Phototriangulation Introduction

Transcription

1 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 sidelap 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. 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 Page 1 of 15

2 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 digitalizedframe, aerial film cameras. Further details are available in SILVEIRA, F. J. C. (2004). Page 2 of 15

3 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 efoto photogrammetric project should go to its the specific tutorial. In the e-foto project manager screen, choose Execute and, as depicted in figure 2. Page 3 of 15

4 Figure 2 The loading of the module of the e-foto software. The Execute command opens the main screen of the phototriangulation module, as in Figure 3. Page 4 of 15

5 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 Page 5 of 15

6 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 < > 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 Page 6 of 15

7 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. Page 7 of 15

8 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) Page 8 of 15

9 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 Page 9 of 15

10 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. Page 10 of 15

11 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 Page 11 of 15

12 Button Image Id w (ω) Phi (ρ) K (Κ) Xo Yo Zo Dw Dphi Dk DXO DYO DZO Id E N H de dn dh Description Image identifier (Omega) or attitude angle of image system rotation around the X axis attitude angle or image system rotation around Y axis rotation angle of the image system around the Z axis angle on the same direction of the flight perspective center X coordinate in ground system perspective center Y coordinate in ground system perspective center Z coordinate in ground system residual for the adjustment of the omega angle residual for the adjustment of the phi angle residual for the adjustment of the kappa angle residual for the adjustment of Xo in meters residual for the adjustment of Yo in meters residual for the adjustment of Zo in meters Photogrammetric point identification East coordinate (UTM) North coordinate (UTM) orthometric altitude or height according to the system adopted for the adjustment residual for the adjustment of the E coordinate residual for the adjustment of the N coordinate 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) Page 12 of 15

13 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 Page 13 of 15

14 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 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 - report, which can be started after processing the phototriangulation. (Figure 10) Page 14 of 15

15 Figure 10 Report II - 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 <END OF TUTORIAL> Page 15 of 15