CGAL users discussion list

[Monique Teillaud <>]

infblnpf wrote:
> Thank you for the information.
>
> General circular arcs unfortunately help me a little only, since I have to
> work with great circles (or segments of those) only.

The spherical kernel offers general arcs, it is not restricted to your 
specific needs, but you can still define arcs of great circles as a 
particular case. So, it seems that these circular arcs are helping you, 
even though you are not using them in their full generality.

> What I meant by the geometric operations was the following: Lets say I would
> like to draw a line from Seattle to Moscow. For each my intention was to
> simply define the cities' coordinates and the line is drawn for me. As far
> as I understand I would though have to define a plane (or other sphere) in
> CGAL first. But this second object is based on Euclidean space, which means,
> that I would first have to determine the Euclidean coordinates from my
> geographic coordinates.

In any case, you already have to do something similar with the cities' 
coordinates, don't you?

If I understand well, a city is given in some kind of spherical 
coordinates (angles). This in fact defines a line in the 3D Euclidean 
space that contains the center of the sphere.
Each line defines a point of type Circular_arc_point_3 of the spherical 
kernel, through intersection with the sphere.

The two lines in R^3 corresponding to two cities define a plane in R^3. 
This is the plane than can be used to define the arc of great circles 
between the two cities.

(In fact a potential problem I see is that CGAL assumes the lines and 
planes to have a rational equation in R^3. (I am putting things in 
simple terms, more details are given in the manual.) But if the sines 
and cosines defining your city are given as doubles, then this is in 
fact already some kind of rational approximation, so, our rational 
framework should hopefully not be a problem.)

> This would still leave me with more geometric operations necessary.

It depends what you call "necessary". Of course a software especially 
tuned at your specific needs would, by definition, be easier to use for 
you. Since the CGAL spherical kernel is more general, then it is 
"necessary" to perform some operations to define your specific objects.

That said, as I already mentioned, we might think of adding some 
functionality in the future, to offer a simpler interface for such a 
use, which is probably of general interest for geographic applications.
(As usual, this is a matter of manpower...)

--
Monique Teillaud
INRIA Sophia Antipolis - Méditerranée
http://www.inria.fr/sophia/members/Monique.Teillaud/