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Lidar data, stored as either LAS files, LAS datasets, or terrains, can be added to a mosaic dataset directly, without converting them to a raster before adding them. The output of the mosaic dataset will be a raster.
Why add lidar data to a mosaic dataset
If you're creating a mosaic dataset with your elevation data, you can add the lidar data. This data can be combined with other elevation data, such as SRTM or NED raster data sources.
Lidar data can be managed within a mosaic dataset by adding the LAS files, LAS datasets, or terrain dataset. For example, you can add all your LAS files, from all your different acquisitions, and use the mosaic dataset as the single source for finding the data as it's needed.
Using the mosaic dataset, you can also serve the lidar data as an image service providing access to the interpolated surface, or even to allow users to download the source files.
How to add lidar data to a mosaic dataset
Lidar data can be added to mosaic datasets using their associated raster types.
- LAS raster type—Use to add lidar data stored as .las files.
- LAS Dataset raster type—Use if the LAS data has been added to a LAS dataset.
- Terrain raster type—Use to add terrains stored in the geodatabase.
When these types of data are added to a mosaic dataset they are converted into raster data. Each raster type adds a unique function which can be edited but not added to or removed from the item in the mosaic dataset.
The output properties are unique to these raster types. Since the inputs involve some sort of interpolation from points, this can be quite computationally intensive and therefore slow to display. There is an option to create caches at the base pixel size for the inputs to improve performance. Without the cache you may have to wait several minutes for some surfaces to display.
LAS file additions to the attribute table
When adding LAS files directly to the mosaic dataset, there are some additional fields that get added to the mosaic dataset's attribute table to support the LAS data.
|LAS specific fields
The version of the LAS file. LAS is an industry format created and maintained by the American Society for Photogrammetry and Remote Sensing (ASPRS).
Total number of points in the LAS file.
Average spacing between points in the LAS file in the units of the LAS dataset.
Minimum point value in the LAS file.
Maximum point value in the LAS file.
Steps to add lidar data to a mosaic dataset
- First create a mosaic dataset with the Create Mosaic Dataset tool.
In the Catalog window
or ArcCatalog, right-click the mosaic
dataset and click Add
The Add Rasters To Mosaic Dataset tool opens.
- Click the Raster Type drop-down list and click one of the following:
- LAS Dataset
There are properties that need to be set when using either of these raster types, therefore, you cannot run this tool without setting those properties.
- Click the Edit Raster Type Properties button .
Click one of the following tabs, depending on the raster type you chose:
- Las Dataset
You will likely want to modify many of these parameters. To learn about each of the parameters on this tab, see either:
- You can accept all the defaults, but you must enter a Pixel size.
The pixel size is the minimum pixel size that will be generated to create the raster. It is better to go with a pixel size that is several times larger than the average point spacing but small enough to identify gaps or voids. Generally, if the pixel size is three or four times greater than the point spacing, the voids in the data should be filled (unless, for example, the voids are due to water). For example, if your data is sampled at 1 meter and your pixel size is 4, you can expect, on average, to get 16 points in a pixel.
- If the pixel size is 3-4 times larger than the average point distance, you can safely use binning. If the cell size is smaller than that, you can try binning with void filling turned off. If the resulting raster mainly contains voids and only a few single data cells, binning generally does not produce a meaningful elevation raster. You need to either increase the pixel size or switch to triangulation. If the resulting raster shows enough content with some salt and pepper voids, and maybe a few larger voids, you can use binning with void filling turned on. Click the Void filling drop-down arrow and select either Simple or Plane Fitting/IDW.
- If you're adding LAS files, you may want to check Treat each folder as a dataset.
This will add all the files in a folder as a single item in the mosaic dataset, which is more efficient for the mosaic dataset. You may choose to check this option if all the LAS files in a folder belong together and have the same spatial reference. For example, they may represent a single data collection (project) that are just stored as tiles.
If the LAS files are unrelated to one another, not part of the same project collection, or don't have the same spatial reference, then don't check this option.
- You may want to modify the Cache folder and Number of cached surfaces as they will take some space on disk and need to be moved with the mosaic dataset if it's moved.
A cache is generated to improve the display time for this data. By default, the cache is stored in a folder next to where the mosaic dataset resides. If you will be modifying the properties of the function to show different views from the same mosaic dataset, then you will want to plan to generate a number of cached surfaces, because one will be created for each modification.
The cache will be created the first time the dataset is drawn; therefore, it will take some time before the surface can be viewed. Once it's cached the surface will display quickly. Otherwise, you can build the cache by building the overviews for the mosaic dataset, or using the Synchronize Mosaic Dataset tool with the Build Item Cache option checked.
- Once you have set all the properties on this tab, you can close the dialog box by clicking OK.
- If you won't be adding more data to the mosaic dataset, you may want to check Update Overviews.
Overviews take time to create and require space for storage, but they will ensure that the statistics area is calculated, the cache is generated, and the mosaic dataset can be viewed at all scales.
- If the coordinate system for the data isn't the same as the mosaic dataset, expand the Advanced Options, click the Coordinate System for Input Data button and select a coordinate system.
You will likely need to specify the coordinate system when adding LAS files.
The coordinate system specified here is used for all the input data. If some of your data is in a different coordinate system, then add it separately, using this same tool.
- Click OK.
Now you can view the mosaic dataset in ArcMap, ArcGlobe, or ArcCatalog. Remember, the cache is created the first time the dataset is drawn; therefore, it will take some time before the surface can be viewed.
If you selected the option to build overviews, then the cache and statistics will be generated. If you did not, then you will have to wait as the cache is generated and since the statistics are likely the entire bit depth, not the range of input values, you should open the Image Analysis window, select the layer, and check DRA to stretch the image based on the values within the extent. If you will not be building overviews, run the Calculate Statistics tool (with a skip factor to save time) to calculate the statistics for the mosaic dataset.
If you will be sharing the mosaic dataset as an image service, it is best to publish the image service, then generate the cache for the mosaic dataset once it's on the server (since the cache is not copied when the mosaic dataset is shared as an image service).
Void filling limitations
If you use the Plane Fitting/IDW void filling method without specifying a Maximum width, then the interpolation will fill all gaps. This includes the gaps that may occur between the points and the extent of the file if the points do not fill the extent of the file. In the example below, the elevation data is shown as a grayscale image and has been rendered as a hillshade with some coloring to help you see the blending. The green lines represent the extent of the LAS file and the red boxes outline areas of blending.
If you overlay the points from each LAS file you can see that the blue points in the LAS file on the right do not fill the extent of the file, and you can see that the blue and the red LAS files have overlapping extents but the points do not overlap.
There are a number of different decisions that can be made to avoid this issue. The first option is to use Simple void filling, or define a value when using Plane Fitting/IDW. This way, the interpolation won't be forced to fill in the gaps along the edge of the file's extent. The examples below use the Simple method.
Another option is to check the Treat each folder as a dataset check box when adding the LAS files to the mosaic dataset. This will add the files as one dataset; therefore, in this case only one footprint would represent these two LAS files. When doing this, there won't be any interpolation along the overlapping extents since the extents are merged. However, void filling will still occur along the edges of the entire dataset.
If using the Plane Fitting/IDW void filling method without specifying a maximum width or using a significantly large value to fill large voids is necessary, then you can run the Build Footprints tool and recalculate the footprints. First, set the void filling to Simple or None. Then, when rebuilding the footprints, use the By Radiometry computation method and likely take the defaults. This will adjust the footprint to fit the extent of the points. Then you can change the void filling method.
To change the void filling method, if there is only one item in the mosaic dataset's attribute table, then click the Raster field for that item, access the LAS To Raster function, and modify the parameter. If you have many items in the attribute table, you can modify them all at once; see Editing function chains in a mosaic dataset.