1st Part – Obtain homologous points for each stereopair that belongs to a photogrammetric

project. If there are, for instance, three images, then there will be two pairs: images 1 and 2,

and images 2 and 3. Afterwards, extract and identify homologous points and calculate their

coordinates according to the object’s (in this case, the terrain) spatial reference system [X,

Y, Z].

2nd Part – Execute the interpolation of such points in order to generate a regular grid, if the

user so wishes.

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Presentation

After running the e-foto software, an opening screen will show up, as shown in image 1. On the main menu, the following options are available: Project, Excecute and Help.

Image 1 – e-foto’s opening screen

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Starting DEM extraction

Step 1: In order to run the DEM extraction, first go to Project Manager, select the “project” option and click on “Load File”.

One window will show up, allowing the user to navigate through directories until s/he finds the folder where the photogrammetric epp file is located. This file should contain all necessary parameter values for both the interior and exterior orientations of the project images.

A window will show up, which will allow the user to browse through directories until s/he finds the folder where the photogrammetric epp file containing interior orientation and exterior orientation parameters is located, according to image 02. For this example, the UERJ_io-eo.epp file will be used.

Image 2 –Photogrammetric Project’s Module screen showing project from which the DEM will be extracted

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Example 1: Extraction of a DEM from 3 project example images taken over UERJ’s Maracanã Campus. These images can be found on the following web address: <http://www.efoto.eng.uerj.br/en/data-and-examples>.

Step 1: Let us start with the automatic extraction of a DEM. On the menu, pick the option “Execute” and, after it “DEM-Extraction”, according to image 03. You can find more details on methods and parameters of DEM automatic extractions in (Silveira, 2011), a pdf faile which comes with this booklet.

Image 3 – Automatic DEM extraction screen

Step 2: Step 2: Adjust automatic extraction parameters. Image 4 shows some tabs. Pick the “Automatic Extraction” tab. Here is a brief explanation about each parameter.

Image radiometric correction >> This option is related to grayscale image correction. The software shows the following options: No corrections / Histogram Equalization / HistogramMatching. The default one is ‘histogram matching’.

Matching method >> This option allows the user to select the method to perform automatic measurement of homologous points. The options available are Cross-correlationand Least-Squares Matching. Do check Least-Squares Matching for this example.

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Region Growing step >> This option allows you to choose the search region with a variable called “step” and defined in pixels during seed generation. The default option is 3 pixels.

Image downsampling >> This option allows reducing the images’ geomatric resolution for the automatic extraction of the DEM. Let us use for this example an image downsampling of 4, which means image resolution will be reduced by four.

Use pairs >> This allows the user to pick two pairs of images. The options are: all pairs (for all photogrammetric project stereopairs), images 016 and 017 or images 017 and018. For this example, pick all available pairs, i.e. ‘all pairs’.

Matching time >> Shows execution time in seconds

Eliminate bad points >> The option allows the removal of points which, after statistical analysis, are proven not to match the other points of the set.

Image 4 – Window with DEM parameters.

Step 3: After adjusting the parameters, press the DEM Extraction button to start the automatic extraction according to image 5. Notice that a status bar will show the extraction

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progress for the first pair of images. Afterwards, the extraction happens for the second pair, according to image 6 as shown.

Image 5 – Automatic DEM extraction for the first pair of images.

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Image 6 – DEM extraction for the second pair of images.

Step 4: After the DEM extraction, let us open the Matching Accuracy Histogram panel. This panel shows the accuracy of the automatic extraction in order to verify if correlations were good or bad. The closer to 1, the more perfect it is. Usually, the highest amount of correlation is located between 0.7 and 0.8, as shown in image 7. Notice how this range covers the majority of points.

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Image 7 – Histogram Analysis for an Automatic DEM Extraction

Step 5: It is possible to save the list with matching points. In order to do that, access Load / Save options and provide the following parameters shown in image 8.

DEM extraction file format (all as text files):- ASCII full (2D + 3D) >> all list and object type information (sequential pair number, pair id, left and right image id, X, Y and Z object-space coordinates, correction value for homologous points and header); - ASCII 2D points >> 2D points and their line and column left and right image coordinates, with no header and no footer;- ASCII 2D points (BLUH) >> the same pairs of points described above, but organized according to the System BLUH (2012) format, with header and footer, plus a pair sequential number;- ASCII 3D points (no index) >> X, Y and Z coordinates of each point in the object space coordinate system, without index, header or footer;- ASCII 3D points (index) >> Sequential index number, X, Y and Z coordinates of each point in the object space coordinate system, without index, header or footer.Grid file Format: (this will be explained later, when it's time to talk about orthorectification)- Binary E-Foto DEM Format;- ASCII 3D points.

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Image 8 – Load/ Save options for image pair homologous points file and for object-space DEM coordinates.

Step 6: You must choose one of those file formats to save the image, then go back to the DEM Extraction panel and click on the “save” button. The window shown in image 9 will show up. The saved file can be reloaded, modified and saved again.

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Image 9 – Saving DEM values for the DEM sub-module

Step 7: The generated model must be examined for the section of the terrain enclosed by the user-defined geographic area, according to image 10. That way, it is possible to identify areas with “holes” (areas where homologous points extraction failed) and add more seeds by using the seed editor, so there'll be a better region coverage filling the gaps that didn't get automatic correlation measurements. On image 10, white lines show superimposition limits for the three example images.

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Image 10 – DEM graphic extraction result.

Step 8: For individual point editing, the seed editor window should be opened. To do that, click on the “seeds editor” button of the DEM Extraction screen, then the window shown on image 11 will show up. It'll show the following options.

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Image 11 – Seeds Editor window

Seeds Editor toolbar showing button functionalities when selected:

MARK button, which allows a seed to be marked on each image.MOVE button, which allows the image to be moved in the viewing area.ZOOM button, which allows the image to be enlarged by using the mouse's left button, dragging and dropping it to create a selection window.ANTIALIAS button, which allows a “blurred” focus to be made, removing temporarily the pixelated aspect of the enlarged image.EQUAL MOVEMENTS button (has to be tested again, because it's not responding correctly at the moment).EQUAL SCALES button, which allows zoom to happen on the same proportion for both images (left and right).FIT LEFT button, for left image screen adjustment.FIT LEFT button, for right image screen adjustment.FIT BOTH button, which adjusts both images to the screen, at the same time.OVERVIEW button, which expands the Detail View window and hides the overview window.NEAR VIEW button expands the overview window and hides the detail view window. defines zoom size in DetailView window

Other edition options:

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Allows the user to pick an image pair for seed editing.Seed position related to the total number of seeds without showing co-related pairs.

Allows the display of all pairs with automatically measured homologous points.

Loads all previously saved seeds.

Saves edited seeds.

Adds a new seed.

Removes seed(s).

Accepts updates and leaves the application.

Leaves the application without saving.

Step 9: Now let us interpolate a regular grid, by using a set of points automatically extracted. To do so, click on the “Interpolation” tab and pick the available options, according to image 12.

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Image 12 – Interpolation edition window.

Grid Interpolation source: Automatic Extraction / Manual Extraction / Both.This lets the user choose automatic extraction, manual extraction or both methods. Let us use automatic extraction for this example.

Grid Interpolation method: These are the available options for interpolating the existing set of points. They are: Moving average / Moving surface / Trend surface /Nearest point. Let us use “Moving average” for now.

Grid area: Min and max X, Y from points. (This option allows the user to automatically define the area from extracted points) / User defined (the user can define such area manually, in case s/he wants to reduce or enhance the grid currently in use. When this option is chosen, only the “Grid user's area” option is available). In this case, let us opt for the automatically defined area.

Grid resolution: Grid resolution (in meters). Default value is 1 x 1 m.Obs.: In the Moving Average Panel, there are some interpolation methods parameters. One important parameter is “Distance”, because it points out to the maximum distance to be taken into account from a specific cell of the grid, for interpolation calculations. It is suggested to use 10, which will allow a good viewing of the interpolated points set.

Step 10: For executing the interpolation, click the “Interpolate DEM to grid” button and the process will start as shown in image 13. Notice there's a status bar which will show the algorithm's progress. Image 14 shows the final results.

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Obs.: As shown in image 9, which shows Load/Save options, it is necessary to choose possible file formats to save the DEM and Interpolation results.

Image 13 – Interpolation progress window.

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Image 14 – Interpolation results window.

Step 11: After the automatic DEM extraction and interpolation of points obtained arranged as a grid, the user can evaluate the DEM quality according to a test, as depicted in image 15. Click on the DEM Quality panel to get to run this test.

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Image 15– DEM sub-module quality evaluation.

The user should provide a certain amount of reference 3D points which will be compared to the DEM. This test compares each Z coordinate value from a reference point to its Z grid value, which is obtained through X, Y bilinear interpolation. The difference between both values shows the altimetry error found for each analysed point.Two types of text files can be provided:Point list (X, Y, Z) >> is a reference point list. In this case, each line contains X, Y and Z coordinate values separated by tabs. This list can be created from reference points (for example, control points) or from 3D points measured by another photogrammetric workstation and converted to text format; andE-FOTO Features (S.P. 1.65) >> Points and lines measured by e-foto's Stereoplotter 1.65.

The example depicted above has a mean grid error of 1.62 m.

Step 12: After having performed Interior Orientation, Exterior Orientation, DEM Extraction and interpolation, it is possible to run the Orthorectification sub-module. If the process was done in different steps, all that is needed is to load saved files (in this case, the photogrammetric project file after the phototriangulation and the DEM file, which had just been created). After that, go to e-foto's Project Manager screen and pick the option “Orthorectification” under the Execute menu. Image 16 shows this process.

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Image 16 – Opening the Orthorectification function.

Obs.: Despite being an operation done independently from the DEM extraction, it is explained as part of the same process because it needs a DEM as an input file. This DEM, however, can come from either e-foto or another software capable of generating a compatible file.

Step 13: Open the file created by the DEM Extraction module. This should be a “dsm” file (which is a file that has already been through the interpolation process). The orthoimage's resolution will be automatically filled, according to what was saved during the DEM extraction. For this example, let us choose the “bicubic” interpolation method. Click on “Run Ortho-Rectification” to start the process. A screenshot of this screen can be seen on Image 17.

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Image 17 – Ortho-rectification.

Step 14: After running the ortho-rectification, a window will show up, depicting the superimposed images according to Image 18.

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Image 17 – DEM and Ortho-rectification final result.

BundLe Block Adjustment Universtiy of Hannover, System BLUH, Disponível em < http://www.ipi.uni-hannover.de/260.html?&L=1>. Acesso em: 18 Jan. 2012.

Coelho Filho, L.C.T., Brito, J. L.N.S. Fotogrametria Digital, Eduerj, Rio de Janeiro, p.151, 2007.

Schenk, T. Digital Photogrammetry. Terra Science, v1, 428 p., 1999.

Silveira, M. T. Consideracoes Tecnicas sobre o Submodulo de Extracao do MDE da Versao Integrada do e-foto (versao educacional). Rio de Janeiro, 2011.

- End of tutorial -

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