the rio de janeiro state university - uerj laboratory of...

The Rio de Janeiro State University - UERJ Laboratory of Photogrammetry and Remote SensingThe E-Foto projectPhototriangulationAuthors: Lia de Souza e Simões Figueiredo and Patricia Farias ReolonRevision: Jorge Luís Nunes e Silva Brito

Introduction

Photogrammetry aims to the reconstruction of the 3D object-space from the 2D

image-space, thus leading to the computation of reliable, indirect measurements of 3D

coordinates in the object-space with known precision. There are three types of

photogrammetric measurements: from orbital, optical imagery; form airborne imagery, and

from close-range photographs. There are also special applications such as the

measurements derived from medical imagery.

The image-space is formed from photographs taken with both forward and side-lap

superposition. The forward superposition is necessary for stereoscopic purposes; the side-

lap superposition guarantees the connection between strips of photographs. This is

mandatory for topographic photogrammetric mapping. The values of those superpositions

usually adopted in photogrammetric projects are 60% in the direction of flight (forward

superposition), and 20% to 30% in the side-lap direction. Those values can vary based

upon technical requirements of a photogrammetric mapping project.

Phototriangulation is the technique of computation of least squares adjustment of

the 3D coordinates of the perspective centers of the photographs in the object-space or in

any external reference frame. This process also calculates the Euler angles or the sensor

attitude parameters for each image of a photogrammetric block of photographs. 3D

coordinates of photogrammetric points, those that are measured only in the image space,

are also accurately computed in the phototriangulation procedure.

There are basically two approaches for computation of phototriangulation: the adjustment

by independent models and the bundle block adjustment procedures. The former method

uses a stereo-model as its basic unit of adjustment; the bundle-block adjustment, also

known as the multiple spatial resection, on the other hand, considers a light-ray connecting

a point in the object-space, the principal point of the camera, and the projection of the

point into the image-space, as its essential element for Leastsquares adjustment

computations. Therefore, a bundle of light rays forms the block of images, which is

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adjusted in a unique procedure. If a block has only one single image, the bundle-block

adjustment procedure is named “spatial resection”.

The e-foto software uses the bundle-block adjustment procedure for airborne

imagery, taken either from digital frame photogrammetric cameras or from digitalized-

frame, aerial film cameras. Further details are available in SILVEIRA, F. J. C. (2004).

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Execution

After starting the free software e-foto its initial screen will look like figure 1 below. In

the main menu we do have the options Project, Execute, and Help.

Figure 1 – Initial screen of the e-foto software.

For starting the phototriangulation module we must go to the Project Manager

screen, and choose the options Project and Load File or Last File. The Load File option

will permit the user to browsing through the file folders until finding the folder which

contains the desired e-foto photogrammetric file (an *.epp file.). The Last Project option

will open the last project used by the user. The user not familiar with the creation of an e-

foto photogrammetric project should go to its the specific tutorial.

In the e-foto project manager screen, choose Execute and Phototriangulation, as

depicted in figure 2.

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Figure 2 –The loading of the Phototriangulation module of the e-foto software.

The Execute command opens the main screen of the phototriangulation module, as

in Figure 3.

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Figure 3 – The main screen of the phototriangulation module.

There are three types of viewports in the main screen of the phototriangulation

module: two viewports are for overviewing of the left and right images currently in use, two

for performing the measurements of photo coordinates of photogrammetric, checking, and

ground control points, and two for the detailed view of measuring areas in both left and

right images.

The meanings of the labels of the tables contained in the lower part of the main

window of the phototriangulation module are listed below:

• Left Image Points: This table contains the list of points measured in the left image.

Going downward in this table, we find the image file name (i.e.,

1997_016_300dpi.bmp), the list of points measured in that image (Id), and their pixel

(Column, Line) coordinates.

• All Registered Points: This table contains the list of points (Id) measured in both

images, their type (Ground Control, Photogrammetric, or Checking Point.) Ground

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coordinates (i.e., UTM coordinates and Height above Sea level) are also listed for

each point.

• Right Image Points: This table contains the list of points measured in the right

image. Going downward in this table, we find the image file name (i.e.,

1997_017_300dpi.bmp), the list of points measured in that image (Id), and their pixel

(Column, Line) coordinates.

• Image Measurements: Given a point selected in the previous tables, this table shows

the list of images where this point was measured (where it appears), as well as the

respective pixel coordinates in each image.

After finishing the photogrammetric measurements it is time to execute the

phototriangulation computations.

NOTE: Starting with the <2016-05-19> version of the e-foto software there will no

longer necessary to indicate flight direction for every image of a block. The e-foto

assumes that every project was flown “N” to “S” and “W” to “E”.

Please notice that hereafter, the e-foto phototriangulation module assumes that

every flight direction is initialized with 0°. If your project does not correspond to this

situation, you must reorganize it such that this condition is satisfied. This approach does

not interfere in the computation of ground coordinates of photogrammetric points of your

block.

Please click on the button to compute the bundle block adjustment of your

photogrammetric project.

In a common photogrammetric working flow the user may need to include some

photogrammetric points in each image of the block. Those points shall be strategically

located, such that they can act either as tie points, connecting one image to its neighbor,

forming strips of photographs, or as passing points, connecting the strips to each other,

thus assembling the whole block of photographs. In this hypothesis it will be necessary to

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go to the point insertion mode. For doing so, the user must click on the Insert

Photogrammetric Point button. There are some buttons designed to facilitate the

phototriangulation point measurement procedure.

The icons of those buttons and their functionalities are presented and explained in

Table 1 below:

Table 1 – The functionalities of photogrammetric measurements in the phototriangulation module

Button Description Moves the image within its screen space, by clicking the

mouse’s left button.

Zooms on the active image, by clicking with the left button

and dragging the mouse over the region.

Toggles antialiasing on

Allows moving both images proportionately, at the same time

Zooms both images at the same time.

Zoom percentage for the left image (upper box) and right

image (lower box)

Restores initial zoom on the left image.

Restores initial zoom on the right image.

Restores initial zoom to both images.

Before selecting the flight direction, and performing the phototriangulation, it is

necessary to measure both ground control points and photogammetric points locations.

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For doing that, please click in the button and locate these points on the images where

they appear. After finishing point measurements, the following screen will show up, as

seen in Figure 4.

Figure 4 – Screen showing the measurements of both ground control and photogrammetric measured

points in their images.

Now we can run the phototriangulation computation routine. To do that, please click

in the button, so the following window will show up. (Figure 5)

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Figure 5 – Screen with available data already selected for running the software, and for the definition of

adjustment parameters.

You can now select which images will be part of the block of images to be

processed and points which will be part of the calculations of phototriangulation. You can

perform the motion picture Images Available to Selected Images by clicking on the

button and moving the entire set, or in the button to move a selected

image.

To include an image that was not already attached to your photogrammetric block,

under “Selected Images” transfer them to Available Images through the movement button

or, to execute this process for all the images, use the button . The same

applies to point selection, by moving Points Available to Selected. If we want to reverse

that with what is Available for Selected and vice versa simply click the button .

For processing the phototriangulation procedure, the user must also set the number

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of iterations to achieve convergence. This is accomplished through the combo Max

interations as well as precision values for convergence in meters, through the combo

Metric convergency and angular value through the combo Angular Convergency.

After choosing images, points and setting parameters for the maximum number of

iterations and metric and angular convergence, you must click in the button as we do

in this example. However, if you do not want to carry out the phototriangulation right now,

just click the button. After pressing the button, you have the option of

calculating phototriangulation in a topocentric local system, by clicking Yes, as shown in

Figure 5b below.

Figure 5b – Screen with option to perform calculations in a topocentric local system.

After the calculation of the phototriangulation is performed, the window shown in

Figure 6 will be displayed, through which it is possible to observe results, evaluate them

and, if necessary, re-running phototriangulation with the setting of new parameters for

maximum number of "iterations" and metric and angular convergence accuracies.

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Figure 6 – Screen with results phototriangulation the environment and photo-integrated.

In Figure 6 we can also observe the indicators of the quality of the adjustment of

phototriangulation, namely:

• Iterations: Number of iterations of the adjustment block that converged, according

to pre-defined tolerances.

• Converged (yes / no): Indicates whether the adjustment block converged or

stopped because the maximum number of iterations has been reached.

• RMSE: Indicates the value of the root mean square error of the ground coordinates

being expressed in meters.

In the middle of Figure 6 there are listed the IDs numbers and the ground cordinates

coordinates of phtogrammetric points calculated and their respective residues. On the

bottom of Figure 6 there are two buttons that allow the user to accept or to discard the

results of his phototriangulation adjustment. The meaning of each one of the columns

shown in Figure 6 is presented in Table below.

Table 2 – The metadata about the phototriangulation module

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Button Description Image Id Image identifier

w (ω)(Omega) or attitude angle of image system rotation

around the X axis

Phi (ρ) attitude angle or image system rotation around Y axis

K (Κ)rotation angle of the image system around the Z axis

angle on the same direction of the flight Xo perspective center X coordinate in ground systemYo perspective center Y coordinate in ground systemZo perspective center Z coordinate in ground systemDw residual for the adjustment of the omega angle Dphi residual for the adjustment of the phi angle Dk residual for the adjustment of the kappa angle DXO residual for the adjustment of Xo in metersDYO residual for the adjustment of Yo in metersDZO residual for the adjustment of Zo in metersId Photogrammetric point identification E East coordinate (UTM) N North coordinate (UTM)

H orthometric altitude or height according to the system

adopted for the adjustment dE residual for the adjustment of the E coordinate dN residual for the adjustment of the N coordinate dH residual for the adjustment of the H coordinate

To save the results of your phototriangulation computation as a "kml" file and to

open it on GoogleEarth, just click in the button on the main screen of the

phototriangulation module. A window will appear so you can choose the file name you

want to use for saving the "kml" file, as shown in Figure 8. Browse the folder where you

wish to save the file and double-click on it. If you have Google Earth installed, it will show

the footprints of each image of your project and the position of photogrammetric control

points georreferenced by Google Earth. (Please see Figure 8)

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Figure 7 – Screen to save the file in the format "Kml" (the Google Earth viewer.)

Figure 8 – Google Earth viewer screenshot depicting points used for the phototriangulation (marked by

green balloons) and photogrammetric images footprints (depicted as purple lines.)

Note: If the results window is closed and if you want to review the results of

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phototriangulation, just click the button on the main screen and a new

phototriangulation and results window will appear, as shown in Figure 9 below.

Figure 9 – Phototriangulation results.

Once accepted the results of the phototriangulation adjustment for all images used

to adjust the block will have their values of exterior orientation parameters saved in the

project file and recorded in their respective formats.

As the generation of phototriangulation reports is of conceers, the user has two

options:

I - Phototriangulation report, which can be started after processing the

phototriangulation. (Figure 10)

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Figure 10 – Phototriangulation Report

II - Phototriangulation full report, witch can be started in the main menu of e-foto or

using the shortcut CTRL+F. There are two options for report generating format: the text

format, and the XML format, the last of which will permit to visualize your results in any

internet browser.

Any contribution for correcting and improving this tutorial is very welcome. Please send

your comments and/or sugestions to the e-foto team at

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

<END OF TUTORIAL>

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