DEM creation using 3D vectors Geomatica 2014 tutorial

DEM creation using 3D vectors
Geomatica 2014 tutorial
The following tutorial demonstrates how to create a raster digital elevation model (DEM) by interpolating
elevation values from millions of points and 3-D structure lines commonly referred to as breaklines. Creating a
DEM in Geomatica with this type of data can easily be done using the following algorithms in EASI:
VDEMINGEST, VDEMSETUP and VDEMINT.
A dataset of the Hamilton, Ontario area supplied by JD Barns was used in this tutorial. The dataset consists of
550,000 points with a nominal spacing of 15m. The 98,000 breaklines consisted of 2,100,000 vertices.
Small subset of reference 3D points and breaklines
VDEMINGEST ingests multiple vector files containing points, breaklines, contours, valleys, ridges, and cliffs,
and merges them into vector segments in a PCIDSK (.pix) format file. The vector segments are formatted
specifically for VDEMINT which uses them to create a raster digital elevation model (DEM).
VDEMSETUP creates an index file containing a list of raster DEM files that will be generated by VDEMINT.
VDEMINT generates a raster digital elevation model (DEM) by interpolating elevation values of points,
contours and 3-D structure lines in vector segments. In addition, 2-D break lines, such as valleys, ridges, and
cliffs with no elevation values are also accepted as constraints for interpolation.
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DEM creation using 3D vectors
Geomatica 2014 tutorial
Project Setup
1. From the Geomatica toolbar, select the EASI application. The EASI environment window will open.
2. Type s vdemingest to view the required parameters in the algorithm.
NOTE: Parameters can be copied into a Notepad file, modified based on your data and saved as a *.eas file.
This file can then be copied into the C:\PCIGeomatics\Geomatica_2014\pro directory and called in the EASI
environment.
3. Modify the vdemingest parameters to suite your data. In the example above, \points\*.* and
\breaklines\*.* was used. This tells the software to use all of the files in the \points and \breaklines
directories.
4. When you are done modifying the Notepad file, save it into the \pro directory as vdemingest_test.eas.
5. In EASI type r vdemingest_test . Hit <enter>.
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DEM creation using 3D vectors
Geomatica 2014 tutorial
6. Once the script has successfully completed you can move on to VDEMSETUP. Make sure the output
vector.pix file from VDEMINGEST has been created and is found in the location you specified. This file
will be used as an input in VDEMSETUP.
7. In EASI type s vdemsetup.
8. Copy the parameters into a Notepad file. Modify them to suite your data and save the file as
vdemsetup_test.eas in the \pro directory.
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The information in this document is subject to change without notice and should not be construed as a commitment by PCI Geomatics.
PCI Geomatics assumes no responsibility for any errors that may appear in this document.
DEM creation using 3D vectors
Geomatica 2014 tutorial
9. In EASI type r vdemsetup_test. Hit <enter>.
10. The process will run fairly quickly and produces an index.txt file. This file will be used as input into
VDEMINT.
11. In EASI type s vdemint.
12. Copy the parameters into a Notepad file. Modify them to suite your data and save the file as
vdemint_test.eas in the \pro directory.
13. In EASI type r vdemint_test. Hit <enter>.
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The information in this document is subject to change without notice and should not be construed as a commitment by PCI Geomatics.
PCI Geomatics assumes no responsibility for any errors that may appear in this document.
DEM creation using 3D vectors
Geomatica 2014 tutorial
14. Once the process has successfully completed, you will notice index.pix is created. This file contains the
generated DEM file created from all of the point and breakline information.
15. Open index.pix in Focus.
16. In the Files tab right click on the channel and select View As Pseudocolor.
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The information in this document is subject to change without notice and should not be construed as a commitment by PCI Geomatics.
PCI Geomatics assumes no responsibility for any errors that may appear in this document.
DEM creation using 3D vectors
Geomatica 2014 tutorial
Accuracy
Tests can to be done on each individual dataset to determine if generating a vector DEM from points and 3D
vector segments is more accurate than extracting a DEM from stereo imagery in OrthoEngine.
For testing purposes, the resulting interpolated DTM (vector DTM) and a DTM generated from stereo imagery
in OrthoEngine were put into a single file with three 32R bands. The two DTMs were subtracted resulting in a
third difference band; the amount of displacement indicates the amount of error in the DTM.
The images below show a view of both DTMs using the Live DEM editor in Focus. As you can see the vector
DTM has a smoother overall appearance compared to the OrthoEngine DTM.
Detail of interpolated Vector DTM
Detail of corresponding OrthoEngine DTM
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DEM creation using 3D vectors
Geomatica 2014 tutorial
The difference image (vector DTM –
OrthoEngine DTM) in this area, shown
to the left, has some of the worst
differences in the entire DTM area.
In the blue area labelled A the vector
DTM is up to 16m below the
OrthoEngine DTM. In the red area
labeled B the vector DTM is up to 15m
higher.
For reference, the uniform green area
on the left has a difference varying
between 1cm and 50cm, and an
average a difference of about 10cm.
Looking at a number of areas in this dataset, it appears that about a quarter of the time the OrthoEngine DTM
was more accurate than the vector DTM. The OrthoEngine DTM tended to be more accurate in flatter areas
than the vector derived DTM. About a quarter of the time the vector DTM appeared to be more accurate.
However, about half the time results were inconclusive due to dark shadows in the imagery or dense tree
cover making it impossible to see the true ground in the stereo imagery used to derive the OrthoEngine DTM.

Differences/errors are almost always in deeply shadowed or heavily treed areas. Both types of DTM’s
make similar errors. However these types of errors are typically unnoticeable in the final ortho mosaics.

3D stereo vector DTMs can model very complex terrain around man made features better. For example
bridges, or steep edges of roadways and retaining walls.

3D stereo vector DTMs look cleaner and smoother and are closer to what many users and cartographers
traditionally expect, even if they may be less accurate than OrthoEngine’s dense DTMs in many areas.
A key aspect to the OrthoEngine method of generating DTMs is labour efficiency. Using imagery to generate a
DTM in OrthoEngine stereo DEM extraction is very efficient compared to manually collecting 3D stereo
breaklines and points which can be a very labour intensive process. The purpose of this tutorial is to show that
if 3D stereo breaklines and points are available, a DTM can easily be created in EASI. The generated DTM will
result in a smooth and accurate output.
Live DEM editing in Geomatica can be used to edit common terrain issues (Layer -> DEM editing in Focus or
OrthoEngine -> DEM from Stereo processing step-> Manually edit generated DEM).
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The information in this document is subject to change without notice and should not be construed as a commitment by PCI Geomatics.
PCI Geomatics assumes no responsibility for any errors that may appear in this document.