The LAS format stores coordinate system information inside the .las file. How this information is preserved depends on the LAS file version and the point record format, which are properties that impact the attributes and values that are stored in the .las file. The current LAS versions are 1.0, 1.1, 1.2, 1.3, and 1.4. The point record formats supported by each version is summarized in the following table:
LAS versions 1.0 to 1.3 store the coordinate system using GeoTIFF tags, whereas LAS version 1.4 supports both GeoTIFF and Well-Known Text (WKT) conventions. The GeoTIFF metadata standard for coordinate system information is a popular convention that was designed to georeference a TIFF raster. Since LAS is not a raster format, it only requires a subset of the GeoTIFF tags to be properly positioned in the real world. In ArcGIS, the GeoTIFF information is interpreted using the Geospatial Data Abstraction Library (GDAL), which is a library for reading and writing various geospatial data formats. The WKT standard is a text-based description of a spatial reference that was adopted by the Open Geospatial Consortium (OGC). LAS 1.4 files with point formats 0–5 can store the spatial reference as GeoTIFF or WKT; whereas the newer point formats 6–10 only support WKT.
The location of the coordinate system information also differs between LAS versions. Each .las files consists of the following elements:
- Public Header Block
- Variable Length Records (VLR)
- Point Data Records
- Extended Variable Length Records (EVLR)—Only available in LAS 1.3
and 1.4
- In LAS 1.3, this is only available for point data record formats 4 and 5 and is intended to store the waveform data packets from the original lidar survey.
- In LAS 1.4, this is supported for all point data record formats and the EVLR can accommodate a wide range of information.
In LAS versions 1.0 to 1.3, the coordinate system is stored in the variable length records portion of the file, which is located near the beginning of the binary file and cannot be updated once it has been written. In LAS 1.4, the coordinate system information can be written in either VLR or EVLR, and the information can be updated by writing a new coordinate system in the EVLR portion and modifying the original one to indicate it has been superseded.
Coordinate system in the LAS dataset vs. individual LAS file
The LAS dataset (*.lasd) can reference multiple .las and .zlas files to support visualization and processing of a large collection of contiguous files. Typically, the data used in one LAS dataset will have the same coordinate system, but files with different coordinate systems can also be viewed through the LAS dataset. The LAS dataset has its own coordinate system that is the one that will be used when processing the data. The LAS dataset's coordinate system is typically derived from the first input .las or .zlas file that is added to it, but this can be modified at the time he LAS dataset is created.
When a LAS dataset is the first dataset that is added to the map or 3D scene, its spatial reference will define the map's coordinate system. Any .las or .zlas file that is referenced by the LAS dataset but has a different coordinate system will be reprojected on the fly to draw in the map's coordinate system. When the LAS dataset is processed by a geoprocessing tool to create derivative data products, such as an elevation raster, the default coordinate system of the output will use the spatial reference from the LAS dataset. This can also be modified through the Output Coordinate System environment setting.
Horizontal and vertical coordinate systems
The horizontal coordinate system defines the x,y positioning of the LAS data, and the vertical coordinate system defines its z-positioning. Having this information ensures that the data is accurately positioned. When the data only has the horizontal coordinate system defined, the z-units are inferred from the horizontal coordinate system. If such data has its x,y coordinates set to a geographic coordinate system, its z-units are assumed to be meters.
Reproject .las and .zlas files
The Project LAS tool can be used to reproject one or more .las and .zlas files from one coordinate system into another. The tool provides an easy way to select the appropriate geographic datum transformation when the data is being transformed from one datum into another. The ArcGIS Coordinate Systems Data package must be installed in order to perform a vertical datum transformation. This software component is provided through a separate installation which can be downloaded from your "My Esri" page. Learn more about installing the supplementary coordinate system files here.
Most geoprocessing tools which create output .las or .zlas files support re-projecting the data through the Output Coordinate System and Geographic Transformations environment settings. Using such tools in lieu of the Project LAS tool can help avoid the creation of unnecessary intermediate data. Consider reprojecting your data through the following tools if you need to leverage the operations described below:
- Thin LAS—Reduce the point density of the data.
- Tile LAS—Subdivide the point cloud into a regular tiling scheme.
- Extract LAS—Clip and filter the point cloud.
Common problems with LAS coordinate systems
The American Society for Photogrammetry and Remote Sensing (ASPRS) standard for the LAS format requires .las files to have a spatial reference. However, this is not always the case. LAS data may be collected in indoor spaces where GPS positioning is not possible. Point clouds derived from photogrammetric techniques may also have no spatial reference defined. LAS format files may also have missing, incomplete, or improperly defined spatial reference information that will cause them to display in the wrong location.
When viewing the dataset properties of .las or .zlas files with missing or unrecognizable spatial reference information, it is listed as having an unknown coordinate system. LAS format files that use GeoTIFF tags to store the spatial reference may also report as being in GCS WGS84 whenever their spatial reference information is partially written but incomplete. This is due to how the GDAL library interprets incomplete GeoTIFF spatial reference data.
Another problem that may be encountered with .las files is that the data may appear to be exaggerated or compressed along the z-dimension. This typically occurs when the linear units that are being used to render the data does not match the units in which the height information was collected. When a dataset only has an x,y coordinate system defined but not a z-coordinate system, the linear unit of the x,y coordinate system is used to display the height information. When such data is reprojected between horizontal coordinate systems of different units, the z-units are not converted because they were never defined. To resolve the scaling distortions that occur in such a scenario, you must define the correct z-coordinate system.
Using .prj files to correct spatial reference issues
In ArcGIS Pro, .las or .zlas files with missing or erroneous spatial reference information can have their coordinate system defined by an auxiliary .prj file. This auxiliary file resides in the same directory as the .las or .zlas file and shares the same base name, but its extension is .prj. The .prj file stores the coordinate system using the WKT convention, and its presence overrides the information in the .las or .zlas file.
You can create the .prj file for an individual .las or .zlas file by using the Define Projection geoprocessing tool. You can also create the .prj file for a collection of .las or .zlas files using the Create LAS Dataset geoprocessing tool by defining the desired spatial reference in the Coordinate System parameter and indicating whether a .prj file will be created for all files or only files that are missing the spatial reference.
When the LAS data was originally georeferenced but the spatial reference information embedded in the file was either missing or incorrectly defined, creating a .prj file with the correct information will be sufficient to have the data aligned in ArcGIS Pro. If the LAS data was never georeferenced, a customizable local map projection can be used to shift and rotate the data along the x,y plane so that it lands in its real-world position. The local projection can be a useful for data representing a small project area, like a building or small neighborhood. Once a dataset has been georeferenced with the local projection, it can be reprojected into a standard coordinate system that is recognized by other applications. Learn more about the local projection.
Similar to the local projection for horizontal positioning, ArcGIS Pro also provides an unspecified height system to define the height units of 3D data. This allows you to correct z-unit distortions that may occur with data that has an undefined z coordinate system and a horizontal coordinate system with a mismatching linear unit.
If .las or .zlas files with a spatial reference defined in a .prj will be used in third-party applications, the coordinate system in the .prj may not be recognized. The coordinate system in the .prj file will need to be embedded in the .las or .zlas files to ensure other applications recognize the corrected information. You can use any geoprocessing tool that outputs new .las files to embed this coordinate information. In addition to the aforementioned tools that output LAS format files, the Convert LAS tool can also be used to create new LAS files in a different LAS version and point record format.