Geometry Boost List: Re: [geometry] Support for geographic coordinate system

Subject: Re: [geometry] Support for geographic coordinate system
From: Adam Wulkiewicz (adam.wulkiewicz_at_[hidden])
Date: 2014-11-11 19:48:26

Hi,

Barend Gehrels wrote:
> Adam Wulkiewicz wrote On 7-11-2014 0:00:
>>
>> vincenty<EllipsoidModel> s(EllipsoidModel())
>> andoyer<EllipsoidModel> s(EllipsoidModel())
>> haversine<SphereModel> s(SphereModel())
>> // or rather
>> haversine<SphereModelOrRadius> s(SphereModel())
>> // for backward compatibility
>>
>> , e.g.:
>>
>> typedef strategy::ellipsoid<double> ellipsoid;
>> strategy::vincenty<ellipsoid> s(ellipsoid(a, b));
>>
<snip>
> Sure, the first version looks much better. And ellipsoid follows the 
> Ellipsoid Concept (or how it will be called)
>
>
>>
>> We could then pass a Sphere model to the haversine<> and handle it 
>> differently than raw radius (as mentioned above).
>
> Sure.

1. The concept

I propose to use a part of already existing nsphere concept for this:
- tag
- radius_type/radius_access
Then various radiuses could be accessed with get_radius<>() and 
set_radius<>().

Eventually there could be optional traits for flattening, eccentricity, 
etc. falling into default ones calculating them from radius values.

Btw, for a (n)sphere I think get_radius<>() should be able to return the 
same radius for all dimension, not only for <0>.
This would be in-line with the spheroids and ellipsoids. This way it'd 
be possible to use spheres in algorithms working for spheroids.
And this also means that for spheroid radius for <0> and <1> should be 
equal and could be different for <2>.

2. The names

Since BG is a general-purpose library we should probably be 
mathematically correct. I know that it's common to call the reference 
Earth shape an ellipsoid in a GIS world but this is a spheroid since it 
has two equal radius values. So I propose to have a Sphere and Spheroid 
or rather ReferenceSphere and ReferenceSpheroid concept because... see 3.

3. The placing/namespace

One problem I see is a collision of Geometries models and those 
Parametric/Reference models. E.g. nsphere Geometry model has a position 
but sphere/spheroid Parametric/Reference model doesn't have one, only 
radius. This might be confusing for the users so I'm thinking about 
picking some namespace for those models (to not place them in the main 
namespace bg or bg::model). Something like bg::parameters or 
bg::reference. Then:

bg::reference::sphere<>
bg::reference::spheroid<>

Or something like that, e.g. shorter.
And then the tags could be:

reference_spheroid_tag
reference_sphere_tag : reference_spheroid_tag

And in the future if we e.g. had model::sphere and Sphere concept, 
sphere_tag could be derived from reference_sphere_tag and used as a 
ReferenceSphere too.

4. Alternative 2&3

They could also be named e.g. sphere_xxx and spheroid_xxx (xxx is some 
word, e.g. params) or reference_sphere/reference_spheroid and placed in 
the main namespace bg.

Regards,
Adam