Mosaic Datasets and LiDAR Data: Advanced Processing
This blog post builds on the techniques discussed in the “Mosaic Datasets and LiDAR Data: An Introduction” blog post. Here we go more in-depth and further explore the functionality that mosaic datasets offer LiDAR data. First, we will create a “Surface with Features” mosaic dataset, then we will create a Surface Height layer, and lastly we will create, examine and compare elevation profiles.
This post was written with ArcGIS for Desktop 10.2.2 in mind, and assumes that you have the 3D Analyst Extension available to you.
We begin by creating a mosaic dataset with surface features. This means that the mosaic dataset of the LiDAR data will show all features on the Earth’s surface such as trees, buildings and even cars.
First, create a new mosaic dataset in a file geodatabase by right clicking on the file geodatabase in ArcCatalog and selecting New > Mosaic Dataset. Ideally, create this mosaic dataset within the same file geodatabase as the Bare Earth DEM and the Shaded Relief mosaic datasets that were created in the previous blog post. Use a similar naming scheme as well, such as “BlockIsland_SurfacewithFeatures_LiDAR_2011.
- Coordinate System = select the proper coordiante system for your area of interest (AOI).
- This is the horizontal coordinate system.
- 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:
- Check the metadata associated with the LiDAR data. If this is unavailable, continue on to the next option.
- Create a temporary LAS Dataset and view the properties:
- Within a folder, create a new LAS Dataset. Do this by right clicking the folder in ArcCatalog and selecting New > LAS Dataset.
- Double click on the new LAS Dataset in the Catalog window. Under the LAS Files tab, choose to Add Filesby clicking the button near the bottom of the dialog box. Navigate to and select a single LAS file (.las) and click OK.
- 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. Alternatively, you can specify the coordinate system of your mosaic dataset (which is the horizontal coordinate system) to be in the same units as the vertical coordinate system.
To add rasters to the mosaic dataset, right click on the mosaic dataset in the ArcCatalog window, and choose Add Rasters from the context menu. Use the following specifications to fill out the tool:
- Raster Type = Table
- Input Data = Bare Earth DEM mosaic created in the previous tutorial.
- Advanced Options:
- Input Data Filter = ObjectID<2.
- This will select the mosaic dataset and exclude the overviews of the Bare Earth DEM dataset. This will work as long as your overviews are the second row of your attribute table.
- Input Data Filter = ObjectID<2.
- Leave the remainder of the tool as defaults values.
- Click OK to run tool.
Select the Footprint feature by opening the Attribute Table of the Footprint layer of the Surface with Features mosaic layer tree in the Table of Contents. Select the first row in the Attribute Table for the Point Cloud Data, and then close the window.
Now, right click the Surface with Features Footprint layer in the Table of Contents and click Selection > Batch Edit Raster Functions in the context menu. Follow the steps below:
- Select Edit Raster Function and click next.
- Click Search next to the Find location by: Function Name, and select the LASToRaster function and click next.
- Click LAS To Raster in the Edit raster function properties list and select the following options:
- Return Types = Any
- Class Types = Any
- Now, click on Update Settings in the Edit raster function properties list and check boxes for ReturnTypesSelected and ClassTypesSelected.
- Click Next, then Finish.
- Clear the selected features with the button from your toolbar.
Next, right click on the Surface with Features mosaic dataset in ArcCatalog and choose Optimize > Build Overviews. You may need to refresh the view first to see the Optimize function. Leave all values as default and click OK to run the tool.
Now we will examine how to enhance the appearance of the mosaicked image using the Image Analysis Window. To open this window, go to the Windows dropdown at the top of ArcMap and select Image Analysisfrom the context menu.
It is important to note that the changes made in the Display section of the Image Analysis Window are NOTpermanent, but we will discuss this in more detail shortly. While many changes can be made to the image through the use of the Image Analysis Window, only changes made in the Processing section can be saved and exported. This can be done by right clicking on the raster generated by the Image Analysis Window in the Table of Contents and clicking Data > Export Data. Once exported, the file will be saved in raster format.
First, bring the Surface with Features layer and the Bare Earth DEM into the Table of Contents, with the Surface with Features layer on top. Next, zoom in on an area of interest (AOI).
- Check boxes next to, and then select both the Bare Earth DEM and the Surface with Features layer in the Image Analysis layers then check DRA (see image below for more details.
- Change the Interactive Stretch method to Percent Clip (green arrow below).
- Change the Resample method to Cubic Convolution (blue arrow below)
Now, toggle the Surface with Features layer on and off to see the surface with and without features (such as buildings). As you can see below, you should be able to see a distinct difference between the two layers.
Now we will begin to create a Surface Height layer. To do this, we will essentially subtract two rasters from one another.
First, double check to make sure the Bare Earth DEM layer is still below the Surface with Features layer (it should be already since we did this in the previous step). Next, open the Image Analysis window.
- Select both of the layers mentioned above in the Image Analysis layer panel at the top of the window.
- Under the Processing section, click the Difference Button (circled in red above).
- Change the symbology on the newly created layer to an easy to visualize color scheme of your preference.
- Remember, to save this raster it is necessary to export it from the Table of Contents.
- To do this, right click the “Diff_Image” and select Data > Export Data.
Below is a sample of the Surface Heights layer (also called the “Diff_Image”). This essentially subtracts the Bare Earth DEM layer from the Surface with Features layer and shows the heights of features in the area of interest. The area shown below is the difference between the two images on the previous page for the Bare Earth DEM and the Surface with Features. Areas shown in red and orange indicate features with greater height differences.
Lastly, we will determine how to examine and compare profile graphs of elevation data.
First, zoom in to an area of interest. Next, in the Image Analysis window, select the “Diff_Image” and check DRA.
- Open the 3D Analyst toolbar and select the Bare Earth DEM image in the dropdown window (purple circle).
- Click the Interpolate Line button (red circle) on the 3D Analyst toolbar and draw a line across the AOI.
- Click the Profile Graph (green circle) button to view the graph of the profile.
To compare this profile graph, to the profile graphs of other layers for the same AOI, select a different image in the 3D Analyst dropdown window and follow the same procedure. This graph provides you with elevation data for the chosen transect.
All graphs can be edited by right clicking a graph and selecting Advanced Options. This is where the user can control the appearance of the graph, the text of the graph (title, subtitle, footer, axis labels, etc.), font options, 3D graph view, and many other options.
You can show multiple profile lines on one graph by drawing multiple lines, then selecting them at the same time by holding SHIFT and clicking them.
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.