Update topobathy LDP to v2

[esilvia]

Several times over the last five years or so we have attempted to compile a list of updates needed in the topobathy domain profile. I'm going to attempt to compile them here.

Classifications

These changes are a composite of what I've gathered from #72, the LWG call earlier this year, and past discussions at various conferences such as ILMF and JALBTCX. Please provide corrections as I'm sure my notes are incomplete.

• Class 40 (unchanged)
  • Current definition: Bathymetric point (e.g., seafloor or riverbed; also known as submerged topography)
• Class 41 (clarified)
  • Current definition: Water surface (sea/river/lake surface from bathymetric or topographic-bathymetric lidar; distinct from Point Class 9, which is used in topographic-only lidar and only designates “water,” not “water surface”)
  • New definition: Water surface of sea, river, lake or other body of water, typically used in refraction. This is distinct from ASPRS Class 9, which is used in topographic-only lidar to designate “water” in general, not “water surface.”
• Class 42 (unchanged)
  • Current definition: Derived water surface (synthetic water surface location used in computing refraction at water surface)
• Class 43 (unchanged)
  • Current definition: Submerged object, not otherwise specified (e.g., wreck, rock, submerged piling)
• Class 44 (REMOVE - several users have identified that it's redundant with Class 43 and that IHO classifications occur on derived products, not within the lidar)
• Class 45 (revised - in practice most folks have been using this as a submerged -- i.e., refracted -- noise class, rather than using it as an indicator of submerged pulse extinction)
  • Current definition: No-bottom-found-at (bathymetric lidar point for which no detectable bottom return was received)
  • New definition: Water column noise (submerged noise within the water column not identified as a real submerged object or identified as submerged backscatter -- i.e., noise points from the bathymetric lidar that are submerged and were therefore refracted)
• Class 46 (NEW)
  • Submerged vegetation (submerged vegetation occurring below the water surface (class 41) and above the bathymetric bottom (class 40), especially seagrass)

ExtraBytes

Most of the ExtraBytes defined in v1 of the LDP are widely unused. The Bathymetry flags ExtraByte is used often, but the rest are largely ignored. Here's a summary:

• (pseudo)-reflectance (REMOVE)
  • It's weirdly named, poorly understood, and -- as far as I know -- unused because there are other standardized way to provide similar attributes.
• sigma xyz (REPLACE - the triple-XB data types were removed in a previous revision of LAS 1.4. Replace with the two xy/horizontal and z/vertical ExtraBytes sigma xy and sigma z)
• water column optical depth (???????)
• figure of merit (unchanged.... apparently it's a CZMIL thing?)
• Bathymetric flags (rename to bathymetric flags to be consistently lowercase)
• refracted range (ADD from standard table)

Admin

The previous version of the LDP should be referenced as Version 1 of the topobathy LDP, while the revised version should be referenced as Version 2 to simplify references in publications and specifications.

LAS files need a method by which the file's contents provide a hint that a LDP was followed during the file's creation. Adding this to the header can't happen without a new version of LAS (e.g., 1.5), but the LDP reference could be added to #82 as a component of the new Classification Lookup VLR, with a lookup table maintained in the wiki.

(Incidentally, this is a good example of why these public GitHub issues threads are so valuable... compiling this information from MANY sources was quite a bit of work.)

[esilvia]

One addition for the admin section: The community seemed to feel that using the GitHub wiki would be a fine place to publish the LDPs, so long as versioned PDFs could be available for download and referenced in standards or publishing documents.

[esilvia]

@nkules Have any followup on this from the topobathy meeting at GeoWeek?

[esilvia]

One change I've received/thought of:

Class 45 -- Water Column Default instead of Water Column Noise. This would mirror the existing Default class (2) and more accurately convey its intended use as non-specific, rather than implying it's all noise. i.e., it could be submerged vegetation unless Class 46 is present.

[nkules]

@nkules Have any followup on this from the topobathy meeting at GeoWeek?

Evon, there wasn't anything to add from Geoweek. I did bring this up in our topobathy workshop, but there were no questions directly related to this. However I am hoping it encouraged all of those attending to join in this discussion! Thank you for keeping on top of this!

[esilvia]

Updates from JALTBTCX 2022:

1. Revise name of Class 40 to Submerged Topography.
2. Add reference to "Submerged Biology" (e.g., coral) in Class 46 description.
3. Incorporate changes to Class 45 from "Water Column Noise" to "Water Column Default".
4. Mark Class 44 as Reserved in case we add a future IHO class.

Lingering questions from JALTBCTX 2022:

1. Note conflict between Class 46 and Class 64 in NOAA/JALBTCX specs. Can we agree on using Class 46?
2. JALTBCX includes Class 65 - Bathymetric Bottom Temporal Change. Should that be added as Class 47.
3. What is the right way to reference the LDP in a standard or tender document? Does anyone have sample language?

I'll post a link to the LDPv2 draft once I get access to the ASPRS GDrive.

[esilvia]

Here is a link to the draft: https://docs.google.com/document/d/1UZYaIHZQp5ljSdSDLRs6sJnY3DJlWSqRdfWfh3N4DwU/edit

On Monday 8/1 I will generate a PDF copy of the draft and forward it to Karen to request feedback from the ASPRS community at large via the weekly newsletter.

[esilvia]

The draft has been updated significantly to incorporate comments from JALBTCX, the LWG meeting, and emails from @parrishOSU and @JohnsonN.

https://docs.google.com/document/d/1UZYaIHZQp5ljSdSDLRs6sJnY3DJlWSqRdfWfh3N4DwU/edit#

I also removed most of the tracked edits, leaving just the ongoing discussion threads.

Some resolution of the lingering questions:

1. Settled on Class 46 for Submerged Vegetation to leverage the fact that classes 32-63 are ASPRS reserved and that's the only reason Class 64 was selected previously.
2. Similarly, added Class 48 for Submerged Temporal Exclusion instead of Class 65 from NOAA/JALBTCX.
3. Added Class 47 for Submerged/Bathymetric Noise – distinct from Class 45 in that it's typically below the Submerged Topography.
4. Improved sample language in a newly-entitled "How to Use This Document" section.

[esilvia]

A few updates were posted to the doc this week in preparation for JALBTCX 2023:

1. Removed italics designating the changes relative to v1.
2. Clarified that v2 was motivated by LAS 1.4 R14 and R15.
3. Resolved comments and notes.
4. Added "Comparison to Version 1.0" section and moved relevant paragraph to that section.
5. Added missing wiki links for Reflectance and Water Column Optical Depth, which were copied from the v1 LDP to the wiki.
6. Added missing reference links to LAS 1.4-R15, LAS 1.4-latest, and the Riegl White Paper.

The publication of this LDP is imminent, pending final comments from JALBTCX.