Horizontal and vertical datums reprojection possible?

[Frederic Brieger]

Hello,

I have a las dataset in NAD83(CSRS) UTM Zone8 with a vertical datum CGVD28 and I want to reproject it to WGS1984 UTM 8N ellipsoid heights. How would i do this using LAStools? And do I understand correctly that las2las is open source and therefore, no random noise will be introduced without obtaining a full license?

Thanks!

[LAStools - efficient tools for LiDAR processing]

Hi Frederic,

yes, las2las64 is within the opensource/freeware part of LAStools. You can use without restrictions and there will be no random noise.

To to a reprojection you have 2 general options:

(a) using the internal routines.

(b) using the PROJ interface.

For both options you can give source and target projection. If the point cloud already contain a CRS information in the header (as WKT or GeoTIFF) you can use this information and only have to specify the requested target. It is always recommended to use EPSG codes to do CRS transformations.

(a1)

:: internal, multiple files, CRS in source
las2las64 -i *.laz -odir outdir -olaz -target_epsg 22208

:: internal, single file , CRS in source

las2las64 -i in.laz -o out.laz -target_epsg 22208

(a2)

:: internal, single file , source CRS in arguments

las2las64 -i in.laz -o out.laz -nad83_csrs -utm 8n -target_epsg 22208

(b1)

:: using PROJ, CRS in source

las2las64 -i in.laz -o out.laz -proj_epsg 22208

(b2)

:: using PROJ, source CRS in argument

las2las64 -i in.laz -o out.laz -proj_epsg 32608 22208

Please see the README for further information.

Cheers,

Jochen @rapidlasso

[Frederic Brieger]

Thanks Jochen,

that is already super helpful! But would the example code you provided not only transform the horizontal datum? Do I need to add -vertical_wgs84 to change the vertical datum from CGVD28 to WGS84 ellipsoid heights?

Thanks!!

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[Michael Perdue]

A word of caution. Be careful when using the projlib for datum transformations from one EPSG code to another. What it does and what you want it to do arn't always the same thing, and might be different than the right thing. I have 3 issues.

1. The EPSG database can be a garbage pile.
2. If it's missing support files (like grid shift files and geoid files) it may resort to a different strategy without notifying the user.
3. Some SRS are incomplete. This means the datum transformation is ambiguous.

Your objective is a good example. EPSG:22208 is missing information. NAD83CSRS requires an epoch. Is it 1997, 2002, 2010 or something custom? So does WGS84. Is it WGS84 original (WGS84 orig ~ NAD83 orig) or is it one of the G series? Neither EPSG code states for either. Do you even know what your target realization and epochs are? They are required information.

If the officially supported transformation requires a grid shift file (probably not for your case) whether the library executes the intended behaviour is dependent on having the necessary support files in the right location. If you have your computer configured, you can get an idea of what is going to happen in a transformation by using projinfo. On my computer:

projinfo -s EPSG:22208 -t EPSG:32608
Candidate operations found: 1
Note: using '--spatial-test intersects' would bring more results (9)
-------------------------------------
Operation No. 1:

unknown id, Inverse of UTM zone 8N + Ballpark geographic offset from NAD83(CSRS)v2 to WGS 84 + UTM zone 8N, unknown accuracy, World, has ballpark transformation

For brevity, I have trimmed a bunch of the output, but the ballpark statement doesn't give me confidence... Ballpark? Unknown Accuracy? This probably isn't what you want. In addition, be careful with the geoids because I have seen people misuse the proj library and end up having it apply or remove a geoid by mistake. As you guessed, it will not remove the geoid in your case.

Admittedly, I am a bit of an ellitest when it comes to geodesy and datum transformations. I don't like black boxes and prefer to take direct control by using lasdatum and lasvdatum. But these are commercial tools so a license is necessary and a black box approach won't work with them either. Whatever you do, check your work. Have 3 known points in both coordinate systems and check the results of your workflow before you trust it. Just my 2 cents.

Mike

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