[PROJ] Vertical deflection: of interest for PROJ ?

[Nyall Dawson]

On Thu, 9 Jan 2020 at 06:38, Even Rouault <even.rouault at spatialys.com> wrote:
>
> Hi,
>
> While integrating different datasets over the last months, I've noticed that a
> few geoids (like the USA or Australian geoids) are accompanied with the angles
> for the vertical deflection. I've read a bit about that, but I'm still unclear
> if such data would be useful for what PROJ does, that is coordinate
> transformations.
>
> For the formula converting between ellipsoidal height to orthometric height,
> H = h - N, sometimes instead of the = one sees a ~= . My understanding of the
> typical deflection angles is that the approximation is legitimate, as being
> largely a sub millimetric phenomenon, given that the cosinus of small angles
> is super close to 1.
> But would there be an impact on horizontal coordinates ?
> For example for a difference of height of 1000 m and a deflection of 50 arc
> second, that would be 1000 * tan(50'') = 0.24 m
> But perhaps the use case for a PROJ-like context could be "given a skyscrapper
> 1 km high whose facade is vertical along the plumb line, given the coordinates
> at the bottom, what are the coordinates at the top ?", which doesn't seem to
> be something people would use routinely :-)

Hmmm... you say that now, but with the ever blurring line between
CAD/GIS/BIM I think it IS likely to be a requirement in the near
future!

Nyall