[PROJ] Converting NAD83/NADV88 heights to HAE

[Patrick Young]

Thanks all for the help!

Dinking around a little bit more, I found through trial and error that
eliminating the +vunits seemed to solve it:

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=WGS84 +units=m +no_defs
565890.95       4192011.75 *22.79*

Somehow, the addition of +vunits=m on the target angers proj:

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=WGS84 +units=m *+vunits=m *+no_defs
565890.91       4192011.91 *55.00*

Adding +vunits=m to the source seems not to matter.  Wish I could say more
on why this is the case; is it a bug or is it a malformed request?

Thanks!
Patrick

On Wed, Jul 1, 2020 at 7:11 PM Brian Shaw <brian.shaw at noaa.gov> wrote:

> 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
>
> _______________________________________________
> PROJ mailing listPROJ at lists.osgeo.orghttps://lists.osgeo.org/mailman/listinfo/proj
>
>
> --
> *************************************
> Brian Shaw
> Rocky Mountain Regional Advisor (CO, MT, WY)
> NOAA's National Geodetic Survey (NGS)
> Phone # 240-533-9522
>
>
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