LAS dataset file structure
There are up to three file types created on disk that are associated with a LAS dataset:
- LAS dataset file (.lasd)
- LAS auxiliary file (.lasx)
- Projection file (.prj)
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 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 such that those which are close to one another in physical record order are also close in spatial proximity, then the index works in a more optimal fashion. The LAS Rearrange utility will perform this sorting. The utility will create new LAS files. The LAS Rearrange utility is located in the 3D GIS gallery resource center.
- If statistics are not calculated, then no spatial index is generated or used.
- LAS datasets created prior to ArcGIS 10.2 will continue to function without any problem. Recomputing statistics for existing LAS datasets will create the LAS dataset auxiliary files. Existing LAS datasets will start functioning faster after recomputing statistics.
- The spatial index also improves performance in ArcScene, but to a smaller degree because data is cached in memory for fast 3D interaction, as opposed to ArcMap, which is constantly rereading it.
- It is recommended that statistics be generated to provide the auxiliary files that will improve the overall performance of the LAS dataset.
- If a LAS dataset is referring to LAS files from multiple locations, the LAS auxiliary files will be created in corresponding locations.
- If LAS files added to a LAS dataset have LAS auxiliary file (.lasx) files, then statistics are already calculated.
LAS auxiliary files may become out-of-date when edits are performed on a LAS dataset. Edits conducted in ArcGIS will result in out-of-date statistical information. These edits will not affect the spatial index in the LAS auxiliary files. The spatial index is automatically updated. Statistics will have to be recomputed to ensure that the statistical information is up-to-date. If a LAS file is edited using another application, then 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 LAS auxiliary files.
Projection File (.prj)
It is common that LAS files will not have a spatial reference or 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 (.prj) defines the coordinate system of a LAS file.
The Define LAS Projection sample tool will create a corresponding projection file (.prj) 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. The Define LAS Projection sample tool is available in the gallery on the 3D GIS resource center website.