Light Detection and Ranging (LiDAR) is an active sensing technology that measures distance from a source device to a target area using laser light. LiDAR sensors collect detailed elevation measurements which are becoming a common source for creating elevation surfaces. This guide will walk you through the process of obtaining LiDAR data from various sources and converting those data to digital elevation models (DEM) and contour lines.
There are several places to learn more about LiDAR and what it can be used for:
LiDAR - Wikipedia
Introduction to LiDAR - NOAA
LiDAR Glossary - Applied Geomatics Research Group
The availability of LiDAR data is becoming more and more widespread. Several sources offer access to free LiDAR data and the listing below contains links to sources of interest to WU users.
This example shows how to select and download data from the MSDIS LiDAR Data access page.
The page features information about Missouri LiDAR data products and links to download tools for LAS or DEM datasets. LAS files provide the greatest level of detail but require more processing in order to create useful products; this guide includes instructions for completing the necessary processing. DEM files are pre-processed digital elevation models of the ground surface and are not discussed.
Click on the LAS Download tool and close the Welcome statement...
The map shows the areas (blue polygons) in Missouri where LiDAR datasets exist.
Navigate the map to find your area of interest:
As you zoom in to your area of interest, you will see the outline of individual LiDAR tiles appear. These show the footprint for the LAS dataset and you will notice some overlap between neighboring tiles. Click on each tile of interest to reveal a 'PopUp' showing the Project and Tile names, then click the link to download the LAS file.
The files you download will all have a ".las" extension and will need further processing before you are able to convert them to contour lines using ArcGIS. See the next content box for ArcGIS processing instructions.
'Raw' LiDAR data (.las files) cannot be viewed in ArcMap but must first be imported into a LAS Dataset using the following procedure.
Open ArcMap or ArcCatalog and use the Catalog window to browse to your project folder...
Right click on the folder and select "New" "LAS Dataset"
Highlight the "New LASDataset" name and replace with something more appropriate.
Right click the LAS Dataset you created and open the Properties
Use the "LAS Files" tab to "Add Files..." - browse to the file(s) you downloaded in the first section to add them to the dataset. Once you are finished loading files, go to the "Statistics" tab and click "Calculate". You should see the Returns, Attributes, and other panels become populated with numbers reporting the Point Count, percent, and minimum and maximum elevation values. Note that the Classification Codes section will indicate if the data provider has classified the points; this will be important for creating digital elevation models (DEMs) which will use Ground points or digital surface models (DSMs) that are produced from First Returns.
Now you are ready to begin working with the LiDAR point cloud, see the next Tab for the steps to display and create products from this dataset.
Open ArcMap with a blank map document to begin working with the LASD.
Add the LASD to the ArcMap map document. Initially you will not see any points but you should see a red box outlining the area where the points will appear. This occurs because there are too many points for ArcMap to display at the full extent of the LiDAR tile. Zoom into a small area within the red box and you should see colored dots showing the LiDAR point cloud. The Table of Contents (TOC) pane will indicate the percentage of points being displayed (see the brown arrow below), the smaller the area you display, the higher the percentage of points shown.
The default display mode for a LASD is using LAS Point Elevations but there are other ways to view these data. Right-click on the LASD in the TOC, open the Layer Properties, and access the Symbology tab. The pane gives you access to teh classification settings for the current point elevation symbology but it also gives access to additional rendering modes. Click the "Add..." button to access the full menu of other symbology options for this layer.
Selecting "Contour with the same symbol" will add another renderer item to the Symbology tab and will create contour lines on the display. Contour lines created from raw LiDAR can be very noisy as illustrated below; there are many vertices and small areas that will make the contour layer difficult to use.
These issues can be resolved by first converting the LiDAR layer into a raster dataset prior to creating the contours. See the next Tab in this series for details.
Once a LiDAR dataset has been created, there are several tools in ArcGIS that can use the point cloud to create raster surface layers or vector contour lines. The best results are obtained when the LiDAR is first converted to raster, then the raster layer is used to create contour lines.
Convert the LASD to a raster surface layer using the LAS to Raster tool
This tool creates a raster by aggregating or interpolating between discrete LiDAR elevation points. For contour creation, the Binning method is preferred using AVERAGE assignment type and LINEAR fill. For the Input LAS Dataset, use the layer from your ArcMap Table of Contents and make sure that you've filtered the LASD to display only 'Ground' points if you are creating a Digital Elevation Model (DEM) or 'First Return' points if you are creating a Digital Surface Model (DSM). Use the Sampling Type and Sampling Size settings to specify the appropriate cell size in your output raster to support the contour interval you are planning to create. If you do not need decimal accuracy, you can set the Output Data Type to Integer (INT).
Converting Raster Surfaces to Contour Lines
The Contour tool creates contour lines connecting the same level of elevation. Now that you have a smoothed raster layer representing a DEM or DSM, you can convert this to contour lines using the Contour tool. Set the contour interval to match your target value and use the Z-factor to adjust the native LiDAR elevation units to some other unit (if desired)
This entry from ESRI Support provides more detail on the processing above.
The contour output obtained from LiDAR data can sometimes be jagged or blocky. This entry discusses some ways to address this issue.