Mosaic dataset to define DTM and DSM

In the previous workflow, the rasters are pregenerated and have the advantage that they can be shared without the server requiring ongoing access to the original LAS or zLAS files. This workflow provides an alternative wherein the rasters are generated on the fly, directly from the LAS or zLAS files. This can provide more advanced capabilities, such as creating additional datasets from classified points, and is faster to set up, but the disadvantage is that the LAS or zLAS files must remain accessible to the servers, which can cause scalability issues.

Workflow for creating DTM and DSM using mosaic datasets

Using this workflow, the raster surfaces can be built when the area is first accessed, and ArcGIS stores the elevation data in a proprietary compressed file format. If an elevation file (for example, in TIFF format) is needed, a new raster for any extent and resolution can be created by exporting from the mosaic dataset.

Create a mosaic dataset—DTM

A mosaic dataset must be created to generate a bare-earth DTM from the lidar data. Presuming your organization will have numerous lidar projects, use a name such as S_Lidar_XXXX_DTM, where XXXX is a project identifier meaningful to your organization (perhaps a project area name, ID number, or combination), and the suffix _DTM will differentiate this mosaic from the similar dataset representing the DSM.

The prefix S_ is a convention recommended to identify this as a source mosaic dataset, meaning that it contains data from a single project or data collection. The concept of a source mosaic dataset becomes important when managing multiple projects, wherein multiple source mosaics are combined in a derived mosaic dataset. Refer to the Elevation workflow (as discussed in Create mosaic datasets and serve the raster data) for details.

Create the new mosaic dataset using the coordinate system of the lidar data. Under Pixel Properties, set the number of bands to 1, and define the pixel type as 32_BIT_FLOAT.

Add rasters

Add the LAS dataset containing your lidar source data files into this mosaic dataset.

Steps:

1. Set Raster Type to LAS Dataset.
2. Open the Raster Type Properties for the LAS dataset, and go to the Settings tab.
   1. Set Return Type to Last and include class codes for Ground and Model Key. You may optionally include Water, unless you intend to show voids in the DTM.
   2. Set Pixel Size, Interpolation Method, and Void Filling as determined by testing above. Leave Maximum width blank.
   3. Set Z factor to 1 unless you need to rescale the vertical units of the lidar data. Presuming vertical units are either feet or meters (according to your project requirements), it is recommended not to change units in the source mosaic. If your full collection will include lidar with different vertical units, the Elevation workflow (as discussed in Create mosaic datasets and serve the raster data) describes rescaling data from feet to meters when projects are merged into the single derived mosaic dataset to manage all data.
   4. Leave the number of cached surfaces at 10 (default).
3. On the Add Rasters properties page, do the following:
   1. Enter the path and name for your LAS dataset.
   2. Turn Update Cell Size Ranges on.
This will configure your mosaic dataset to create full-resolution elevation datasets for the DTM on the fly when any area is accessed.

Steps:

Next you need to create footprints for the data. A proper footprint represents the extents of the valid data and is necessary to avoid interpolation across concave areas that may exist. As noted above, a project extents polygon may already exist or should be created, after which you should use the Import Mosaic Dataset Geometry geoprocessing tool to copy that polygon into the footprint.

Steps:

Next, it is recommended to define and then build overviews. This will build reduced-resolution datasets to improve performance.
1. In the Define Overviews tool, do the following:
   1. Under Overview Tile Parameters, set Pixel Size to 5 times your DTM resolution, set Overview Sampling Factor to 2, and leave Number of Levels blank.
   2. Under Overview Image Parameters, set Resampling Method to Bilinear and Compression Method to LZW.
2. After defining overviews, run the Build Overviews tool.
3. Run Calculate Item Visibility to ensure the data displays properly at all scales.
   Note:

   As above, this is another step that will require processing time, so it is recommended to test a small area and then schedule a convenient time for processing the full dataset.

When complete, this source mosaic dataset will provide the DTM generated from the lidar data, and this mosaic can be used as input in the Elevation workflow described in Create mosaic datasets and serve the raster data.

Create a mosaic dataset—DSM

The workflow for creating the first-return DSM is essentially the same as that for the DTM, except for the changes noted below.

Steps:

1. In the Raster Type Properties for the LAS dataset, on the Settings tab, set Return Type to First and turn on all class codes.
2. If your desired pixel size for the DSM (determined above) is different from the DTM, enter the appropriate value.

All other settings should be the same as those for the DTM; complete the workflow above to build the DSM for this dataset.

At this point, the basic workflow is complete for one lidar project. You can repeat it for other lidar projects and then proceed to the Elevation workflow in Create mosaic datasets and serve the raster data.