LiDAR – Light Detection and Ranging
LiDAR utilizes laser pulses to image the earth, generally from aircraft or terrestrial vehicles. The Laser pulses strike an object; the pulse is reflected by the surface and then received by a sensor within the unit. LiDAR devices are becoming small enough that they can be flown on UASs, but in general, are still flown by manned aircraft for remote sensing purposes. LiDAR is also used in self driving automobiles. LiDAR is an active measurement system since it emits the energy that is returned to the instrument through the reflected pulses. By knowing the speed of light, the time between the transmission and the reflectance, a distance measurement can be determined. Some of the LiDAR techniques are similar to Radar, but in the visible and near visible spectrum. There are two basic types of LiDAR used in remote sensing; topographic for land surveying using NIR (near IR) and bathymetric used for measuring sea and river beds depths using a green laser[1].
In a LiDAR system there are tens of thousands of pulses (or more) being generated per second. These returned reflections generate a point cloud when viewed. The distance to the object is the distance from the aircraft to the object, thus higher elevation objects have a shorter distance to travel since they are closer to the aircraft. The distance to the aircraft is a product of the speed of light times the reflection time divided by two. The division by two is required since the time is a measurement from emission to return which is twice the distance to the object. The time for return is very short and depends on the altitude of the aircraft above the surface (z), which will be established, as well as the x and y position of the plane using GNSS technology. The speed of light is:
The position of the aircraft in all three directions is determined by the use of the GNSS network of satellites. An aircraft at 1,000 meters would have a pulse time of only about 6 microseconds.
The aircraft position (pitch, roll and yaw) is determined and corrected using the Inertial Measurement Unit (IMU), which is composed of three gyroscopes. If a single set of pulses with the same time reading is received then it is likely from the surface of the earth, but if multiple timed pulses are received it might be the reflectance of trees and also the ground, thus the shorter timed pulses would be eliminated if the purpose is to map the surface.
Generally, LiDAR will use either UV, visible or NIR as the source radiation. The returned point cloud can be an extremely large data set. The point cloud can be converted into a digital elevation model (DEM) that can be converted into TINS, Hillshade, contour lines and slope measurements. LiDAR can be used to filter out vegetation from the data set and thus give a true image of the surface of the earth. Using multiple wavelength lasers the depth of some water features can be measured. Flying LiDAR is expensive and thus, in general, is only flown periodically, but accurate height measurements are useful to both the scientist and the general population. For example, a hiker can obtain accurate elevations for a proposed trip, floodplain determination; first responders looking for missing/lost individuals can have a better understanding of the terrain. LiDAR can also be used to understand the potential for landslides and measuring surface changes.
Recently LiDAR was used in the dense jungles of Guatemala to discover an ancient city that may have had more than a million inhabitants. The project was done with support from the National Geographic and PACUNAM Foundation. The team mapped more than 2100 square kilometers of the Maya Biosphere Reserve[2]. This research is completely changing the understanding of the Mayan society. Much of the Earth’s surface has never been mapped using LiDAR.
Information Sources:
Information for this lesson was obtained from the GST 105 Model Course of the National Geospatial Technology Center of Excellence, a National Science Funded project and https://en.wikipedia.org/wiki/Lidar.
References:
[1] https://oceanservice.noaa.gov/facts/lidar.html
[2] https://news.nationalgeographic.com/2018/02/maya-laser-lidar-guatemala-pacunam/
Information Sources:
Information for this lesson was obtained from the GST 105 Model Course of the National Geospatial Technology Center of Excellence, a National Science Funded project and https://en.wikipedia.org/wiki/Lidar.
References:
[1] https://oceanservice.noaa.gov/facts/lidar.html
[2] https://news.nationalgeographic.com/2018/02/maya-laser-lidar-guatemala-pacunam/
Process
The process of taking a LiDAr data set and converting it into a DEM can be done in many different geospatial applications including ArcMap Desktop and ArcGIS Pro.
ArcMap Desktop Methodology
This exercise was created by Demetrio Zourarakis. This exercise requires ArcMap for Desktop version 10.5.1 or higher with the 3-D Analyst and Spatial Analyst extensions enabled.
MATERIALS
ArcMap Desktop Methodology
This exercise was created by Demetrio Zourarakis. This exercise requires ArcMap for Desktop version 10.5.1 or higher with the 3-D Analyst and Spatial Analyst extensions enabled.
MATERIALS
- Image Services (add a GIS Server connection in Catalog Window):
- The Kentucky Aerial Photography and Elevation Data Program and KYGEONET web mapping services to download the .laz tile at: http://kygeonet.ky.gov/govmaps/KyFromAboveGallery/map.html?webmap=b5ff91df6309491090c20333c8f58f52
- Download this folder and decompress it to the C: drive on your computer.
II- Procuring a KYAPED .laz tile
- Access the KyFrom Above Map Gallery (http://kyfromabove.ky.gov/). Under Web Maps, click on “Download Point Cloud Data”
- Search for (upper right hand corner) 1000 Community College Dr., Louisville, KY
- Click on the image and download the N087E223.laz tile, saving it to the c:\Student\Data\LiDAR folder, refer to Figure 1.
- Download the laszip.zip file saving it to the c:\Student\Software folder, refer to Figure 1.
- Decompress the laszip executable file in the same folder
- Run the laszip.exe executable, see Figure 2
- See Figure 2, Click on the Browse button (left side) to expand the window, change directory to c:\Student\Data\LiDAR
- Add and DECOMPRESS (right side) the N087E223.laz tile and select Quit
- Using File Explorer, examine the c:\Student\Data\LiDAR and notice the difference in size of the .laz and .las files
III- Creating a LAS Dataset in ArcGIS for Desktop
- Open the LiDAR_DEM which is located at c:\Student\Software\
- The HistoricImagery2016 service from the LOJIC services @ ags1.lojic.org
- The Ky_NAIP_2016_2FT service from the kyraster.ky.gov services
- Navigate using a bookmark named SW_Campus_JCTC
- Located in the Data Management toolbox, the LAS Dataset toolset, select the Create LAS Dataset tool
- Run the Create LAS dataset
- Add the N087E223.las file
- Choose as output the c:\Student\Data\LiDAR folder with a name of “N081E226” for the lasd file
- Select the Coordinate System, the KY Single Zone State Plane Coordinate System (FIPS 1600, NAD83, US Survey Feet)
- Select Create PRJ For LAS Files from the pull down
- Check compute statistics.
- Click OK
- Zoom to the LAS dataset
- Open the layer properties
- Select the Display tab
- Input 5,000,000 points for Point Limit (Do not use commas)
- Pull the slider to Fine for Point Density
- Check Use Scale For Full Resolution and input 1000
- Check LAS file extents and use a red outline
- Select the Filter tab
- Select the Ground button
- Select Apply and OK
- Open the catalog
- Locate the LAS Dataset (.lasd)
- Go to the LAS Files
- Note the point spacing (nominal point distance (NPD))
- Close properties window
IV- Interpreting, exploring and visualizing the LAS Dataset
- Using the Effects toolbar (Customize/Toolbars),
- Select the swipe tool, see Figure 3
- Go to the red line at the top of the point and drag down (can also be done from the bottom or sides)
- To turn off the Swipe Pointer click on the Select Element Arrow
- In the Catalog window access the Properties of the N081E226.lasd
- Set the Vertical Coordinate System (select the Z Coordinate System tab)
- Select the Vertical Coordinate system folder
- Next select North America
- Then select NAVD88 (height) (US Feet), click OK
- Using the catalog window select the properties of the LAS dataset file, open the LAS files tab. Explore the contents of the LAS tile loaded.
- Version
- Point Count
- Point spacing
- Vertical range (Z values)
- Click on the ellipsis under Statistics to view additional information
- Close the window
- Zoom to the extent of N081e226.lasd
- On the Table Of Contents (TOC), on the properties window select the Filter tab
- To see the filter effect apply must be selected after the property
- Filter for Ground using the Predefined Settings
- Buildings should be unclassified by this filter
- Filter for Non-Ground using the Predefined Settings
- Trees and buildings should be classified by this filter
- Filter for Unassigned by selecting the Unassigned check box
- To see the filter effect apply must be selected after the property
- Ensure that the 3D Analyst Extension is activated (selected under the Customize tab)
- Turn on the 3D Analyst tool bar by going to the Customize tab
- Turn on the LAS dataset toolbar by going to the Customize tab
- Select Ground by using the Filters pull down
- Using the pull down to the left of Filters select elevation to create a TIN
- Note: the higher areas are in yellows and reds (assuming default colors are used)
- Open the Properties for N081e226.lasd
- Open the Symbology tab
- Select add
- Select Edges with the same symbol
- Close Add Render Window
- The results should show the edges of the triangles created in number 8
- Add unique symbolization for LAS point elevation
- Cross Section (Profile)
- Zoom to the hill on the north edge of the tile
- Using the LAS Dataset Tool Bar select the Profile View Tool (Second button from the right)
- Select a north anchor point for the profile (single click)
- Select a south anchor point that goes through the hill for the profile (single click)
- Expand the single line very slightly to the east and click
- A profile should appear
- Select the Measurement Tool on the Profile view
- In the Profile view select the lowest elevation point
- In the Profile select the highest point
- Note the difference
- Close the Profile view
- On the LAS Toolbar select the 3D View (button to the far right)
- Using the scroll wheel on the mouse zoom to the hill
- Place the cursor near the hill, click and hold, rotate and tilt the image
- Open the open the TOC Properties of the layer (N081e226)
- Open the Filter tab
- Select all classes (check box)
- Close the Layer Properties
- In the 3D view refresh the image
- Close the 3D view
V – From points to surfaces and terrain derivatives
- Using the LAS Dataset Toolbar
- Use the filter pull down and select Ground
- Using the Catalog create a new file Geodatabase
- Open the Toolbox
- Select Conversion Tools
- Select To Raster
- Select LAS Dataset to Raster
- A new window will open
- Input the LAS Dataset using the pull down
- Output saved in the file Geodatabase
- Name the file DEM
- The value field, select Elevation
- Select Interpolation Type to be Triangulation
- Set method to Triangulation by Natural Neighbor
- Sampling value change to 2 (2 feet)
- Select OK
- A DEM should be displayed, make sure the LAS Dataset is not selected
- Open the TOC properties of the DEM
- Select the Symbology tab
- Select Classified (left side)
- Change the number of classes to at least 16
- Select a color ramp and apply
- Select the Windows tab, select the Image Analysis window
- Highlight the DEM
- Under the Processing Pane select Shaded Relief
- Use the Swipe in the Effects Toolbar to compare two layers