Step 2: Exterior Orientation

Overview

Step 2 is the second dialog in the Project Wizard.

In this dialog the positions of the fotos in a photogrammetric project are introduced.

If you are editing an opened project

You can skip this step if you are editing a opened Strabo project. In this case this step is not important anymore, because all (approximate) positions of the images have already been entered earlier. However you still have the possibility to add new images to the project using the same type of file content.

If you are creating a new project

In case of a new project, it is the purpose of this step, to at least enter approximate coordinates for the projection centers of all images you wish to enter in the project. Fotoname, X, Y and Z coordinate, Omega, Phi, Kappa are minimum obligatorily.

In some cases there will be a file available that contains already the GPS or GPS/IMU information created during the flight.
GPS/IMU data contain normally very good approximations for both projection centers and orientation parameters.
If GPS and IMU are accurate values, you will be able to start immediately data collection, after succesfully finishing the last step of the project wizard.
If this is not the case, approximate values for the projection centers are enough to startup the project, and to visualize relative positions between the given imagery.

Two Possible Ways for Creating a New Project

  1. There is a file available containing GPS or GPS/IMU data:
    • as the result of the GPS or/and IMU measurements during the flight ,
    • as a manually created file, e.g. by checking and reading manually the image's position on a small scale map.
      The flying height can be taken as the approximate Z-value for the projection centers.
  2. There is no file available, and you have only limited information about the relative positions between the images
    • press the Edit/New button to create a file on the fly.

These data are entered using a text file containing GPS/INS data per photo.

See more details about the format of this file at the end of this page.

Wizard Guidelines

The dialog looks like:

It is the purpose of this dialog to finally provide a set of EO parameters (either approximate or accurate values) for all the images you want to process in this project.

The list should contain: imagename, X,Y,Z, Omega, Phi, Kappa for all images.

If the EO parameters cannot be provided at this point, only approximate values suffice. The exact EO parameters can afterwards be calculated using the Aero Triangulation software available in Strabo.

Indicate Angle Unit Type

First indicate in which unit the angles are expressed.

Choose between:

  1. DEG (a straight corner is 90 degree).
  2. GRAD (a straight corner is 100 degree).

Define External Orientation File

You can either select the file containing the EO (External Orientation) data (a .txt file), or even create a new file on the fly.

Open Existing File

  1. Pressing the button to the right of the text field to open a File selection box.
  2. Select the appropriate text file containing EO (External Orientation) containing either exact or approximate EO data for each image:
    EO data can be:
    • XYZ coordinates that are approximated measurements from e.g. a small scale map and approximate Omega, Phi and Kappa.
    • more accurate GPS/IMU data (IMU are orientation parameters) from flight registration measurements.
  3. The filename will appear in the textlabel field on the wizard's dialog:

Create New File

If you do not have an EO file, you can create one on the fly:

  1. Press the Edit/New button.
    The next dialog will appear:
  2. Press the Yes button, to start a text editor:
    • Create a filename on a location of your choice using the fileselection box that will popup.
    • Press the Save button on the fileselectionbox to start the actual text editor:
  3. Enter approximate values for the images:
    Here is an example showing the necessary minimum of EO data :
    • The name of the image.
    • Approximate projection center coordinates X and Y.
      These values can be GPS data, or they can be derived manually from a topographic map.
    • Z equal to the flying height or coming from GPS data.
    • Zero values for omega and phi.
    • Kappa must reflect the orientation of the camera taking into account that the West-East direction equals 0 degrees.

This means that it is possible that the main flight direction is 0 degrees (flight in West-East direction) and at the same time kappa equals 180 degrees, in case that the x-axis of the camera coordinate system is East-West oriented.


  1. Pressing the OK button will save the data into the given filename, and the filename will also appear in the text labelfield on the wizard dialog.

View or Edit Existing File

After defining the EO data you can have a look at the available data :

  1. Press the edit/new button and edit the data (if still necessary): here is another example containing XYZ and orientation data as result of an earlier executed adjustment procedure (similar to GPS data):

In case of Microdrone imagery

In case imagery comes from a Microdrone mission, the MdCockpit software from Microdrones provides an EO data file, containing data from each position images are triggered on. Please extract this file using the MdCockpit software and save it (a txt file).

This file contains :

  • photo-numbering
  • GPS time stamp and timecode
  • coördinates in lat/lon
  • height of the drone from the ground
  • camera attitude values called: roll, pitch and yaw
  • mount attitude values called Cam. servo

To start introducing the Microdrone imagery, use the button “Adapt Microdrone Data” to read this file containing preliminar EO data. This file will then be transformed (adapted) into the format that Strabo needs, and will be placed automatically in the text field.

There are some differences in output between the MD4-200 and MD4-1000 drone, as well as in the case different settings for the camera mount were used when the flight-plan was generated. The software will ask for a compensation parameter when the Microdrone data is loaded.

To determine this compensation parameter, you need to manually review the file which was provided by MdCockpit. There you will see that there is a 'Pitch' value and a 'Cam. servo' value. The Cam. servo is used to indicate whether the mount used pitch compensation or not. The main thing to remember, is that the sum of the pitch, Cam. servo and compensation parameter needs to be approximately 0°.

For the MD4-200 drone, the following is an example:

Photo GPS Time Timecode Latitude Longitude GPS-A. Height Roll Pitch Yaw Cam. servo
001 11:22:06 88.992 +51.30315027 +4.39560821 87.7 44.5 -2.0 +0.8 +55.2 +90.0

Although the drone indicates a pitch compensation of 90°, the MD4-200 drone was programmed with the CSS parameter set to -100. To enable Strabo to correctly load the data, you need to provide the system with a compensation parameter of -90 to indicate that the pitch is vertically down (Cam. servo and pitch are added with this parameter to result in 0° pitch).

For the MD4-1000 drone with a Microdrones mount, the following example indicates a clear difference in Cam. servo parameters:

Photo GPS Time Timecode Latitude Longitude GPS-A. Height Roll Pitch Yaw Cam. servo
001 11:22:06 88.992 +51.30315027 +4.39560821 87.7 44.5 -2.0 +0.8 +55.2 -0.7

The drone thus indicates a pitch compensation of -0.7°. This drone was also programmed with a CSS parameter set to -100. To use this downlink file, we need to provide a compensation parameter of 0.

In the case a different mount is used, as for example the APS-mount, we can get the following results:

Photo GPS Time Timecode Latitude Longitude GPS-A. Height Roll Pitch Yaw Cam. servo
001 11:22:06 88.992 +51.30315027 +4.39560821 87.7 44.5 -2.0 +0.8 +55.2 +78.2

After calibrating the mount, the drone was programmed with a CSS parameter of -79. From the output, we can conclude that the compensation parameter in this case should be -79 (adding this parameter to the pitch and Cam. servo will result in approximately 0°).

Afterwards press the “Apply” button to continue with the project creation.

Import Visionmap Data

Strabo supports the Visionmap A3 imagery External Link. There is a special approach for these images. See Preparation of Visionmap Imagery

Apply External Orientation

After preparing the EO data, press the apply button to introduce the EO data into the project.
The following message will popup showing the amount of defined images.
Here is an example with 3 images:

The project overview will be drawn and shown in the OrbitGIS dialog, and the wizard will show the next step.
Here is an example of a project with 125 images:

File Formatting

When the text file is not properly created or formatted, you will be informed with the next dialog:

  • By pressing OK, the EO data will not be accepted and you will not be able to go to the next wizard step.
  • First repair the file and re-read the file until it is accepted.

Formatting the text file containing Exterior Orientation data:

This text file must contain one line per photo containing the following data:

  1. A project wide unique photo name.
  2. X-Projection center approximation expressed in terrain system units.
  3. Y-Projection center approximation expressed in terrain system units.
  4. Z-Projection center approximation expressed in terrain system units.
  5. Omega approximation expressed in the selected angular unit (enter 0 when not known).
  6. Phi approximation expressed in the selected angular unit (enter 0 when not known).
  7. Kappa approximation expressed in the selected angular unit.
    Kappa can be e.g. 0 DEG for west-east flights and 180 DEG for east-west flights.
    The values entered here should reflect the camera orientation, these can be different than the strip oriëntation.
    Angles are expressed positive anti-clockwise.
  8. e.g. 2401 165211.012 546511.152 1240.122 0 0 180
  9. The data may be tab- or space-separated.

Goto the next wizard step

  1. Go to the next wizard step by pressing the button on the wizard dialog.
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