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

Dimensions and size of LAS files

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.

TipTip:
  • 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.

Related Topics

6/3/2013