Developing a Topobathy Digital Elevation Model (DEM)
LiDAR, or Light Detection and Ranging, is a method used to remotely study the surface of the earth. LiDAR data is most commonly collected through the utilization of airplanes and helicopters; these carry the instrumentation necessary to collect these data. This specialized instrumentation consists of a laser, a scanner and a GPS receiver. This equipment collects information about the surface of the earth through a series of pulses of light, creating a three-dimensional representation of the surface.
Airborne topobathy LiDAR is collected in a similar manner to the traditional terrestrial LiDAR described above, however in addition to the near-infrared wavelength (1000 – 1500 µm) laser, a blue-green wavelength ( 532 µm) laser is utilized. This blue-green wavelength laser allows for penetration through the water column.
In early 2014 the National Oceanic and Atmospheric Administration (NOAA) National Geodetic Survey (NGS) collected topobathy LiDAR for 2,775 miles of the Atlantic Coast from New York to South Carolina. This provides coverage of the highly damages areas of Hurricane Sandy in late October 2012, including coastal National Parks such as Assateague Island National Seashore in Maryland and Virginia, Fire Island National Seashore in New York, and Gateway National Recreation Area in New York and New Jersey.
Using the following steps, users will learn how to create their own topobathy digital elevation model (DEM). This involves selecting the correct classification codes so that both bare earth (ground) elevations and bathymetric bottom (submerged topography) elevations are visible.
If you haven’t downloaded any elevation data, please refer our blog post titled “Obtaining Elevation Data from NOAA Digital Coast.” This will walk you through where high quality elevation data can be downloaded for free, and the necessary steps to do so. Within this workflow we will be working with the 2014 NOAA NGS Topobathy LiDAR for Fire Island National Seashore.
Once you’ve downloaded some LiDAR, please follow the steps below to begin creating your high-quality topobathy DEM!
- Create a new LAS Dataset (LASD – .lasd) within the file folder containing the LiDAR data.You don’t have to create it in the same folder, I just do this to keep my data organized; this is totally my personal preference.
- Double click the newly created LASD to open the properties dialog. Navigate to the LAS Files tab. At the bottom of the dialog, select Add Folders… (circled in red below); add the workspace (folder) containing all of your LAS files for the area of interest. Once these have been added, click Apply.Note: When you choose to add a folder, you only select the folder containing the data (see green arrow below) and click Add (see green circle below).
- Next, navigate to the XY Coordinate System tab to check the projection information of your data. For the NOAA NGS 2014 Topobathy LiDAR, this should be NAD 1983.
- Now, we need to calculate statistics for the LAS files.This will provide information on point counts per classification and return, minimum and maximum elevation values for each classification and return, as well as many other useful statistics. It is necessary to do this, so that the vendor-specified classification codes are properly associated with the data.
Navigate to the Statistics tab. In the bottom right corner, click Calculate. Depending on the number of input LAS files, this can take some time.
This will begin calculating statistics for each input LAS file. Within the directory containing the LAS files, a new LASX (.lasx) file will be created; this file houses the statistics information, and one will be created for each input LAS file.
Once statistics have been calculated, the Statistics tab will look like this:
Click Apply, and OK to save these changes and exit the LASD dialog box. Drag and drop the LASD into your ArcMap document.
- Next, in the Table of Contents, double click the LASD to open the Properties dialog box. Navigate to the Filter tab.Within the Classification Codes portion of the dialog (see below), check off boxes for 2 Ground and 26 Reserved. This will filter out any return that does not fall within these two classifications.
The classification code 2 Ground corresponds to all ground returns, also commonly referred to as bare earth. To determine which classification code corresponded to the bathymetric data, it was necessary to consult the metadata provided by NOAA NGS. Within the first process step (contained within the Data Quality Informationsection), the vendor listed all classification code definitions. Code 26 Reserved is “bathymetric bottom and submerged topography.”
By selecting these two classification codes, it will enable us to create a topobathy raster displaying both bare earth (ground) elevation values and bathymetric (submerged topography) elevation values.
- Within the Search pane of ArcMap, search for the LAS Dataset to Raster tool (alternatively, this can be found within the Conversion Toolbox). Using the following parameters, fill in the tool dialog (see below for example):
- Input LAS Dataset = previously created LAS dataset
- Output Raster = Select output location
- If writing raster to a file geodatabase, no file extension is needed.
- Interpolation Type – Binning
- Cell Assignment Type = Average
- Void Fill Method = Natural Neighbor
- Output Data Type = FLOAT
- Sampling Type = CELLSIZE
- Sampling Value = 1Once the dialog is completed, click OK to run. It could take a few hours to output a raster dataset (depending on size of dataset and processing speed of computer).
- Once completed, add the raster to ArcMap to ensure it was outputted correctly.
This blog posting was developed with the support of a competitive grant (cooperative agreement number P09AC00212; task agreement number P13AC00875) from the National Park Service in partnership with the North Atlantic Coast Cooperative Ecosystems Studies Unit.