[PROJ] proj8 and NAD27 <-> NAD83 transform

[Even Rouault]

Kirk,

this is tricky topic.

Prerequisite knowledge: when transforming between datums A and B, PROJ 
can obviousl use transformations present in its database that have A as 
source and B as target (or the reverse), as well chaining 2 
transformations going through one intermediate datum C: A -> C followed 
by C -> B. It can't do A -> C -> D -> B.

In the NAD83(2011) to NAD27 case, there is no direct transformation 
registered between both datum. So PROJ has to go through the 
intermediate datum way, and NAD83 can be a possibility (likely the only 
one, but I didn't check).

For NAD27 to NAD83, for the US, PROJ only handles NADCON4 grids for 
Conus and Alaska. NADCON5 is registered in our database, but we don't 
have the code yet to handle it.

For NAD83 to NAD83 (2011), there is a single transformation registered 
in EPSG, EPSG:8971, and it is only valid for Gulf of Mexico and it is a 
0-meter shift between both datums.

So when chaining the 2 steps and intersecting the validity extent of the 
available transformations, NAD27 to NAD83 (2011) has a known accuracy 
only for Gulf of Mexico. For other areas, PROJ can't really know that 
taking as approximation NAD83 ~= NAD83 (2011) and using the NAD27 to 
NAD83 transformation would give something that is (likely) better than 
just giving up and considering that NAD27 -> NAD83(2011) is a no-op with 
a unknown accuracy.

If NADCON5 grids were implemented, we could potentially use the NAD83 -> 
NAD83 (HARN) -> NAD83 (NSRS2007) -> NAD83 (2011) transformations that 
are valid for CONUS, but that would require extra logic in code or 
adding concatenated transformations in the database so that PROJ doesn't 
have to figure all those intermediates!

If for your purpose NAD83 ~= NAD83 (2011) is a reasonable approximation, 
you could potentially add an entry in the database similar to EPSG:8971 
but with a validity area that encompasses whole CONUS.

Hope that makes sense

Even

Le 02/06/2021 à 21:33, Kirk Waters - NOAA Federal via PROJ a écrit :
> Dear proj list,
> I'm having a bit of trouble with proj and I'm hoping someone can tell 
> me where I've gone wrong. For context, my primary issue is converting 
> a raster from NAD83 state plane to NAD27 state plane using GDAL, which 
> uses proj under the hood. It looks like it's proj that is causing me 
> problems. I used conda to install a fresh environment with proj8 and 
> proj-data on a RHEL7 machine.
> Here's the conversion that I think is giving the wrong answers (EPSG 
> 6570 is NAD83(2011) South Carolina State Plane (ft) and EPSG 32033 is 
> NAD27 South Carolina South):
>
> cs2cs EPSG:6570 EPSG:32033 << EOF
> 2320610.000 353815.000 0
> EOF
>
> which returns:
> 2320626.52      353751.41 0.00
>
> Using the NGS NCAT online program, that should be:
> 2320568.869 353687.842
>
> I did the transform in parts and found that the issue is in the 
> reference frame transform between NAD83(2011) lat lon and NAD27 lat 
> lon, where no change happens, but it should. That is:
> cs2cs EPSG:6318 EPSG:4267
> with inputs of
> 32d48'4.701"N   79d57'23.546"W 0.000
> has outputs of
> 32d48'4.701"N   79d57'23.546"W 0.000
>
> If I use NAD83 (EPSG 4269) instead of NAD83(2011), things look right 
> and I get output of:
> 32d48'4.066"N   79d57'24.227"W 0.000
>
> Trying to change from NAD83(2011) to NAD83 (EPSG 6318 to 4269) gives 
> no change, so I'm assuming PROJ doesn't know that transform. I'm 
> guessing that because it doesn't know that transform, it can't figure 
> out that it should do the NAD83 to NAD27 part, so instead of being a 
> couple meters off, it's off by 50 meters or so. If I do the transforms 
> in parts, SP83 to NAD83(2011), NAD83(2011) to NAD83, NAD83 to NAD27, 
> NAD27 to SP27, I get the right answer within the couple meters due to 
> the missing transform.
>
> Is the transform from NAD83(2011) to NAD83 something I just missed 
> installing? Without it, it looks like my GDAL process will be painful. 
> Is there a better way to handle it?
>
> Kirk Waters, PhD                     | NOAA Office for Coastal Management
> Applied Sciences Program      | 2234 South Hobson Ave
> 843-740-1227 (empty office)   | Charleston, SC 29405
> 843-324-2203 (cell during COVID)
> coast.noaa.gov/digitalcoast <http://coast.noaa.gov/digitalcoast>
>
>
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