Georeferencing LAS files with LAS 1.4

Author:Howard Butler
Contact:hobu.inc at gmail dot com
Date:2/28/2011

Summary

This document proposes how to handle describing spatial reference systems in ASPRS LAS 1.4. LAS 1.4 prescribes that software developers add and prefer to use OGC WKT coordinate system description in the form of VLR records in LAS files. Softwares can choose to continue to write GeoTIFF keys to support older applications, but they should prefer the OGC WKT if it is found in the file.

Rationale

LAS 1.3 and below have used GeoTIFF keys as described by the GeoTIFF specification, but use of the GeoTIFF keys within LAS has a number of deficiencies:

  • GeoTIFF is typically tied to the EPSG database and does not provide simple mechanisms for extended definitions.
  • Only a few softwares provide support for working with GeoTIFF keys directly.
  • The GeoTIFF specification has no organization backing its continued development.
  • GeoTIFF does not offer a mechanism to represent transformations between datums, while OGC WKT offers the TOWGS84[] mechanism
  • GeoTIFF does not offer the possibility of fitted coordinate systems.

WKT

The OpenGIS Coordinate Transformation Service Implementation Specification provides the “Well-known Text Representation of Spatial Reference Systems” (WKT) language in section seven. WKT is much more expressive than GeoTIFF keys, has an organization providing institutional heft and progress behind it, and a number of open source and commercial softwares are available to support parsing and interpreting WKT output.

There are a few dialects of WKT of which to be aware. The most common dialect is the “ESRI WKT” dialect, which does not include TOWGS84 and authority nodes, but there are others including “GDAL”, “CSMap”, “Oracle 9”, “Oracle 10”, and “GeoTools.” While the basic form of WKT in these different dialects (other than ESRI) is the same, the names used for different items, such as PARAMETER, can often be spelled differently. It should be assumed that all WKT provided in LAS files should be of a dialect that is not the “ESRI WKT” dialect, which lacks some important features – notably TOWGS84 and AUTHORITY nodes.

Implementation

WKT can be specified in two optional Variable Length Record (VLR) entries with the LASF_Projection User ID. The Math Transform WKT can be used to apply transformations to the Coordinate System WKT.

Math Transform WKT

Math Transform WKT (section 7.1 of the OGC Coordinate Transform Specification) can also be specified to apply to a FITTED_CS-style coordinate system.

OGC Math Transform WKT strings shall be provided in a VLR with the following attributes:

  • User ID: LASF_Projection
  • Record ID: 2111

The following conditions should also apply:

  • Because VLRs can only be 65536 bytes size, an OGC Math Transform WKT string in a VLR data block is functionally limited to 64K in size.
  • The OGC Math Transform WKT VLR data shall be a null-terminated string.
  • The OGC Math Transform WKT VLR data shall be considered UTF-8.
  • The OGC Math Transform WKT VLR data shall be considered C locale-based, and no localization of the numeric strings within the WKT should be performed.

Note

Math Transform WKT’s widespread use is not expected, but it is provided for completeness and flexibility.

Coordinate System WKT

OGC Coordinate System WKT strings shall be provided in a VLR with the following attributes:

  • User ID: LASF_Projection
  • Record ID: 2112

The following conditions should also apply:

  • Because VLRs can only be 65536 bytes size, an OGC Coordinate System WKT string in a VLR data block is functionally limited to 64K in size.
  • The OGC Coordinate System WKT VLR data shall be a null-terminated string.
  • The OGC Coordinate System WKT VLR data shall be considered UTF-8.
  • The OGC Coordinate System WKT VLR data shall be considered C locale-based, and no localization of the numeric strings within the WKT should be performed.

Note

Though not specifically mentioned in previous LAS specifications, coordinate system information (in GeoTIFF or WKT) applies to the final coordinate data that are produced by transforming the raw coordinates by the scale and offset specified in the header for the given dimension.

Transition

Older softwares may want to continue to use GeoTIFF keys, and it is not desirable to have an abrupt discontinuity in the description of coordinate systems for LAS data. Additionally, while many softwares may not need the features of OGC WKT, its usage represents the strongest case for interoperability with other software – especially software in the GIS and CAD domains that take advantage of the OGC WKT specification.

Softwares that currently write GeoTIFF VLR records into LAS files can continue to do so in LAS 1.4. It is acceptable to write only GeoTIFF VLR records into an LAS file, while softwares that choose to write OGC WKT VLRs must also write GeoTIFF VLRs to promote backward compatibility. If a file is found to have both OGC WKT and GeoTIFF VLRs, the OGC WKT description, which is more expressive, should be preferred if possible.

Resources

The ASPRS LAS specification attempts to offload specification of coordinate system description and transformation by standing on the shoulders of other specifications in the ecosystem. To that end, there are options for both software and support for an LAS developer to take advantage of. Those below are provided for information purposes only, and are not expected to be canonical. Use your favorite search engine for more details.

Software

As mentioned previously, a number of open source and commercial softwares are available to help an implementor of ASPRS LAS utilize Coordinate System and Math Transform WKT in their software.

Some common open source softwares include:

Some commercial options include:

Questions

If you have questions about how to produce WKT, please participate on the MetaCRS mailing list. Examples of WKT may also be found on the http://spatialreference.org website.