Michael Barton wrote:
> >>>> I was just wondering why aspect is measured from the east and increased
> >>>> counterclockwise instead of the north and clockwise? It seems to run
> >>>> counter to what is done in other GISs.
> For my 2 cents worth, it seems to make a lot more sense for a
> *geographic* information system for all output to follow the same
> spatial organizational standards. I understand the history of the east
> is 0 convention for parts of GRASS. And I also appreciate the
> importance of not breaking code-dependent features within versions.
> However, that does not mean that we should keep a non-standard way of
> measuring direction for select modules (like r.slope.aspect), while
> measuring from north for others, simply because the program started
> out that way. Version changes are a time when we can rethink and
> standardize different modules that have evolved independently. Scripts
> are likely to break across the 6/7 transition for other reasons anyway.
>
> That said, a functionally simple approach that would not be quite so
> disruptive would be to add a flag to switch to count from east mode
> (with count from north as the default) or even a flag to switch to
> count from north mode (with count from east being the default if
> backward compatibility is indeed very important in this case).
I would suggest generalising it to scale and offset, so that you can
use degrees, grad, radians, etc. E.g.:
scale offset description
360 0 degrees CCW from east
-360 90 degrees CW from north
2*pi 0 radians CCW from east
-400 100 grad CW from north
We also need a flag to indicate whether angles are signed or unsigned,
e.g. 0..360 vs -180..180. Off the top of my head:
struct angles {
double scale, offset;
int is_signed;
};
double G_import_angle(const struct angles *f, double a)
{
return (a - f->offset) * (2*M_PI) / f->scale;
}
double G_export_angle(const struct angles *f, double a)
{
a = f->offset + a * f->scale / (2*M_PI);
return a - f->scale * floor(a / f->scale + f->is_signed ? 0.5 :
0);
}
We would also want a function to set the conversion parameters from a
string, so that common cases can be named (e.g. "deg,geo" for
degrees/CW/north, "rad,math" for radians CW from east).
Then, modules such as r.slope.aspect would get an angles= parameter,
with the existing interpretation as the default.
--
Glynn Clements <gl...@gclements.plus.com>
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