Extracting a DEM from stereo-image pairs

This topic describes extracting a digital elevation model (DEM) from stereo-image pairs for production of orthorectified images.

Producing standard orthos

The DEM Extraction Airphoto module generates a digital surface model (DSM) from overlapping aerial stereo-image pairs. The module also provides an option to convert the DSM to a digital terrain model (DTM).

Note: For more information about the module parameters, see the DEM Extraction Airphoto module.

Extracting a DEM from stereo imagery is a processing-intensive operation. The following are various methods you can use to achieve the highest-quality DEM for your project.

Method 1
This method is the fastest and requires the least amount of effort, but the resulting DTM may contain significant errors, especially in hilly or mountainous areas. This method is recommended when speed, rather than accuracy, is the primary concern.
1. Generate a raw DSM by using the DEM Extraction Airphoto module.
2. Create a DTM from the raw DSM by selecting Create DTM from DSM in the module.
Method 2
With this method you can use the innovative DEM Editing feature of CATALYST Professional Focus to rapidly convert a DSM to a DTM without external 3-D stereo requirements. You can use DEM Editing to, for example, flatten roads and lakes, reconstruct bridges, and remove buildings and trees. In many cases, a few hours of editing can generate an accurate DTM ready for immediate use in standard ortho production.
1. Generate a raw DSM by using the DEM Extraction Airphoto module.
2. Manually convert the DSM to a DTM by using DEM Editing in Focus.
Method 3
A more traditional approach, this method can produce a very high-quality DTM, but requires the most labor and access to 3-D stereo editing software (such as DAT/EM).
1. Generate a raw DSM by using the DEM Extraction Airphoto module.
2. Export the DSM to third-party 3-D stereo editing software.
3. Use the 3-D stereo editing software to edit points, add break lines, and perform other tasks.
4. Merge the 3-D points, break lines, and other vector data back into a raster DTM.
Method 4
This method is a hybrid approach, where some of the work is performed using DEM Editing in Focus to identify problem areas. You then use the 3-D stereo editing software for isolated problem areas. This reduces the amount of labor required.
1. Generate a raw DSM by using the DEM Extraction Airphoto module.
2. Manually convert the DSM to a DTM by using DEM Editing in Focus.
3. Export the DTM to 3-D stereo editing software.
4. Use the 3-D stereo editing software to edit points, add break lines, and perform other tasks.
5. Merge the 3-D points, break lines, and other vector data back into a raster DTM.