# [Proj] How grids work in PROJ4

Mikael Rittri Mikael.Rittri at carmenta.com
Mon Jan 16 05:50:28 EST 2012

```Joaquim,

I think Thomas's point wasn't that your H/h convention was wrong,
just that there are two conflicting conventions used in the literature.
(I have noted that the "Guide to coordinate systems in Great Britain",
http://www.ordnancesurvey.co.uk/oswebsite/gps/docs/A_Guide_to_Coordinate_Systems_in_Great_Britain.pdf
uses H for ellipsoid height and h for orthometric height.)

You also wrote:

> It happens that I just tested with our Portuguese geoid with following command
>
> cs2cs +proj=latlong +ellps=WGS84 +towgs84=0,0,0 +geoidgrids=GeodPT08.gtx
> +to +proj=latlong +ellps=WGS84 +towgs84=0,0,0
> -8 37 0
> 8dW 37dN 51.910
>
> which is fine and means that the left side altitude was interpreted as ellipsoidal height
> and the output is the geoid undulation at that point.

Well, that's one way to interpret the result (and a correct one). That is, the geoid
undulation is +51.910 m, so the geoid surface is 51.910 m above the ellipsoid
surface here.

However, I believe the intended interpretation is different: The presence of
+geoidgrids before +to means that the input height is an orthometric height,
while the absence of +geoidgrids after +to means that the output height is
height above ellipsoid.  This interpretation is more general, since the geoid
undulation is, in general, not output explicitly by cs2cs, although it can be
calculated as ellipsoid height minus orthometric height.

So as I understand it, your suggested +H/+h parameters are unnecessary,
since the presence of +geoidgrids means orthometric height, and its absence
means ellipsoid height.

Regards,

Mikael Rittri
Carmenta
Sweden
http://www.carmenta.com<http://www.carmenta.com/>

________________________________
From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of J. Luis
Sent: Sunday, January 15, 2012 9:12 PM
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] How grids work in PROJ4

On 15-01-2012 13:58, Thomas Knudsen wrote:
2012/1/14 Michal Seidl <michal.seidl at gmail.com<mailto:michal.seidl at gmail.com>>

[-- TEXT DELETED --]

So following Joaquim's suggestion of +H and +h options is quite risky. Out of context you can never be sure what is meant by H and h.

/Thomas

Ai, this is embarrassing for me. Off course I mistakenly swapped the H/h as it never crossed my mind going against the notation followed by the "Physical Geodesy" bibles.

Joaquim
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