Lesson 10c3: Clipping LAS Data and Creating Derivative Products in Glo

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By the end of this lesson, you will be able to load multiple LAS files, define and use a polygon area of interest to subset LAS files, and generate and export raster surfaces and contours derived from the LiDAR point cloud. If necessary, please review Lesson 10c1 for guidance with importing LAS files, filtering, and visualizing LiDAR point clouds in Global Mapper.
 

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Length: 00:17:00

Location Taken: Reston, VA, US

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Lesson 10c3: Clipping LAS Files and creating derivative data in Global Mapper. Lessons 10c1 and 10c2 introduced the basics of interacting with lidar point cloud data in Global Mapper. This lesson will cover cropping LAS tiles and generating derivative data in Global Mapper. If necessary, please review lesson 10c1 for guidance with importing LAS files, filtering, and visualizing lidar point clouds in Global Mapper.

By the end of this lesson, you will be able to load multiple LAS files, define and use a polygon area of interest to subset LAS files, and generate and export raster surfaces and contours derived from the lidar point cloud. In this tutorial we are using uncompressed LAS data, but The National Map also offers compressed data, which is compatible with Global Mapper as well.

If you’re interested in learning more about using USGS lidar data in other software packages, additional videos show how to use lidar data in ArcGIS Pro and LP360.

For this lesson, we will be using the same six tiles of LAS data from Lesson 10c1. If you have not done so already, please download the “USGS_LPC_CO_SoPlatteRiver_Data_for_Lessons.zip” file from our FTP site at : ftp://rockyftp.cr.usgs.gov/Training_Data/.

After you’ve downloaded the lidar data, extract the zip file into a folder on your local computer to use during the lesson.

The National Map has a download client where you can find USGS products including elevation data such as lidar point clouds at https://viewer.nationalmap.gov/basic/. If you are interested in learning more about downloading products on The National Map, be sure to check out our training videos located at http://www.usgs.gov/NGPvideos.

This exercise specifically uses Global Mapper version 20.0.0 with the lidar module installed. However, any version that is version 18.0 or newer should suffice for this tutorial.

Launch Global Mapper.

Make sure that your ‘LIDAR’ license is active before proceeding. Under the ‘Help’ menu, click on the ‘License Manager….’ In the next screen make sure the ‘LIDAR’ module is toggled on, then click ‘OK’.

Note that Global Mapper preserves settings from previous use sessions, so the defaults in your interface may differ slightly from what is shown in this tutorial.

From the splash page, select “Open Data Files. In the ‘Open’ window, browse to the location of the lidar point cloud LAS files that were downloaded from the FTP site. Select all 6 original LAS tiles and click ‘Open’.

We will load the LAS tiles in their entirety. Therefore, when the ‘Lidar Load Options’ window appears, make sure all point data are loaded by clicking the ‘Select All’ button for point classifications and return types. In the bottom left corner, toggle on the option to ‘Use These Options for All Lidar Files in the Current Group.’ Then click ‘OK’. Each of the 6 tiles will be loaded using the same settings. This may take a minute to process.

When the lidar data load in the view window, make sure all classes are enabled. To do this, click the ‘Filter Lidar Data’ button, then select ‘Enable All’ and click ‘OK’.

By default, in the 2D view window the LAS tiles are displayed with ‘Color Lidar by Elevation’ symbology in an ‘Atlas Shader’ color palette. Where blue corresponds to the lowest elevation, and red corresponds to the highest elevation values. Because Global Mapper preserves settings from prior sessions, if you have previously altered the symbology selections, your display may appear differently. If needed, use the drop-down menus to display the LAS tile with the settings seen here.

When working with many large LAS tiles, it is sometimes helpful to clip point cloud data to a specific area of interest to save processing time and storage space. Global Mapper uses polygon features to define the area of data to clip. The polygon feature can be an existing shapefile such as building footprints, or the polygon can be created directly in the Global Mapper 2D window. We will use the Digitizer (Create) Toolbar to create an area of interest polygon in Global Mapper.

If the Digitizer (Create) toolbar is not present on your screen, turn it on by selecting the ‘View’ menu, then click ‘Toolbars’. Active toolbars have a check mark next to them. If needed, toggle on the Digitizer (Create) toolbar now.

We will be creating an area of interest that omits the foothills on the west part of the tile. On the Digitizer (Create) toolbar click ‘Create Area Feature’.

Now, left click to create vertices and draw a polygon that encompasses the area of interest and avoids the foothills. Right click to finish drawing the New Area Feature.

A ‘Modify Feature Info’ window pops up to allow you to define a name for the feature, specify style, attributes, and the feature layer to save the polygon to. In the name field type ‘AOI’ for Area of Interest. Under the Feature Layer drop-down select ‘User Created Features’. Accept the default options for style and attributes and click ‘OK’.

You will now see a new ‘User Created Feature Layer’ appear in the ‘Control Center’ and the polygon labeled with ‘AOI’ appears in the 2D window.

To clip the LAS tiles loaded in the 2D window we must first select the polygon or polygons that will define the clipping boundary. We only have one polygon to select, however, these selection options can be used for selecting multiple polygons as well. You can use the ‘Digitizer Tool’ to select one polygon at a time by clicking on each feature.

Or you can select all polygons within one or more layers from the ‘Control Center’. Right click on the ‘User Created Features’ layer and click the ‘Select’ option to select all features within the layer. This will select our newly created polygon.

With the newly created polygon selected, right click in the 2D window and select ‘Crop/Combine/Split Functions’, then ‘CROP’.

In the ‘Crop to Selected Area(s) Setup’ window, use the ‘Feature Layer’ drop-down to select ‘Create New Layer for Feature’. Accept the remaining defaults and click ‘OK’. When the ‘Enter Layer Name’ prompt appears, type ‘Clipped LAS Tiles’ and click ‘OK’.

The ‘Layer Projection’ window that pops up allows you to define the projection information for the newly clipped layer. This window automatically reflects the current projection of the workspace. We will use this default. Click OK.

After a few seconds the new layer is added to the 2D display and ‘Control Center’. To view the new layer, turn off all layers by clicking on the check mark next to ‘Current Workspace’. Then turn on the ‘Clipped LAS Tiles’ layer.

This new layer can now be used as an input LAS file to any lidar processing functions.

Note that this layer is only saved in the memory of the current Global Mapper session. To save the newly created layer to an external file, right click on the tile name in the ‘Control Center’. Select ‘Layer’, then click ‘EXPORT’.

Select ‘Lidar LAS File’ as the ‘Export Format’ and click ‘OK’.

A tool tip pops up to inform you that the LAS file will be exported with the current display projection. This will export the LAS tile using the current horizontal projection but will not save a vertical projection. Click ‘OK’. In the ‘Lidar LAS/LAZ Export Options’ window click on the ‘Vertical Coordinate System’ drop-down and select ‘NAVD88’ to match the input data. Accept the remaining default options and click ‘OK’.

Now, navigate to where you saved the training data LAS tiles and save this exported tile as ‘Lesson10c3_ClippedLAS.las’. This may take a few seconds to export. The newly clipped lidar layer is now saved as a LAS file. You can now perform any lidar visualization, classification, and processing on just the data contained within your area of interest using the same work flow as you would for a single lidar tile.

Global Mapper provides the ability to create and export Elevation Grids and Contours derived from the point cloud. You could create a derivative product from a clipped LAS such as the ‘Lesson10c3_ClippedLAS.las’ file that we just created. Or you can create a derivative product from all loaded LAS tiles. We will use all six LAS tiles to create derivative products. Turn on all layers by clicking the ‘Current Workspace’ toggle, and then turn off the ‘User Created Features’ and the ‘Clipped LAS Tiles’ layers in the ‘Control Center’.

We’ll start by creating a Digital Elevation Model or ‘DEM’. Click the ‘Create Elevation Grid’ button on the ‘Analysis’ toolbar. This tool generates gridded elevation surfaces from input vector features. In this case, we will use the LiDAR point cloud as the input.

Next, make sure all six LAS tiles are selected in the ‘Select Layers’ window. Click ‘OK’.

In the ‘Description’ box type ‘DEM’. The ‘Vertical Units’ are detected from the input dataset and, as such, are in ‘Meters’. However, you could change them to ‘Feet’ if you desire the output grid to be in feet.

The ‘Grid Method’ defines how vector values will be used to generate the output grid. The default is ‘Triangulation’ which creates a triangulated irregular network using the LiDAR points. The ‘Binning’ methods create a raster output by overlaying a regular grid and calculating the minimum, maximum or average within each bin. We are interested in a bare earth DEM, so select ‘Binning (Minimum Value – DTM)’ from the drop-down menu.

The ‘Grid Type’ drop-down specifies the values used to calculate the grid. We will use the default ‘Elevation Values,’ however this drop-down provides additional options such as ‘Intensity Values’, ‘Height Above Ground’ and others.

Any lidar filter applied in the view window will carry over to this tool. We can also define the points used through the ‘Filter Lidar Points to Use by Elevation/Class/Color’ button directly in the tool interface. In the ‘Filter Lidar Points’ window, select ‘Clear All’ and then select class ‘2-Ground’ and click ‘OK’. The grid calculation will now only refer to ground points when creating an output.

The ‘Grid Spacing’ option defaults to automatically detecting spacing based on the point spacing of the input dataset. In this instance the ‘Automatic Spacing Multiple of Point Spacing’ is set to ‘5’. Optionally, you can also set the grid spacing to a desired resolution using the ‘Manually Specify the Grid Spacing to Use’ option. If you choose this option in the future, make sure the grid spacing is not less than the point spacing of the lidar data. We will accept the default automatic option.

This tool uses interpolation to fill any gaps from the input dataset in the ‘Elevation Grid “No Data” Distance Criteria’ setting. The ‘Grid Tightness’ slider allows you to specify how loose or tight to make the interpolation. Loose settings enable interpolation to fill areas with ‘No Data’ values whereas tight settings leave ‘No Data’ holes in the DEM. We will use a value around ‘25’ so the DEM output is continuous where buildings exist.

Accept the remaining Grid Options defaults. The ‘Tiling’ tab provides the user options to split up the data on an export. We want to output our results as a single tile, so keep the ‘No Tiling – Just One Export File’ option selected.

The ‘Grid Bounds’ tab provides several options to subset the dataset for the grid creation. We will accept the default to use ‘All Loaded Data’. Now click ‘OK’.

This will quickly calculate an output layer stored in the workspace memory. This raster is a bare-earth layer derived from the ground points of the lidar point cloud.

We can repeat the process to create a Digital Surface Model which will also contain above ground objects such as buildings and vegetation. Again, click the ‘Create Elevation Grid’ button. Make sure all six LAS tiles are selected in the ‘Select Layers’ window and click ‘OK’.

In the ‘Description’ box type ‘DSM’. Now change the ‘Grid Method’ to ‘Binning (Maximum Value -DSM)’.

Click on the ‘Filter Lidar Points to Use by Elevation/Class/Color’ button to define the points to use. To create a surface model we want to use above ground points while excluding all possible noise points. If you recall from lesson 10c2, these LAS tiles contain noise as well as misclassified noise points due to changes in LAS versions. Therefore, in the ‘Filter Lidar Points’ window, unclick class ‘7- Low Point (Noise)’, class ‘11- Road’ and class ‘18 – High Point (Noise)’.

The tool remembers the remaining settings from the previous run, so click ‘OK’ to generate the DSM raster layer.

Once the DSM layer generation is complete, click the ‘Full View (Home)’ button to center the layers. Then click the ‘Image Swipe’ button to compare our generated raster layers. This allows users to swipe the top layer out of the way to view the layer underneath.

In the ‘Select Layer to Swipe’ menu, select ‘DSM’ since that layer is currently on top in the 2D view window, then click ‘OK’.

Now click and hold the left mouse button on the edge of the tile and drag the cursor towards the center. You can see the DSM layer containing trees and buildings pulled back to reveal the underlying bare-earth DEM below. You can perform this ‘Image Swipe’ from any side to visually compare the two layers.

To get a more detailed comparison of the DSM and DEM, zoom into an area with buildings and perform the image swipe again.        

Global Mapper can also use these derived raster layers to generate contours. We want our contours to be based on the bare -earth surface and not above ground features; so, in the ‘Control Center’ pane deselect all layers, then select the DEM layer. Click on the ‘Create Contours’ button on the analysis toolbar.

The ‘Contour Generation Options’ window enables users to define the contour generation parameters. We will create contours with 5-meter intervals. These intervals will have minor contours every 5 meters and major contours every 10. The ‘Elevation Range’ and ‘Resolution’ options are detected from the input dataset. For most contour generation, it’s suitable to accept the defaults for these fields. Many additional options exist within this tool to constrain contour generation parameters, define the tiling scheme, and spatial extent of contours, but for our purposes we can leave them unaltered. Click ‘OK’ to create the contours. A new vector line layer will be generated within the Global Mapper workspace.

The generated contours will be superimposed with labels on top of the DEM surface created earlier. All layers we created are stored within the Global Mapper workspace but are not saved as an external file. To save files use the same steps performed earlier to export the clipped LAS tile. The export menu is accessed by right clicking on an active layer, selecting ‘Layer’, then ‘EXPORT.’ From there you can select an appropriate file format and export parameters and then save the file to a folder of your choosing.

This concludes Lesson 10c3: Clipping LAS Data and Creating Derivative Products in Global Mapper.

In this lesson, we discussed how to load multiple LAS files, use polygon features to clip LAS files, and generate and export derived raster surfaces and contours. If you are interested in learning more about using lidar data, please see the additional USGS lidar training videos in ArcGIS Pro, LP360 and Global Mapper at:  http://www.usgs.gov/NGPvideos.