When a LAS dataset is created, associated files of any of the following can be created.
- LAS dataset file (.lasd)
- LAS auxiliary file (.lasx)
- Projection file (.prj)
- Optimized LAS file (.zlas)
- LAS pyramid file (.slas)
Each file type is described below.
LAS dataset file
When a LAS dataset is created in ArcGIS Pro, a .lasd file is created and stored on disk. This .lasd file is a pointer to .las files and surface constraints used to construct the LAS dataset. The .lasd file does not import lidar point data from .lasfiles into the .lasd file. It only stores references to .las files and surface constraints.
LAS auxiliary file
A LAS auxiliary file (.lasx) is created for each .las file when statistics are calculated on a LAS dataset. The .lasx files contain the statistical information and spatial index for each .las file. The .lasx file is given the same name as the source .las file and is stored in the same location on disk. For more information about statistical analysis and LAS datasets, see LAS dataset statistics.
The .lasx files provide a spatial index structure that improves the overall performance of a LAS dataset. One benefit of LAS dataset spatial indexing is that it allows ArcGIS Pro to work with larger .las files and .las files located across a network. Also, once the .lasx files are created, they can be moved alongside the .las files and LAS dataset to provide a quick method to access .las files and statistics.
The effectiveness of spatial indexing is dependent on several factors, including file size, data location (local drive or network), and the spatial distribution of points in a .las file. The more random the point distribution, the less effective the spatial index. Generally, if the points in a file are sorted so that those that are close to one another in physical record order are also close in spatial proximity, the index works more efficiently. The Rearrange points parameter for the Extract LAS tool performs this sorting. The tool creates .las files with this sorting completed, and there is no need to extract or clip the new .las files.
- If statistics are not calculated, no spatial index is generated or used.
- It is recommended that statistics be generated to provide the .lasx files that will improve the overall performance of the LAS dataset.
- If a LAS dataset refers to .las files from multiple locations, the .lasx files are created in corresponding locations.
- If the .las files added to a LAS dataset have .lasx files, statistics are already calculated.
- .lasx files may become out of date when edits are performed on a LAS dataset. Edits conducted in ArcGIS Pro result in out-of-date statistical information. These edits do not affect the spatial index in the .lasx files. The spatial index is automatically updated. Statistics must be recomputed to ensure that the statistical information is up to date. If a .las file is edited using another application, statistics and the spatial index file will all be out of date. Recompute statistics to ensure that the statistical and spatial index information is up to date in the .lasx files.
Typically, .las files do not have a spatial reference or have an incorrect spatial reference defined in its header section. These situations are common and result in poor data quality. Including a corresponding projection file (.prj) for each .las file eliminates this problem. A .prj file defines the coordinate system of a .las file.
The Create LAS Dataset tool creates a corresponding .prj file for each .las file. The corresponding .prj file is given the same name as the .las file. The .las file is not modified, and any spatial reference information stored in its header is preserved. The new coordinate system information is only recognized in ArcGIS Pro and takes precedence over the spatial reference stored in the .las file header.
Optimized LAS file
Optimized LAS files (.zlas) are compressed files and are read directly into ArcGIS Pro. For more information, see LAS Optimizer. .zlas files improve archiving, sharing, and use of LAS format lidar. No decompression is necessary in ArcGIS Pro, you can make edits to the classification of .zlas files. Because the data is also spatially indexed and contains statistics, it's more efficiently accessed and easier to use than .las files, especially across a network.
Optimization methods include the following:
- Lossless compression
- Spatial indexing
- Statistics generation
- Efficient direct use with no need for decompression
LAS pyramid file
When a pyramid is built for a LAS dataset, a folder of files is created in the same folder where the LAS dataset is stored. The naming convention for the pyramid folder is the .lasd name plus the suffix .slas.