This tutorial outlines the process of extracting a digital elevation model from digital airphotos using Geomatica OrthoEngine .
Initial Project Setup
- Open OrthoEngine from the Geomatica toolbar
- Navigate to File > New
- In the Project Information window, specify a Filename, Name, and Description
- Select Aerial Photography as the Math Modelling Method
- Select Digital/Video for the Camera type
- Select Compute from GCPs and tie points for the Exterior orientation
- Click OK
- In the Set Projection window:
- Specify the Output Projection
- Specify the Output pixel spacing and line spacing information
- Click Set GCP Projection based on Output Projection
- Click OK
- In the Digital / Video Camera Calibration Information window:
- Enter the Focal length, Chip size, and Y scale factor (This information can be found in the camera_calib.xml file).
- Note that you can also import the camera_calib.xml file by clicking the Import from File… button
- Click OK
- On the OrthoEngine toolbar, switch the Processing step to Data Input
- Click Open a new or existing image
- Click Add Image…
- Navigate to the location of the images, select all photos, and click Open
- Click Yes to importing the data into PIX format to optimize processing
- Click OK to the Multiple File selection window
- The imported photos will all appear as PIX files in the Open Image window
- On the OrthoEngine toolbar, click the Import exterior orientation data from text file
- In the resulting window, verify the Input projection, Input angle unit, and EO vertical datum
- Enter the Accuracy information (If this information is not provided in the exterior orientation data, it is safe to estimate under 1 metre for eX, eY, and eZ, or under 1 degree for eOmega, ePhi, and eKappa)
- Click Browse and navigate to the exterior orientation data file (“eo.txt”)
- Select Import
You can optionally choose to collect GCPs and TPs for your project to improve the math model. Information and instructions on how to collect GCPs and TPs is outlined in the Airphoto Orthorectification and Mosaicking tutorial:
Epipolar Image Creation
- On the OrthoEngine toolbar, select DEM From Stereo as the Processing Step
- Click Create Epipolar Image
- For Epipolar selection, selecting Optimum pairs is sufficient.
- Click Add Epipolar Pairs To Table
- Click Generate Pairs
- Click Close
- On the OrthoEngine toolbar, select Extract DEM automatically
- Click the Select box beside each epipolar pair.
- In the DEM Extraction Options section:
- Select SGM (Semi-global matching) as the Extraction method
There are two extraction methods available. NCC (Normalized cross-correlation) and SGM (Semi-global matching). SGM is based on newer technology and produces higher-quality results with fewer errors and higher detail, but processing time is increased. However, if lower-resolution DSMs are adequate, and faster processing time is preferable, consider selecting the NCC method. More information about these extraction methods is available in the Geomatica Help.
- Verify the Output DEM vertical datum
- Select 2 as the Pixel sampling interval
- Select Low as the Smoothing filter
- Check Epipolar tracking
The quality of the DSM extraction is highly dependent on how accurately the epipolar lines between stereo images are aligned. Errors in the epipolar alignment can vary across a stereo pair resulting in a DSM where there are large patches of poor-quality elevation values. This is especially true when DSM’s are generated at full resolution. Errors can occur even if the epipolar lines are shifted by a single line. The Epipolar Tracking option enables tracking of changes in the epipolar line over the stereo pair and can automatically compensate for small gradual errors. Epipolar tracking increases processing time (typically by 20% to 30%) and there is a small possibility of introducing errors in an otherwise good DSM.
- In the Geocoded DEM section:
- Select Create geocoded DEM
- Specify the Output file name and its location
- Set the X and Y Resolution to 1 meter
- Specify your preferred Output option
- Use last value: Uses the last value to replace the pixel values in the overlapped area in the existing geocoded DEM by the pixel values of the geocoded DEM that is added to the file.
- Average: Replaces the pixel values in the overlapped area by the average pixel values between the existing geocoded DEM and the one that is added to the file.
- Highest score: Uses the highest score to replace the pixel values in the overlapped area by the pixel value with the highest correlation score between the existing geocoded DEM and the one that is added to the file.
- Blending: Uses the mosaicking method to mosaic the DEMs together with blending between DEMs. This is the default value.
- Merge (new with Geomatica Banff): Available when SGM (Semi-global matching) is selected for DEM Extraction Options, this option merges a set of geocoded DEMs from a stereo airphoto or satellite project into a single DEM. The merged DEM is typically of higher quality, because each geocoded DEM has a different viewing geometry. Occlusion in one DEM can be filled from another DEM. In addition, the multiple DEM elevations of the same ground pixel can be used to detect blunders. Averaging multiple good elevations helps to increase vertical accuracy.
Merge also attempts to straighten building edges, if applicable. That is, with Merge selected, you can select the Clean up building edges with filter size check box, and then in the box to the right, type or select the size of the filter to use. The recommended value is 13.
- Click Extract DEM
- View the finished product in Focus (optional)
DEM Editing (Optional)
To further edit the produced DEM to smooth out irregularities and improve the accuracy, consider the following Live DEM Editing tutorial:
DSM to DTM Conversion (Optional)
A DSM (also referred to as a DEM) extracted from stereo images represents the earth’s surface and includes all objects on it, for examples, buildings and trees. Many applications require a DTM which represents the bare ground surface without any objects. To convert a DSM to a DTM through manual editing is a very time-consuming process. An automatic DSM to DTM conversion program was developed at PCI Geomatics.
Once the DEM is extracted you can choose to convert the DEM to a Digital Terrain Model (DTM). DTMs only includes the elevation of the ‘bare earth’ with vegetation, buildings and other man-made features removed (though roads and bridges are typically retained). A DTM is the preferred product to use in an orthorectification workflow. For more information on DSM to DTM conversion please see this tutorial: