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by mouse. It is important to refer the table entries carefully to the zoomed centers of the fiducial mark. Small deviations may case large errors in the exterior orientation that will be established later.

If the GRASS monitor is not open, you need to exit i.ortho.photo, open a monitor, and restart the i.ortho.photo module. After selecting this menu entry (no. 5), the aerial photo has to be selected in the monitor. The list of fiducial marks will be automatically displayed. The orthophoto module now expects the assignment of the fiducial coordinate points to the fiducial marks in the image by digitizing.

Using the zoom function, enlarge the first mark (mark 1 in the upper left corner of the gs13.1 photo), until the bright point is visible as composed from multiple pixels. Generally, if no bright point is visible, the scanning resolution was set too low and you will have to start again from scratch by rescanning the aerial photo. In our sample data, the quality of the photo gs13.1 is unfortunately very low, the fiducial marks in the corners are very difficult to identify. For now you may guess their position - this photo is from 1971, nowadays images are of course of much better quality. The center of the bright point within the enlarged fiducial mark is digitized using mouse by clicking into it. Then the related entry (row) in the fiducial marks table must be selected by double clicking on it. Now both image and fiducial marks coordinates are also shown in the terminal window. The marker in the GRASS monitor will turn to green color once the control point is accepted. This procedure has to be done for all fiducial marks.

The menu item ANALYZE in the monitor allows us to verify the digitizing accuracy. The RMS-error should be less than half pixel size. A misplaced point can be deactivated through a mouse double click on the corresponding control point in the ANALYZE table. This fiducial mark must then be re-digitized again. Once all fiducial marks have been successfully assigned to table entries, select QUIT to go back to the i.ortho.photo main menu.

We skip the menu step 6 as it is only needed for rectifying oblique aerial photos as described in Section 10.3.3.

7. Compute ortho-rectification parameters. Now we define the exterior orientation which relates the aerial photo to the target coordinate system using ground control points. Reasonable rectification results can be obtained with around twelve control points well distributed over the image. The related elevations, which are crucial for the creation of an orthophoto, are automatically read from the elevation data raster map that we have chosen earlier. To start, select again the aerial photo in the GRASS monitor for display. Then click on the Plot Cell entry and into the right part of the monitor to display the reference map. For the example session, you may again select the map roads.



Similarly to i.points, the corresponding control points are digitized by zooming into a map portion and digitizing the GCPs using mouse both in the aerial photo and in the reference map. When a ground control point pair has been marked in both the aerial photo and the reference map you have to confirm this with a new mouse click (left button: y, right button: n) as explained in the terminal.

Recommended control points are road intersection centers and the centers of objects (buildings etc.). However, be careful when using buildings. An aerial photo will show both the roof and the bottom of a building. Due to the central perspective in the unrectified image only the footprints of buildings may be used, not the displaced roofs. If you want to create a true orthophoto instead of a pseudo orthophoto, the DEM needs to contain the building heights. In this case, the building roofs have to be geocoded, so that both the buildings lower and upper edges are georeferenced.

When more than four GCPs have been digitized, their accuracy can be verified using the ANALYZE menu item. It displays a table of all control points with their RMS error. Each error value is calculated for the control points from the current transformation equations (depending on the camera type, etc). The value is computed for the target LOCATION, so it is given in the projection units of the target LOCATION (usually meters). If a control point appears heavily deviated according to the transformation equations, the row will be shown in red color. Such a control point can be deactivated by double clicking the entry in the ANALYZE table. The total RMS-error is acceptable if it is less than half target resolution of the aerial photo, e.g. below half a meter. Note that the RMS error significantly depends on the digitizing accuracy of the fiducial marks (see above, menu no 5).

If GCPs have been measured by other means, e.g. using a GPS, these coordinates should be entered. Each ground control point is marked within the aerial photo and then KEYBOARD is selected as an input method instead of the default SCREEN . When sufficient number (twelve or more GCPs well distributed over the image) of ground control points have been digitized, you can leave the GCPs identification mode with QUIT and return to the main menu.

8. Ortho-rectify imagery files. Finally, the ortho-rectification process of the aerial photo can be performed by selecting the menu item (8). Enter a new name for the target orthophoto, in our example we will choose gs13. If several aerial photos have been stored in the image group (menu item (1)), they will be all listed here (e.g., multiband aerial photos). Because the defined control points are valid only for the current aerial photo, a new name for the target LOCATION for this photo will be entered here. After leaving this screen, the transformation equations are internally generated ( Computing equations ).



Then another query follows: Selecting 1. Use the current window in the target location will start the rectification based on the settings in the target LOCATION. Selecting 2. Determine the smallest window which covers the image allows us to override the settings in the target LOCATION from here, due to the image size and boundaries. The pre-defined values in this screen result from the current aerial photo and the ortho-rectification parameters. Eventually, the ground resolution should be modified to appropriate values and the photo boundaries should be extended to appropriate rounded coordinates.

GRASS will send an email when the rectification process is completed and the new orthophoto generated. Meanwhile, you can exit GRASS and perform other tasks on the computer. Once the email arrives, GRASS can be started with the target LOCATION to view and verify the new orthophoto. An overlay with the reference map will show the quality of the rectification.

10.3.3 Generating orthophotos from oblique aerial photos

The procedure is similar to the one described in Section 10.3.2, but we insert an extra step to take into account the flight and aircraft position parameters, that are needed to generate orthophotos from oblique aerial photos. This step is added after the computation of image-to-photo transformation.

6. Initialize exposure station parameters. In this step, the flight path parameters such as the aircraft tilt and crab, and the camera coordinates and altitude above sea level have to be defined (to be taken from aerial photo auxiliary data). The following three values for the camera position have to be specified (for an example see below):

X: East aircraft position;

Y: North aircraft position;

Z: Flight altitude above sea level

Further the (approximate) tilt of the aircraft has to be specified. This is defined by the angles Omega (roll), Phi (pitch), Kappa (yaw). They represent (see Fig. 10.5, Hildebrandt, 1996:149, Schowengerdt, 1997:95):

Omega (roll): Raising or lowering of the wings (turning around the aircrafts axis);

Phi (pitch): Raising or lowering of the aircrafts front (turning around the wings axis);

Kappa (yaw): Rotation needed to align the aerial photo to true north: needs to be denoted as +90° for clockwise turn and -90° for a counterclockwise turn.



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