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The Rio de Janeiro State University - UERJ Laboratory of Photogrammetry and Remote SensingThe E-Foto projectThe Extraction of a Digital Surface Model (DSM) Authors: Patricia Farias Reolon and Lia de Souza e Simões FigueiredoRevision: Jorge Luís Nunes e Silva Brito

Introduction

In the context of the E-Foto project a Digital Surface Model (DSM) stands

for a cloud of points which ground 3D coordinates (X,Y,Z) or (E,N,H) are

automatically computed by digital photogrammetry. Such a process is

computationally intensive. It involves a set of overlapping images forming a

block of photographs, the knowledge of both interior and exterior orientation

parameters of each of those images, the automatic finding of conjugate or

homologous point pairs in their overlapping areas, and the computation of the

space-intersection algorithm for each pair of conjugate points, thus leading to

the resulting cloud of points, whose ground coordinates are calculated.

The DSM Extraction module of the e-foto software can be executed

when you already have the parameters of the interior and exterior orientation for

all images in your project. From the DSM model it is possible to generate an

ortho-image or an orthomosaic from the surface area covered by a DSM.

The DSM extraction process is divided into three or four steps:

I. Manual configuration of automatic extraction algorithms and their

respective parameters;

II. Automatic searching for the corresponding (homologous) points of each

stereo-pair belonging to a photogrammetric project. If, for example, we have

three images in our project, we could have two stereo-pairs: images 1 and 2,

and Images 2 and 3. Then, we have to extract and to identify the homologous

points and to calculate the value of its coordinates in the object space (X, Y, Z)

or (E,N,H), depending upon the desired ground coordinate system you have

chosen in your project creation and management module;

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III. Manual edition of the DSM, by monoscopic measurements of conjugate

point pairs in problem areas, where the automatic measurements may have

failed; and,

IV. Optionally you may execute the interpolation of a regular grid from the

cloud of points that forms the DSM. This is an optional step, which is a pre-

requirement to generate an ortho-image or an orthomosaic.

The DSM Extraction

After running the e-foto free software it will appear its start up screen, as

shown in Figure 1. In the main menu you must choose the option Load File or

Last Project to load the file containing the interior and exterior orientation

parameters previously calculated. With your project already loaded you must

choose the option Execute and then DSM Extraction or press the keys

CTRL+D simultaneously.

When the DSM extraction module is opened you will see the screen as in

Figure 2.

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Figure 1 – The e-foto start up screen.

Figure 2 – The start screen of the DSM Extraction Module

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In the Automatic Extraction Options (Figure 3) tab you will see the

following parameters to be adjusted, and a brief explanation of them.

Figure 3 – The Automatic Extraction tab and its Options

Image radiometric correction: it concerns to the correction of image

gray levels. Possible options are: No corrections / Histogram Equalization /

Histogram Matching. The default option is the method “histogram matching”.

Matching method: this option allows the user to select the method to

perform automatic measurement of of homologous or conjugate point pairs. The

options are: Cross-correlation and Least-Squares Matching. We have used the

“Least-Squares Matching” method in this example.

Region Growing step: Delimitation of the searching area with the

variable "step" defined in pixels in the seed insertion process. The default

option is 3 pixels.

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Image downsampling: this variable reduces the geometric resolution of

the images for the implementation of automatic extraction of a DSM. We have

used in this example an “image downsampling” of 4, that is, reducing at four

times the resolution of the original images.

Use pairs: this is the option for choosing the image-pairs in your project.

The options are: all pairs (for all pairs of photogrammetric images in use in your

project), or any specific image-pair. We will use the option “all pairs” in our

example.

Matching Time: it shows the run time of the DSM extraction, in seconds.

Eliminate bad points: this option is used for removal of the stitches,

which, after statistics analysis, are at odds with the other points of the set.

After setting the parameters, we must choose the DSM Extraction Panel

button to start the execution of the automatic extraction algorithm, as shown

in Figure 4. The status bar of the extraction progress indicates that the extraction

is being performed for the first pair.

After the execution of the DSM extraction, we will observe the Matching

Accurary Histogram panel. This is the accuracy of the automatic extraction, to

check the quality of the correlations. The number 1.0 indicates the perfect

matching accuracy, ie, the closer the value is to 1.0 the better the performance

of the process. Generally the value is between 0.7 and 0.9 of accuracy, as can

be seen in Figure 5.

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Figure 4 – The DSM extraction for the first pair of images

Figure 5 – The matching accuracy histogram

Now we can save the list of obtained matches. For doing that we must go

t o Load/Save options tab (Figure 6). You can choose between the following

parameters for the DSM extraction file format:

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ASCII full (2D + 3D): it contains all the information about the points,

such as: sequence number; the pair id; the id of the left image and of the right

image; column and row of the left image; column and row the right image;

X, Y and Z or E, N, H of the points in the ground reference system.

ASCII 2D points: coordinates of points in the 2D image-space.

ASCII 2D points (BLUH): the same pairs of points described above only

in compatibility with the standard format of application software BLUH, with

header and footer, plus the sequence number (index) of the conjugate point-pair;

ASCII 3D points (in the index): records only the coordinates X, Y and Z

or E,N,H of the point in the ground reference system.

3D points ASCII (index): pair sequence number (index), coordinates X,

Y and Z or E,N,H of the point in the ground reference system.

For the Grid File Format you can choose between two options:

Binary DSM Grid, or;

ASCII 3D points.

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Figure 6 – Parameters of the Load/Save options

After configuring the file formats to save your DSM extraction or Grid, you

must click on the save button . A window will be opened where you can name

your DSM. Later this file can be loaded, modified and saved again.

You need to visually inspecting the generated DSM for the interest area of

the surface, as in Figure 7. From this observation we can identify areas with

“holes” in the automatic extraction of homologous point-pairs. Thus you can

manually launch (by monoscopic measurements) new seeds in these areas

through the seed editor, to obtain a better coverage of the region that has not

been fully explored by the region growing algorithm. The regions that appear in

black are the uncorrelated areas. In figure 7 we can also observe the white lines

that indicate the limits of the superposition of the images.

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Figure 7 – Results of the Automatic DSM extraction and the limits of each stereoscopic model

(white lines)

To edit the points, we must open the seed editor window. For doing that,

you should click on the button in the Editors panel on the DSM Extraction

window, and it will open the seed editor as shown in Figure 8. You can see the

options for the seed editor in table 1.

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Figure 8 – The Seed editor Window (by means of monoscopic measurements)

Table 1 – Functions of the buttons on the Seed Editor

Button Function

MARK: allows the user to mark a seed in each image.

MOVE: allows the user to move the image inside the view window.

ZOOM: allows the display magnification by using the left button of the

mouse and drag it, creating a zoom area on the screen.

ANTIALIAS: allows the user to do a blurred focus removing,

temporarily, the square form of the pixel. By clicking again, the pixel

goes back to its normal viewing.

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Button Function

EQUAL MOVEMENTS: makes both images to perform equal

movements.

EQUAL SCALES: allows the user to zoom both images in the same

scale proportion.

FIT LEFT: fits the left image in its window.

FIT RIGHT: fits the right image in its window.

FIT BOTH: fits both images simultaneously in their windows.

OVERVIEW: allows the expansion of the Detail view, hiding the

overview window.

NEARVIEW: expands the overview window, hiding the detail view

window.

DETAILVIEW ZOOM: defines the zoom of the detail view window.

Table 2 shows the additional functionality and options of the seed

editor:

Table 2 – Additional functionalities of the seed editor

Button Function

Allows you to choose the pair of images where

you want to add one or more seeds.

Position of the seed regarding the total number

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Button Function

of seeds without showing the correlating pairs

Allows the exhibition of all pairs of homologous

points automatically measured.

Loads the previously saved seeds.

Saves the edited seeds.

Adds a new seed.

Removes the seeds.

Accepts the update and exits.

Exits without saving the updates.

Now we will do the interpolation of a regular grid from the cloud of

points automatically extracted. To do this, we must go to the Grid Interpolation

tab and edit/choose the available options, as shown in Figure 9.

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Figure 9 – The Grid Interpolation tab

These are the available options for interpolation of a GRID from a cloud of

points::

Grid interpolation source: The options are: Automatic extraction/

Manual extraction/ Both. Uses the data from the automatic extraction, manual

extraction or both, respectively.

Grid interpolation method: Those are the interpolation options for the

cloud of points. The options are: Moving Average/ Moving Surface/ Trend

Surface/ Nearest Point.

Grid Area: Defines the limits of the Grid to be interpolated, in the ground

reference system. The available options are:

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◦ Min and Max X, Y from points: This option allows that the area could

be defined automatically from the extracted points;

◦ User defined: the user can manually define the area of interest, in the

hypothesis that he/she wants to decrease or increase the grid in use.

When this option is chosen, it is released to editing the option: Grid

user's area, as shown in figure 9.

Grid resolution: Defines the grid resolution in meters. The default value

1.00 by 1.00 meter.

NOTE: Inside the Moving Average panel, we have some parameters of the

interpolation methods. What deserves attention is the distance parameter,

which indicates the maximum distance to consider of the set of points used,

from a given grid cell, for calculating the interpolation of it. We suggest the

value of 10, which allows a good viewing of the points of the interpolation.

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Figure 10 – The Grid interpolation process in progress

To execute the interpolation, we should now click on the button

(Interpolate DSM to grid) of the DSM interpolation panel and the interpolation

process will begin, as shown in Figure 10. Please note that there is a status bar

displaying the progress of implementation. In figure 11 we can see the final

result. Please notice that we have not included any break-line file for

improvement or the DSM in our example. Although being an additional

refinement of a grid interpolation, an user is encouraged to try using this option

for improvement of his/her results.

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Figure 11 - The final result of the Grid Interpolation

After the automatic extraction of the DSM and the interpolation of the

obtained points in a grid, the user can evaluate the quality of the DSM generated

through a test, as seen in Figure 12. To perform this test, click in the DSM

Quality panel.

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Figure 12- The DSM Quality Checking Window.

The user will provide a certain amount of 3D reference or checking points

to be compared with those corresponding, extracted in the DSM. In this test, it

will be compared the values of the “Z” coordinate of each reference point with

their respective “Z” coordinate from the grid, obtained by the bilinear

interpolation of the X, Y coordinate of that point in the grid. The difference of

these values indicates the elevation error found for each point being analyzed.

The results of a DSM quality checking procedure may be saved two types

of files, both in text format, as follows:

Point list (X, Y, Z): It is the list of reference points, where each row

contains the values of the X, Y, Z coordinates of a tab separated point. This list

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can be created from reference points (example: Ground Control Points) or 3D

points measured in other photogrammetric workstation and converted to this

format;

E-FOTO Features: Points, lines and polygons measured in the

Stereoplotter module. (*.spf format)

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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