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Mosaic Datasets and LiDAR Data: An Introduction

Mosaic Datasets and LiDAR Data: An Introduction

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.

While the most typical way to utilize LiDAR data is through the use of LAS Datasets, it is also possible to manipulate these data through the use of mosaic datasets. This powerful way to store and manage LiDAR data is extremely effective as it provides the user with added functionality to enhance the resulting raster and produce a polished, high quality final product.

If you haven’t downloaded any elevation data, please refer a recent 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. While this blog post refers to downloading elevation data for Fire Island National Seashore, you can alternatively search for and download the elevation data that will be used in this workflow. Pan to Block Island, Rhode Island and select the “2011 USGS LiDAR for the Northeast.”

First, create a new mosaic dataset within a file geodatabase by right clicking on the file geodatabase in ArcCatalog, and selecting New > Mosaic Dataset. This will open the Create Mosaic Dataset tool where details about the input LAS files will be specified. The first step is to name the mosaic dataset; a best practice for this would be to select a name that specifies the collection area, type of data and the date of collection (ie. BlockIsland_BareEarth_LiDAR_2011). Next, use the following specifications to fill out the remainder of the tool's fields:

  1. Coordinate System = NAD 1983 UTM Zone 18N (for ASIS, FIIS, & GATE)
    1. This is the horizontal coordinate system.
  2. Click OK to finish creating the mosaic dataset.

To determine the units in which the elevations will be represented, it is necessary to know the units of the vertical coordinate system. To determine this, use one of the following methods:

  1. Check the metadata associated with the LiDAR data. If this is unavailable, continue on to the next option.
  2. Create a temporary LAS Dataset and view the properties:
    1. Within a folder, create a new LAS Dataset. Do this by right clicking the folder in ArcCatalog and selecting New > LAS Dataset.
    2. Double click on the new LAS Dataset in the Catalog window. Under the LAS Files tab, choose to Add Files by clicking the button near the bottom of the dialog box. Navigate to and select a single LAS file (.las) and click OK.
    3. Once this appears in the LAS Files tab, click the  button. This opens the LAS File Properties and Statistics dialog which provides statistical information, in addition to information on the Z (elevation) units. Here, you can see that the vertical Z units are in meters (see below).

Note: If you discover that the vertical coordinate system (Z units) is not the same as the horizontal coordinate system units, it is okay. We will address how to account for this later in the procedure.

Next we will determine the average point spacing of the LAS files. It is important to do this, as you will need this information in the next step to determine the appropriate pixel size for the mosaic dataset. To do this, search for and open then Point File Information tool. Use the following specifications to fill out the tool:

  1. Select all LAS files as inputs.
  2. Select the output name and location for the new dataset.
  3. File Format = LAS
  4. File Suffix = .las
  5. Select the coordinate system of the input data.
  6. Check “Summarize by Class Codes” – this is optional. It will add an additional field to the output attributes table specifying return code. This will allow the user to determine the average point spacing for each different return code.
  7. Click OK to run tool.

This will create a polygon shapefile. To determine the average point spacing, open the attribute table. Sort the Class field and select all records with the proper return code (2 = ground returns {based on ASPRS class code definitions}, and all records for “all” returns). Find the field labeled Pt_Spacing, right cliclk on the heading and select Statistics. This will open a Statistics pop-up that will provide the Mean (or average) point spacing. Note this value!

To add rasters to the mosaic dataset, right click on the mosaic dataset in ArcCatalog, and choose Add Rasters from the context menu. Within this tool fill in the following specifications:

  1. Select the Raster Type to be LAS.
    1. A warning icon will appear next to the Raster Type subsection; click the Edit Raster Type Properties button next to the raster type selection box (see below).

  2. Click the LAS tab within the Raster Type Properties dialog box.
  3. Return type = Any
  4. Class type = 2. This specifies LiDAR returns that've been categorized as "bare earth" assuming your LAS data conform with recent ASPRS LAS file format specifications.
  5. Pixel Size = Calculated by multiplying the average point spacing by 3 or 4.
    • This ensures that all gaps and voids and filled in the dataset.
    • Enter the calculated pixel size into the appropriate field in the Raster Type Properties dialog box.
    • Void Filling = Plane Fitting/IDW
    • To change from vertical units of meters to feet, the Z factor can be changed; refer to the table before for conversion factors.
  6. Check treat each folder as a dataset box near bottom of dialog box.
  7. Click OK to return to the Add Rasters dialog box.

Next, finish filling out the Add Rasters dialog box.

  1. Select Workspace in the Input Data dropdown menu and browse to, and select the folder containing the LAS files.
  2. Under the Advanced Options section select the same coordinate system as used in the Create Mosaic Dataset tool.
  3. Click OK to finish adding LAS files (see below for the completed Add Rasters dialog box).

Next, right click on the mosaic dataset and choose Optimize > Build Overviews. Leave all values as default and click OK to run.

You have just created a Digital Elevation Model (DEM) mosaic dataset from LiDAR data!

Now, we will work on creating a Shaded Relief mosaic dataset. This process will be much shorter as we will be creating a referenced mosaic dataset. This type of mosaic dataset will directly reference the Digital Elevation Model mosaic dataset that we just created, and will also use all of the specifications used to create it.  

To begin, in the ArcCatalog window, right click the DEM mosaic dataset and select Create Referenced Mosaic Dataset from the context menu.

This opens the Create Referenced Mosaic Dataset tool. Change the name of the output mosaic dataset to be something like BlockIsland_ShadedRelief_LiDAR_2011 and click OK.

Next, in ArcCatalog, right click on the Shaded Relief mosaic dataset and open the Properties dialog box. Once open, click the Functions tab.

  1. Right click Mosaic Function > Insert and select the Shaded Relief Function (circled in red below).
  2. Accept all defaults and click OK.

Note: If you are interested in exaggerating the Z factor - you can do this here.

It is important to note that many other functions such as Slope, Aspect and Hillshade (circled in green above) can be added to separate mosaic datasets, or can be substituted in for the Shaded Relief Function.

There are also some important caveats to note when working with referenced mosaic datasets. They will prohibit the user from building overviews, adding new rasters and calculating pixel size ranges. While this may not be ideal in some situations, the use of referenced mosaic datasets can also help to significantly reduce the amount of time needed to work with data.  Another important caveat is that if the original mosaic dataset that the referenced mosaic dataset refers to is moved or deleted, the referenced mosaic dataset will become invalid.

While creating a referenced mosaic dataset is useful in some cases, they may not be ideal in all situations. If a user does not want to create a referenced mosaic dataset, simply follow the first procedure and create a new mosaic dataset, then apply the Shaded Relief Function as described in the steps above.

 

 

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. It is part of a larger document available for download on the IRMA Portal.

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Authored by

erica
 

Last updated

10/15/2015 - 14:39