Displaying LAS-Format, Point Cloud LIDAR Data: Saga & FugroViewer
famous Serpent Mound area in Ohio offers a good terrestrial example for
comparing various platforms' visualization abilities. The
well-known GRASS image is compared to versions provided by Saga and FugroViewer. This somewhat
new technology is important due to possible uses with small reef
characterization, ship or plane wreckage identification, marine
Preliminary Reading (in
OceanTeacher, unless otherwise indicated):
|1. Recently, high-resolution
relief data (including very high-resolution imagery of physical objects,
such as statues!) has been published in various new formats that portray the
XYZ-nature of the subjects. One popular such format, LAS, frequently
used now to capture very high resolution earth relief, will be examined
here. A famous dataset, the Serpent Mound relief, used widely for
demonstration purposes will be examined in 3 different softwares that
accommodate LAS data, with a view toward getting the best possible images.
These data were obtained from a LIDAR survey of the area.
There is a
bewildering complex of data formats and media for LIDAR, because these data are quite
valuable commercially, and all the survey companies want to be the field
survey analog to ESRI. The closest thing to a standard format is the
LAS format for LIDAR
data published as point shapes. Hence the narrow focus of this
exercise. If you want to brave the cruel world of other, commercial
formats, good luck.
|2. This is a well-known image
of how GRASS displays the Serpent Mound data.
The view here is toward the SOUTH,
but all images below are north-facing, i.e. with the river correctly shown
on the left.
The mound, in Ohio, USA,
is the world's largest serpent representation; read about it at the link
above. The overall vertical relief is only about 2-3 meters, but GRASS
does an excellent job of using hillshading to emphasize the serpent with a
This author is not a GRASS expert, so he only refers you to the
OSGEO exercise link above for more information. The excellence of
this image clearly confirms that GRASS is the tool of choice for these data,
and the standard against which other platforms may be judged.
|3. Run the latest version of
Saga, and make sure you've set it up with the recommended properties, as in
|4. Download the compressed
LAS data to your folder DATA > BASEMAP > RELIEF and unzip it.
NOTE: This is a filtered LiDAR dataset, containing just ground
points (bare earth model). It would be nice to have a fully classified
dataset, including also vegetation, buildings etc. for further visualization
|5. Select TOOLS >
IMPORT/EXPORT > LAS > IMPORT LAS FILES. For INPUT FILES,
navigate to and select the LAS file you just obtained.
The only ATTRIBUTE
item we think you should check is INTENSITY.
This attribute, when
can be used to create a greyscale image showing the amount of laser light
reflected from the surface, similar to a near-infrared channel. Later, if you want to become a LAS expert,
of course, you should study the other attributes on your own.
So just click OK to load the file.
|6. Select the file you just
loaded (on the DATA tab) and make these settings. Then click APPLY at
the bottom of the page.
7. Now that the settings are applied, open the file as a map (i.e. double
click on it). It is impossible (or very difficult) to see any evidence
of the mound with this magnification. If it were visible, it would be
just below the major tributary stream that flows in from the right.
|8. In Saga, you can
right-click on the data object in the data menu and select HISTOGRAM to see
the distribution of elevations.
Aside from the obvious water surfaces at 169-175 m, the majority of the
land has only a relief of about 32 m (175-207 m). There is a tantalizing hint of a
feature with heights of around 206 m, within a relatively small range of
local elevation values. But nothing really "pops out" here.
|9. To see this dataset in
3 dimensions (3D), select VISUALIZATION > 3D VIEWER > POINT
- For POINT CLOUD, select the LAS file
- For COLOR, select the Z value (of the typical X, Y, Z triplets that
make up point clouds). In this case Z would be the elevation (in
- NOTE: For this type of file, you cannot use the
other "3D" functions that appear along the top margin of the Saga page,
and also in the VISUALIZATIONS tools.
Then click OK.
10. Initially, the 3-D map is viewed from directly above, so it still
appears to be flat.
|11. At the top left, there are
these controls to move the solid image. These are not easy to use, or
completely intuitive. So take some time to play with them. The
units seem to be percentage of a full rotation, so 50 for example, means 180º.
NOTE: The navigation in the 3D View is most easy with the mouse
(all mouse buttons and mouse wheel).
NOTE: When you increase the EYE DISTANCE, objects appears
closer; when decreased, then further away
|12. When you use the ZOOM tool
and look carefully, you can find the serpent mound as you see here.
It's very indistinct, so we'll try to make it more visible in the various
|13. First we can consider
working with either a TIN version of the data or a grid, to improve
- We could use the GRID > GRIDDING > TRIANGULATION module to create a
TIN. It is working, but takes a long time to compute. The
triangulation in SAGA is not optimized for datasets with so many points.
We won't try this here.
- Another approach is to use the GRID > SPLINE INTERPOLATION >
MULTILEVEL B-SPLINE INTERPOLATION module, to create a grid. This
optimized for large scattered datasets and is considerably faster.
We will try this here.
|14. Open the second tool, and
select the Serpent Mound data. Make these choices for tool execution; then click OK.
NOTE1: Most of these are
selected for you, but when you enter the CELLSIZE of 0.5, Saga recalculates
many of them.
NOTE2: As the dataset is quite large, it maybe
worth to use a cell size of 1m for gridding or to add an intermediate step
cutting the point cloud in order to increase the performance in the 3D
viewers. The latter could be done with TOOLS > SHAPES > POINT CLOUDS > POINT
CLOUD CUTTER tool or its interactive version.
|15. After only about 1 minute
this new grid appears.
|16. You could work
immediately with this new grid,
the 3D Map Viewer (Parameter 'Resolution' set to 1000). There are certainly
a lot more visualization options, but maybe this is a good starting point.
|17. But there is a special
tool that we should use instead, to get the best visualization of this new
|18. Select TOOLS >
VISUALIZATION > GRIDS > TERRAIN MAP VIEW.
Then make these choices, and
NOTE: There is an alternative method to TOPOGRAPHY, called
MORPHOLOGY, which provides a shading based on Topographic Openness and
Slope. This alternative is computational intensive (unless you reduce the
'Radial Limit' parameter). You can explore it later on your own.
|19. You can see that a new
grid object appears with the automatic name ANALYTICAL HILLSHADING.
|20. You can open it
immediately to see what it looks like. The shading is very apparent,
apparently calculated for a sun in the northwestern quarter.
|21. And if you magnify the
region of the Serpent, here's how it appears.
|22. You can experiment on
your own with colors, palettes, etc.
|23. Now let's move on to
another software option. Run the latest version of FugroViewer.
|24. Select FILE > OPEN LIDAR FILES
and navigate to and open the LAS file for Sepent Mound.
|25. Click on the 3D control.
26. Now a second view window opens. 2D is still on the left; 3D
is on the right.
27. Select the ZOOM tool. Then right-click on the left
map, and draw a box with your cursor to select and magnify the central
region, as you see here. The right map will zoom automatically to the
|28. Click the DISPLAY COLOR SHADED TIN
control along the top row of controls.
29. Now the serpent is very sharply emphasized, in both the 2D and 3D
|30. You can use very refined
controls over the TIN image. Select SETTINGS > TIN DISPLAY SETTINGS
to see them. On your own time you can explore how they work.
now, let's increase the vertical ("Z") exaggeration by increasing the value
to 7.5. Then click APPLY.
|31. After increasing the
vertical exaggeration you can also zoom in closer to see the mound itself,
as you see here. The detail is magnificent in this 2-D view, due to
the heavier shadow.
NOTE: There are very few XYZ points on the river
water surface, so you see just a few -- but quite large -- triangles. The triangles
on land are hundreds of times smaller, so you can't see them.
32. How does FugroViewer get such an excellent 3-D image? It uses a
special feature that allows the user to view the XYZ data as individual
triangles constructed from adjacent physical data points, called a TIN image.
Here is a very tiny part of the Serpent Mound data (2-D on the left; 3-D on
the right), to show you what's involved.
|33 The author posits that we
will see more point cloud data in our work, both for relief and possibly
also for ocean analyses. You should take time to learn more about how
Saga and FugroViewer work with LAS-type files. Make sure to save your
work in the usual ways when ready to exit.