[PROJ] Converting NAD83/NADV88 heights to HAE

[Brian Shaw]

Hi Patrick

NOAA does maintain the NAD 83, NAVD 88 and geoid models, looks like the 
typo might be in the readme from anothers email since NGA maintains 
WGS84 and EGM08.

I apologize as Im not completely sure on the format of your command but 
I think I have it you are passing UTM coordinates in NAD83 to geoid18 to 
get an NAVD88 height except that instead of passing an ellipsoid height 
you are passing an orthometric height.

To calculate the ellipsoid height you would use the formula h=H+N where 
h (ellipsoid height, in this case NAD 83), H (orthometric height, in 
this case NAVD 88) and N (geoid height, in this case GEOID18).

You should be providing an ellipsoid height to the geoid to get an 
orthometric height.  Maybe its because its the end of my work day that I 
cant figure out how to do this in one step but I believe if you provide 
0 as the height this will then give you the geoid height for that 
location (in geoid18) and you can just add this to your orthometric 
height to get the ellipsoid height.  You should get a negative number 
since the geoid is all negative for CONUS.

more information about geoid18 can be found here:
https://geodesy.noaa.gov/GEOID/GEOID18/

Hopefully this helps if I think of a way to do it in one command Ill 
respond again if someone else doesnt provide a solution.

Cheers
Brian

On 7/1/2020 5:49 PM, Patrick Young wrote:
> Hello,
>
> I am trying to figure out how to convert some elevations I have into 
> heights above ellipsoid via the cs2cs tool.  I am using proj 7.0.1, 
> and I've run projsync --all to snag all the grids.
>
> Here is an example of what isn't working for me:
>
> echo 565890.947 4192011.754 55 | cs2cs +proj=utm +zone=10 +datum=NAD83 
> +units=m +no_defs +geoidgrids=us_noaa_g2018u0.tif +to +proj=utm 
> +zone=10 +datum=NAD83 +units=m +vunits=m +no_defs
> 565890.95       4192011.75 55.00
>
> I am confused as to why the elevation value isn't changing for me.  My 
> (extremely limited) understanding is that I am asking for 55m to be 
> converted from a height referenced against the NADV88 geoid to one 
> referenced against the NAD83 ellipsoid.  Am I using the wrong geoid 
> grid?  From the grid readme, it mentions "us_nga_g2018u0.tif : 
> Conterminous US, referenced to NAD83(2011). Vertical datum: NAVD88" 
> (presumably the nga <-> noaa is a typo in the readme) so I think I'm 
> using the right one.
>
> Thanks for any help,
> Patrick
>
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-- 
*************************************
Brian Shaw
Rocky Mountain Regional Advisor (CO, MT, WY)
NOAA's National Geodetic Survey (NGS)
Phone # 240-533-9522

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