There are up to four file types created on disk that are associated with a LAS dataset:
- LAS dataset file (.lasd)
- LAS auxiliary file (.lasx)
- Projection file (.prj)
- Optimized LAS file (.zlas)
The following sections describe each of the LAS dataset file types.
LAS dataset file (.lasd)
When a LAS dataset is created in ArcGIS, a .lasd file is created and stored on disk. This LAS dataset file is a pointer to LAS files and surface constraints used to construct the LAS dataset. The LAS dataset file does not import lidar point data from LAS files into the LAS dataset file. It only stores reference to LAS files and surface constraints.
LAS auxiliary file (.lasx)
A LAS auxiliary file (.lasx) is created for each LAS file when statistics are calculated on a LAS dataset. The LAS auxiliary files contain the statistical information and spatial index for each LAS file. The new LAS auxiliary file is given the same name as the source LAS file and is stored in the same location on disk. For more information on statistical analysis and LAS datasets, see Statistical analysis for LAS datasets.
The LAS dataset's auxiliary files provide a spatial index structure that improves the overall performance of a LAS dataset. A few noticeable benefits of LAS dataset spatial indexing is to allow ArcGIS to work with larger LAS files and LAS files located across a network. Also, once the auxiliary 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. These factors include file size, data location (local drive vs. network), and the spatial distribution of points in a LAS file. The more random the point distribution, the less effective the spatial index will be. 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 in a more optimal fashion. The Rearrange LAS Points option from the Extract LAS tool will perform this sorting. The tool will create new LAS files with this sorting completed and there is no need to extract or clip any of the LAS file.
Projection file (.prj)
Typically, LAS files will not have a spatial reference or will have an incorrect spatial reference defined in the header section of the LAS file. These situations are common and result in poor data quality. Having a corresponding projection file (.prj) for each LAS file eliminates this problem. A projection file defines the coordinate system of a LAS file.
The Create LAS Dataset tool will create a corresponding projection file for each LAS file. The projection file will be 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 and will take precedence over the spatial reference stored in the LAS header.
Optimized LAS file (.zlas)
Optimized LAS files (.zlas) are compressed files and are directly usable in ArcGIS 10.2.1 and later through the LAS dataset. For more information, see: LAS Optimizer. The LAS Optimizer improves archiving, sharing, and use of LAS format lidar. No decompression is necessary in ArcGIS unless edits to the classification need to be done. Because the data is also spatially indexed and has statistics, it's more efficiently accessed and easier to use than regular LAS, especially across a network.
Optimization is accomplished in the following ways:
- Lossless compression
- Spatial indexing
- Statistics generation
- Efficient direct use without need for decompression