[PROJ] Needs a sanity check on Vertical Transformation

[Brian Shaw]

Not sure if this helps but here are a few notes.

GEOID18 supersedes GEOID12B in CONUS and Puerto Rico.  GEOID12B should 
be used in Alaska, Hawaii, Guam, and the Commonwealth of the Northern 
Mariana Islands.  American Samoa should not be used for anything as 
ASVD02 is no longer valid after earthquakes.

A note on Geoids, Horizontal (Geometric) Datums and Vertical Datums.  To 
get an orthometric height (aka elevation) in a particular vertical datum 
you should use coordinates in a particular geometric datum with lat,lon, 
and ellipsoid height along with a geoid model to get an orthometric 
height.  For the US here are the appropriate datums and geoid models.  
Note that the corresponding geoid models should be used based on the 
datum of your coordinates. Also note the the original NAD 83 (1986) was 
a 2D datum while all other realizations were 3D, aka geometric.

Also note that if you are concerned about accuracy, WGS84 should be 
avoided if at all possible.   WGS84 has a 3-5 meter uncertainty so any 
time you use coordinates or use it as a pivot in transformations you 
introduce this uncertainty into your coordinates.

Table on geoids and NAD 83 realizations

NAD 83 (realization) 	Geoid Model 	Regions
NAD 83 (1986) 	N/A 	
NAD 83 (HARN) 	GEOID96/GEOID99 	
NAD 83 (FBN) 	GEOID03 	
NAD 83 (CORS96) 	GEOID03 	
NAD 83 (NSRS2007) 	GEOID09 	
NAD 83 (2011) 	GEOID12B 	HI, AK, Guam, CNMI
NAD 83 (2011) 	GEOID18 	CONUS, PRVI


Cheers
Brian


On 11/1/2019 4:33 PM, Even Rouault wrote:
> On vendredi 1 novembre 2019 13:12:11 CET YK_climate wrote:
>> Hello,
>> I have been trying to perform vertical transformation with Proj for a few
>> days now, I think I am done but I am in some help to be sure I didn't make
>> some stupid mistakes!
>> I am looking at getting coordinates in WGS84-Ellipsoid to WG84-EGM96,WG84-
>> EGM2008 and to NAD83(2011)GEOID12B.
>>
>> Here is what I have working so far :
>> WGS84-EGM2008 : cs2cs EPSG:4979 EPSG:4326+3855
>> WGS84-EGM96    : cs2cs EPSG:4979 EPSG:4326+5773
>> Is this correct?
> Yes
>
>> So naturally I thought that GEOID12B would be as easy :
>> NAD83(2011)-GEOID12B    : cs2cs EPSG:4979 EPSG:6319+5703 <= no vertical
>> change
>> NAD83(2011)-GEOID12B    : cs2cs EPSG:4979 EPSG:5498         <= no vertical
>> change
> As the target SRS here use the NAVD88 height vertical CRS, but in EPSG, there
> is no transformation from NAVD88 to WGS84, then PROJ < 6.2.1 didn't do any
> vertical change.
>
> Starting with PROJ 6.2.1, it will use the NAVD88->NAD83/NAD83(2011) grids, so
> GEOID12B if that's what you've available, as a fallback.
>
> $ ~/proj/install-proj-6.2.1/bin/projinfo -s EPSG:4979 -t EPSG:5498  --spatial-
> test intersects
>
> Candidate operations found: 294
> -------------------------------------
> Operation n°1:
>
> unknown id, NAD83(2011) to NAVD88 height (1) + Inverse of NAD83 to WGS 84 (1),
> 4.02 m, USA - CONUS - onshore, at least one grid missing
>
> PROJ string:
> +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert
> +xy_in=deg +xy_out=rad +step +inv +proj=vgridshift +grids=g2012bu0.gtx
> +multiplier=1 +step +proj=unitconvert +xy_in=rad +xy_out=deg +step
> +proj=axisswap +order=2,1
>
>
>
> Even
>

-- 
*************************************
Brian Shaw
Geodesist
NOAA/NOS/National Geodetic Survey
Phone # 240-533-9522

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