LAS dataset considerations

Several factors should be considered prior to building a LAS dataset. The following is a list of best practices and recommendations for LAS datasets.

Types of lidar data supported

• Support for LAS datasets in ArcMap and geoprocessing is intended primarily for airborne lidar.
• Use either terrestrial or airborne lidar in ArcScene for a 3D experience.
• Airborne lidar will be more useful if the data has been classified (for example, ground versus nonground).
• ArcGIS supports LAS versions 1.0, 1.1, 1.2, and 1.3. Additionally, LAS version 1.4 files that are 1.3 compliant, containing point record formats 0 through 5, are supported. LAS version 1.4 files containing point record formats 6 through 10 are not supported.
• The classification codes detailed in ArcGIS are based on LAS version 1.1 or greater. Consider using the Change LAS Class Codes geoprocessing tool to update classification codes residing in the LAS files of outdated LAS versions. The tool can be used to ensure that such data conforms to the current standards.

Dimensions and size of LAS files

• For best performance, the client machine should have direct access to data on local internal drive, not across network or slower external drives.
• Build a LAS dataset using LAS files from the same project (that is, specifications, study area). LAS datasets are not meant to compile data from disparate sources and study areas. For that purpose, use mosaic dataset.
• Generally, it is beneficial for a LAS dataset to be constructed with LAS files that are tiled. The LAS files should not contain lidar data in large strips or flight lines unless the LAS files have been indexed and contain a LAS auxiliary file (.lasx) for each LAS file.
• Calculate statistics to provide the LAS dataset auxiliary files (.lasx). These auxiliary files contain a statistical analysis and spatial index calculated for each LAS file. The spatial index improves the overall performance of a LAS dataset.
• Generally, it is a good idea to keep LAS file sizes around 500 MB when LAS auxiliary files (.lasx) are created. However, there are many situations where LAS files are much larger, and the spatial index in the LAS auxiliary files allows ArcGIS to better handle these very large LAS files. A size limit becomes less significant in ArcGIS if there is a spatial index present. The size recommendations are for practical data management issues of larger file sizes, for example file/disk fragmentation.
• For best performance, when statistics are not calculated, the LAS tiles should be contiguous, not overlapping, and roughly 200 MB or less in size.

Spatial reference

Several considerations exist when working with LAS files and spatial reference in ArcGIS. The LAS specification requires that spatial reference information always be included in the header information of all LAS files. ArcGIS will read and use the spatial reference information if one exists in the LAS files.

ArcGIS assigns the spatial reference found in one of the LAS files added to a LAS dataset as the spatial reference for the dataset. If no spatial reference information is found in any of the participating LAS files, the spatial reference for the dataset will remain Unknown.

Having a spatial reference defined in the LAS dataset does not necessarily indicate that all the LAS files participating in the LAS dataset have that spatial reference. They may be in a different project or have no projection defined; both of these situations are common and result in poor quality.

Unfortunately, it is very common to obtain LAS files without spatial reference information. The following discussion describes best practices on how to apply a spatial reference to the LAS files referenced by the LAS dataset in ArcGIS.

If your LAS files have either missing or incorrect spatial reference, you can

1. Create a projection file (.prj) file for each LAS file to define the spatial reference for each LAS file. There is a sample tool available in the gallery on the 3D GIS resource center to automate this process.
2. Rewrite the LAS files to contain the proper spatial reference information. There is a sample tool available in the gallery on the 3D GIS resource center to automate this process.

It is important for subsequent analysis and visualization in ArcGIS that the spatial reference information is known for each LAS file. Without a spatial reference, certain functions and workflows may not work as expected in ArcGIS. While on-the-fly projection of LAS data is not encouraged (for performance reasons), the spatial reference must be defined for each LAS file for it to work. Defining the spatial reference at the LAS dataset level alone is not enough. If your LAS data is not being displayed, chances are the map is in a different projection than the LAS data and the spatial reference for the LAS files is not defined properly.

Tip:

• It is recommended that LAS data be delivered and consumed in a projected coordinate system, for example, UTM or NAD83 State Plane. LAS data that is captured in Geographic coordinates can be displayed, but some functionality may fail or be suboptimal.
• It is recommended that you add an accompanying projection file (.prj) file along with a LAS file referenced by the LAS dataset if no spatial reference is present or if the LAS file has an incorrect spatial reference. If you are unable, or unwilling, to fix the issue with the LAS file, use a .prj file for each LAS file to define the spatial reference. It is important for subsequent analysis and visualization in ArcGIS if the spatial reference information is known for each LAS file.
• Set units to millimeters or inches if point spacing is very small (for example, less than 0.01 if in feet or meters, which can happen with stationary terrestrial scanners).
• Use orthometric heights, not ellipsoidal heights.