Marine Data Literacy 2.0

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 Home > 7. Bathy/Topo Data > 7.7 Point Cloud LAS

7.7 Displaying LAS-Format, Point Cloud LIDAR Data: Saga & FugroViewer

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 shadow.

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 2.1 Saga Preliminary Setup
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 options.

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 plotted, 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 CLOUD VIEWER.
  • 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 m)
  • 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 180.

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 steps below.
13.  First we can consider working with either a TIN version of the data or a grid, to improve visualization. 
  • 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 tool is 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 initially automatically 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, with 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 grid.
18.  Select TOOLS > VISUALIZATION > GRIDS > TERRAIN MAP VIEW.

Then make these choices, and click OK.

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 same area.

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 views.

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.

Right 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.