[Proj] Proj-5.0.0 cct path to gridfiles

Kristian Evers kreve at sdfe.dk
Tue Feb 20 12:31:15 EST 2018

> On 20 Feb 2018, at 17:20, stevejking <stevejking at talktalk.net> wrote:
> Thanks for to suggestions, but the grids are lat/long grids realised on the
> Airy ellipsoid and go from OSGB lat/long to Etrs lat/long and height. So
> think I’d need to apply the grids in the reverse direction/reverse sign (if
> that’s possible in the api) to first get Airy ellipsoid lat longs, then step
> to my final projection. They work with cs2cs so that fine for now.

When doing gridshifts PROJ does not care what ellipsoid your coordinates relate to.
It is simply offsetting the coordinate by whatever value(s) is at a the corresponding grid cell.
This is independent of the API that is used (hint: they use the same functions internally).

The general idea I have expressed in the previous mail is correct. Or, at least it replicates what you’ve
done with cs2cs. If you disagree I am afraid you’ve misunderstood how cs2cs works, which is
totally understandable as it is in no way transparent what is happening. This is one of the reasons
why the pipeline framework is superior - it communicates better what is happening in a transformation.

> I’ll try to summarise the steps I did, as I’m not likely at my computer
> today.
> I installed RC3 using visual studio 2017 community edition. From a VS prompt
> (I’d need to check, maybe re install as can’t recall if it was 32 or 64 bit
> prompt, or might have a check in Dependency Walker). I used the soon to
> depreciate nmake commands listed in the readme (two or three lines).

I have tried replicating this now. If I don’t have PROJ_LIB set to a directory with proj_def.dat, various
init and grid files I can’t do anything. Which make sense cause how would PROJ know where to look?
On Unix-based systems things are a bit simpler as PROJ always looks in /usr/local/share/proj for
what it needs and grids are installed there by default. No such thing exists on Windows. SO, the
solution is to make sure PROJ_LIB points towards your grids folder. I’ll add that to the README.

> I used a regular command prompt, navigated to the bin directory, then typed
> the cs2cs and cct strings as you list above.
> Steve
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