New LAS Enhancements Support Topographic-Bathymetric

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Back in the late 1990s, LiDAR professionals were becoming increasingly cognizant of the need for a standard file format for point cloud data that would serve as an alternative to vendor-proprietary formats or cumbersome XYZ text files. Industry stakeholders sought an open, publicly-available binary format that would preserve important metadata and enable point cloud data to be transferred from sensors to processing and analysis software. This identified need led to the development of the LAS file format.

In 2003 the American Society for Photogrammetry and Remote Sensing (ASPRS) published the first official specification for the LAS format, version 1.0. Since then four updates to the LAS spec have been published by ASPRS (the current version is 1.4), and the LAS file format has become somewhat of a de facto standard for airborne LiDAR data. In LAS, individual points within a point cloud are stored in what is known as a Point Data Record, and each point can have an associated class (e.g., ground, low/medium/high vegetation, building, rail, transmission tower, etc.) and other attributes.

Despite–or, perhaps, because of–the widespread use of LAS, certain segments of the broad LiDAR community are interested in enhancing the format to meet their application-specific needs. For example, groups using LiDAR for power line mapping, coastal mapping, or other specific applications may have a need to define their own point classes and attributes. Thanks to a recent initiative of the ASPRS LiDAR Division, this customization of LAS is now possible.

In early August, 2013, the LiDAR Division introduced the concept of a LAS Domain Profile (link), enabling expansion of the base LAS 1.4, such that specialized communities can extend the format to their needs. Domain Profiles are made possible by a couple of new features in LAS 1.4: Specifically: 1) the expansion of the number of available point classes to 256, and 2) the introduction of a new variable length record (VLR) structure, referred to as "Extra Bytes."

Unlike the VLRs from previous versions of LAS, the Extra Bytes are added to each point record. This allows for point-by-point attributes to be recorded, such as positional uncertainty (or "error") and processing flags. An important aspect of LAS Domain Profiles is that, while new classifications and attributes can be added, existing ones explicitly cannot be removed or altered. This would be problematic in a unified specification, but community-specific expansion on a common framework resolves this issue.

The first approved and published LAS Domain Profile was designed to support the coastal LiDAR community (link). While merged data sets that bridge the land-water interface, providing highresolution coverage of the backshore, intertidal, and shallow near shore zones, have been available for years from programs such as the USACE National Coastal Mapping Program, the recent developments of a new class of shallow water bathymetric LiDAR has led to an increased need for standardization of these merged coastal data sets.

At the moment, LAS 1.4 does not provide innate support for bathymetry. For example, of the 19 classifications defined within the specification, none of them refer to submerged topography. This has led to inconsistent classification by the community at large. As an example of the discrepancies that can arise, the U.S. Army Corps of Engineers (USACE) and the Joint Airborne LiDAR Bathymetry Technical Center of Expertise (JALBTCX) use a point classification of 29 (currently unassigned in the official LAS specification) to denote submerged topography in their SHOALS and CZMIL bathymetric LiDAR data. Meanwhile, NOAA’s Digital Coast, maintained by the Coastal Services Center (CSC), uses a classification of 11.

This inconsistency is rectified in the Bathy-Topo Domain Profile, which was developed with input from all of the JALBTCX interagency partners. The Bathy-Topo Domain Profile creates six new point classifications for bathymetric points and five new attributes (Table 1). Figure 1 shows an example of assigning point classes from the Topo-Bathy Domain Profile to data collected by NOAA’s National Geodetic Survey (NGS) near Ft Lauderdale, Florida.

The Way Forward
With the approval of the Bathy-Topo Domain Profile, ASPRS also published a template for other communities to create their own Domain Profiles (link). The goal is to eventually have a catalog of profiles from various communities maintained on the ASPRS website. If certain classifications or attributes are common to many of the communities, those would be absorbed into the classifications that are currently reserved for ASPRS use when the base LAS specification is next updated.

Of course, the specification alone is of little value, if no one has the ability to use it. To this end, the most critical next step is for LiDAR software manufacturers to add support for domain profiles, including the current Topo-Bathy Profile. With software support, domain profile implementation will lead to greater consistency within communities, better LAS support for specialized fields, and increased longevity for the current LAS specification.

Russell Quintero is a Lieutenant in the NOAA Commissioned Officer Corps. He has a M.S. in Hydrographic Science from the University of Southern Mississippi and is currently assigned as the NOAA Representative to the Joint Airborne LiDAR Bathymetry Technical Center of Expertise.

A 1.432Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE