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DEM Extraction

PCI Geomatics -

The following is a brief tutorial showing a step by step procedure for extracting a digital surface model (DSM) from stereo imagery and converting to a digital terrain model (DTM).  In this tutorial, Pleiades 1A imagery was used, however this workflow is similar for DEM extraction from all stereo optical data.

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. 

The data used in this tutorial is a sample Pleiades primary data set consisting of panchromatic, multispectral, PMS and tri-stereoscopy images of Melbourne, Australia: https://www.intelligence-airbusds.com/en/23-sample-imagery . A great innovation of the Pleiades system is to offer high resolution stereoscopic coverage capability. The stereoscopic coverage is realized by only a single flyby of the area, which enables collection of a homogeneous product quickly. In addition to the “classical” forward and backward looking stereoscopic imaging, Pleiades can acquire an additional quasi- vertical image (tri-stereoscopy), thus enabling the user to have an image and its stereoscopic environment. In general, a forward and backward looking stereo pair produces the highest accuracy, but this combination's use is limited to areas with gentle terrain. A nadir and forward/backward looking stereo pair can be used in most types of terrain.

Initial Project Setup

1. Open the Geomatica OrthoEngine application.

2. Click File > New

3. Give your project a Filename, Name and Description

  • Select Optical Satellite Modeling as the Math Modeling Method
  • Select Rational Function (Extract from image) under Options
  • Click OK

4. Input the appropriate Output projection and GCP projection information for your project

Data Input

5. Select Data Input as the Processing step

The data to input will be the back scan image and the forward scan image. When viewed in Focus the images will look similar to the images below.

  • Click Open a new or existing image

  • Click New Image
  • Navigate to the location of the data. Select the DIM_PHR1A_XXXXXXXXXX.xml file in the image folder
  • Select Yes when asked if you want to import the data file to a .pix file for optimized processing

  • Select a file name and location for your output .pix file
  • Select Yes when asked if you want to create overviews now
  • Repeat step 5b to 5f to add all images to the project

6. To view the output image select the image from the Open Image window and click Quick Open. Close both windows and move on to step 7

GCP Collection

7. In the OrthoEngine toolbar select GCP/TP Collection as the Processing step

  • Select Collect GCP’s Manually, two panels will immediately open (GCP Collection panel and the Open Image panel)

  • In the Open Image panel, click on the first image so it turns blue and click on Quick Open
  • In the Open Image panel, click on the second image so it turns blue and click on Quick Open

  • Adjust the two viewers so that they both can be seen beside each other along with the GCP Collection panel

  • In the GCP collection window select Manual entry as the Ground control source
  • Input a DEM for the data or use demworld.pix found in the etc directory C:\PCI Geomatics\Geomatica 2013\etc
  • Check mark Compute model
  • Collect GCP’s for the image
  • Repeat step 7b- h for all images in the project

Note: Importance of GCPs
A very important step in the DSM extraction workflow is ensuring that you collect very accurate GCPs so that the geometric model of the two images are updated so that when the image models are applied to the imagery, they will accurately align with one  another.

If the two images do not line up during the DSM Extraction process, the output elevation layer may have high levels of error.

DEM Generation

8. In the OrthoEngine toolbar select DEM From Stereo as the Processing step

  • Click Create Epipolar Image

  • Select left and right image
  • With both images selected, click the Add Epipolar Pairs To Table button
  • Click Generate Pairs
  • Click OK to the pop-up message that states the epipolar pairs completed successfully
  • Close the Generate Epipolar Images panel

9. In the OrthoEngine toolbar select DEM From Stereo as the Processing step

  • Click Extract DEM automatically

  • Check the epipolar pair by checking the Select box associated with that record

Under DEM Extraction Options:

  • 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.

  • Check mark 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.

  • Select a Pixel sampling interval of 2
  • Select Create Geocoded DEM
  • Select an output file name and location
  • Set the X and Y resolution to 1 meter for both
  • Click Extract DEM

 Note: DEM creation will take a long time as these images have a high resolution. Occasionally you may get artifacts in water bodies. This is normal as the algorithm processes the DEM using smaller tiles. These artifacts would need to be edited manually using our DEM Editing Tools

Pleiades Melbourne extracted DSM

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 prefered product to use in an orthorectification workflow. For more information on DSM to DTM conversion please see this tutorial: support.pcigeomatics.com/hc/en-us/articles/360015130032-DSM-to-DTM-Conversion

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