- The USGS has developed a “content complete” version of the Base LiDAR Specification.
- This is to be considered the minimum requirements to be included in the national LiDAR dataset.
- The data must be fully LAS compliant.
Karl Heidemann from the USGS gave an excellent presentation at ILMF10 on the Base LiDAR Specification that is now up to version 13. (The link is to version 12.) He noted that this version is considered “content complete”, but that there is still editing to be done. The spec is considered the minimum requirements for a LiDAR data collection in order to have it potentially become part of the national LiDAR dataset. Karl noted that the $15 million in ARRA spend led to the need for the spec.
Some of the details include a minimum 2 meter post spacing, support for multiple returns and a requirement that the grid be essentially the same dimension in both directions.
The data must be provided in fully compliant LAS 1.2 or 1.3 format. Karl noted that very few data sets that he sees meet this requirement.
The other key item is that the data be supplied with hydro enforcement, meaning that the DEM be topographic. This will result in bodies of water having the correct visual appearance.
The specification is an excellent starting point and worth your investigation. Please provide Karl with your comments.
A point of clarification is needed on the much misunderstood subject of hydro-enforcement and hydro-flattening:
Hydro-enforcement is a modification to a traditional topographic DEM that allows water to flow across the surface as it does in the real world (using culverts and other structures). This results in a hydrologic DEM. The most apparent difference between the two surface types is that in a hydrologic DEM, the road surfaces over culverts are removed, leaving trenches across the road surfaces. Hydrologic surfaces are used by water resource engineers in modelling; Topographic DEMs are the traditional foundation of elevation mapping and contour generation.
The USGS-NGP Lidar Guidelines and Base Specifications DO NOT require hydro-enforcement; rather, they require a HYDRO-FLATTENED Topographic DEM.
The confusion on this topic may stem from the USGS-NGP requirement that water bodies be “flattened”, making them appear more like a traditional photogrammetrically-derived DEM. Hydro-flattening is a cartographic alteration to a lidar-only DEM to improve its aesthetic appearance. A hydro-flattened DEM is still a topographic DEM.
The lidar industry has chronically suffered from a lack of commonly recognized, precise terminology. This has contributed substantially to this misunderstanding. The ASPRS is currently compiling a Lidar Manual which among other things should establish a more consistent vocabulary for the broader lidar community. Look for it this fall! In the meantime, I hope to develop a PowerPoint Presentation that explains the issue in more detail, and when finished, I’ll make it available to the public on the CLICK site.
Gene, many thanks to you for your kind words about the presentation and “the Spec”, and moreover, for helping spread the word through your excellent site!
-Karl