[GRASSLIST:5954] Re: Krovak / PROJ4 (+axis option?)

[Frank Warmerdam]

Radim Blazek wrote:
> Hi Frank,
> 
> as you probably know, Krovak defines X=-y, Y=-x, i.e. X is vertical axis, 
> oriented down and Y is horizontal oriented to the left. But all
> GIS/GPS/... systems must use usual coordinate system where 
> coordinates are in 3. quadrant.
> For example:
> 14.39862613611E 50.465671925N  LL
> X=1001120.05 Y=738666.75       Krovak
> x=-738666.75 y=-1001120.05     In usual coor system and all GIS
> 
> Because:
> 1) Evenden's implementation of Krovak (PJ_kocc.c) returns by default
>    values in 3. quadrant (-738666.75 -1001120.05) and Krovak X,Y
>    is returned only with +czech option
> 2) All GIS applications require coor in 3. quadrant and there 
>    is no reasonable way to switch axis outside PROJ4 only for Krovak
>    (For GRASS for example, I used code PJ_krovak.c from PROJ4
>     but modified the code to get negative signs.)
> 3) Current behaviour in PROJ4 is wrong anyway in my opinion, because
>    returns 738666.76,1001120.13 (switched signs but not axis)
> 
> I suggest to change PJ_krovak.c in PROJ.4 as you can see below.
> That means to get values that most of applications require.
> 
> Radim
> 
> 136,137c139,140
> <       xy.y = ro * cos(eps) / a;
> <       xy.x = ro * sin(eps) / a;
> ---
> 
>>      xy.y = -ro * cos(eps) / a;
>>      xy.x = -ro * sin(eps) / a;
> 
> 199,200c202,203
> <       xy.x=xy.y;
> <       xy.y=xy0;
> ---
> 
>>      xy.x = -xy.y;
>>      xy.y = -xy0;

Radim,

Rather than have a +czech switch specifically for the Krovak projections, and
instead of altering the sign conventions from the "usual", I would like to
introduce a generic mechanism at the pj_transform() level for alternate axis
orientations.

Something along the line of a +axis switch, where "+axis=en" means that
x axis = 'e'ast, and y axis = 'n'orth.  Then a coordinate system string
could specify alternate axis orientations as +axis=sw indicating that
X is a "southing" (or negative northing) and Y is a westing (or negative
easting).  I think this would be the right organization for Krovak right?

My thinking is that this isn't a "projection" issue, but rather at a higher
level a "coordinate system" issue which is why I think it should be handled
by pj_transform() and not effect the low level projections routines.

I would add that Krovak is not the only coordinate system with unusual
axis conventions but this is an area I have been "sweeping under the carpet"
in most of my work in PROJ.4, and OGRSpatialReference, so far.

I would also note that I intend to adopt Gerald Krovak code at some point.

I have no schedule for implementing generic axis support or migrating to
use Geralds Krovak (or simplified libproj).  I will not have time to even
look into this for several weeks.  Thus, for your own purposes you should hack
your local GRASS built in a manner suitable to your needs in the meantime.

I have taken the liberty of cc:ing this reply to the PROJ list where I hope
any discussion of a +axis switch can take place.  GRASS folks might also be
interested as it would presumably be the way that alternate axis orientations
are selected in GRASS.

Best regards,

-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent