
Geometry : 
Subject: Re: [geometry] Support for geographic coordinate system
From: Krzysztof Czainski (1czajnik_at_[hidden])
Date: 20141106 13:18:46
20141106 17:06 GMT+01:00 Adam Wulkiewicz <adam.wulkiewicz_at_[hidden]>:
> Hi,
>
> I'm planning to begin the implementation of the support for geographical
> CS and would like to hear your opinion.
Hi Adam,
I just wanted to say that I'm glad you're working on this. I used the
geographical projections from extensions in a project I finished a year
ego. The project used a Boost release 1.54.0 (I think) with a snapshot of
Boost.Geometry from svn trunk up front in include dirs list. To unittest
our use of the projections, we generated some test data in Matlab, and the
tests confirmed accurate results of projections.
> Esspecially from people already using the geographical extensions.
> Basically, to support this CS we need a distance strategy and side
> strategy. They're already implemented (more or less) though in the
> extensions so not officially released yet. To release them we need to think
> about the interface first. The geographical strategies should have an
> interface similar to the one used in spherical strategies. Or the other way
> around, spherical strategies interface could be extended somehow to match
> the geographical version. In all cases the strategies should be backward
> compatible. Currently we have:
>
> CARTESIAN:
>
>  cartesian side has no ctor
>
> side_by_triangle<> s;
>
>  cartesian distance has no ctor
>
> pythagoras<> s;
>
> SPHERICAL:
>
>  spherical side has no ctor since it doesn't need to know the radius
>
> spherical_side_formula<> s;
>
>  spherical distance (haversine) requires a radius of type passed to the
> strategy as template parameter, and by default radius = 1.0 is used (unit
> sphere)
>
> haversine<RadiusType> s(radius);
> haversine<RadiusType> s; // radius = 1.0 by default
>
> so to calculate the distance on a sphere the user can pass a strategy with
> radius or use the default one and multiply the result by the radius:
>
> distance(p1, p2, haversine<RadiusType>(radius));
> distance(p1, p2) * radius;
>
> GEOGRAPHICAL:
>
>  a spherical side strategy is just used by default which I guess should
> be changed. I think the most precise would be a strategy finding the side
> using the geographic courses found using the inverse Vincenty's formula but
> this requires some testing
>  for distance there are 2 strategies, both taking
> detail::ellipsoid<RadiusType>, andoyer also has a ctor taking a flattening
> of type RadiusType (like detail::ellipsoid<>). If 1 parameter is passed
> (flattening) then the radius is set to 1.0 but if 2 parameters are passed
> then A and B radiuses are set. Furthermore by default Earth's radiuses are
> used. So the situation looks like this:
>
> vincenty<RadiusType> s(detail::ellipsoid<RadiusType>()); // A =
> 6378137.0, B = 6356752.314245
> vincenty<RadiusType> s(detail::ellipsoid<RadiusType>(f)); // A = 1.0, B =
> [some fraction for oblate spheroid]
> vincenty<RadiusType> s(detail::ellipsoid<RadiusType>(a, b)); // A = a, B
> = b
> andoyer<RadiusType> s; // A = 6378137.0, B = 6356752.314245
> andoyer<RadiusType> s(f); // A = 1.0, B = [some fraction for oblate
> spheroid]
>
> It's confusing (this 1 and 2 paramers difference) and inconsistent with
> spherical strategies.
> Furthermore flattening should be stored using some Floating Point type and
> currently it's just RadiusType. E.g. for integral RadiusType the default
> ctor would initialize flattening with 0.
>
> PROPOSAL:
>
> 1. I propose to keep it simple for now, don't release detail::ellipsoid
> and change the default parameters to make the strategies close to the
> spherical ones:
>
> vincenty<RadiusType> s(); // A = 1.0, B = [fraction], F=
> [fraction,default]  unit WGS84 spheroid
> vincenty<RadiusType> s(a); // A = a, B = [calculated from a using default
> flattening], F= [fraction,default]
> vincenty<RadiusType> s(a, b); // A = a, B = b, F= [calculated from a and b]
>
> This way the calls:
>
> distance(p1, p2, vincenty<RadiusType>(a));
> distance(p1, p2) * a;
>
would be equivalent (assuming that RadiusType is not integral type?).
>
If I recall correctly, we didn't use the distance algorithm in the end. But
at some prototyping stage we used distance(p1, p2, vincenty<RadiusType>()),
expecting the distance on Earth. You're proposed change would break this
expectation, but I don't think this is a problem.
>
> By default FlatteningType would be:
>  for floating point RadiusType <=> RadiusType
>  for integral RadiusType <=> promoted to FP of the same size (int>
> float, long long> double)
>  for user defined types <=> UDT
> The FlatteningType could be optionally passed as a second template
> parameter of a strategy however I'm not sure about it since it wouldn't
> play nice with possible future improvement 3 (see below).
>
> ALTERNATIVES (FUTURE ADDITIONS)
>
> 2. To decrease the probability of a mistake the strategy could take
> wrapped parameters (only?). This way it'd also be possible to create an
> ellipsoid from A,B A,F or B,F pair:
>
> // below are functions returning wrapped values
> vincenty<RadiusType> s(major_radius(a), minor_radius(b));
> vincenty<RadiusType> s(major_radius(a), flattening(f));
> vincenty<RadiusType> s(minor_radius(b), inverse_flattening(if));
>
> 3. Desing concepts for models parameters. And then always pass a type
> adapted to model's parameters concept to the strategy, instead of
> RadiusType. Actually a single number could be adapted to e.g. sphere
> parameters concept (radius) but why reserve it specifically for sphere?
>
> haversine< parameters::sphere<RadiusType> > s(parameters::sphere<
> RadiusType>(radius));
> vincenty< parameters::ellipsoid<RadiusType> > s(parameters::ellipsoid<RadiusType>(a,
> b));
> vincenty< parameters::ellipsoid_af<RadiusType> >
> s(parameters::ellipsoid_af<RadiusType>(a, f));
> vincenty< parameters::ellipsoid_unit_flattening<RadiusType> >
> s(parameters::ellipsoid_unit_flattening<RadiusType>(f));
> vincenty< parameters::ellipsoid_wgs84_1996<RadiusType> >
> s(parameters::ellipsoid_wgs84_1996<RadiusType>());
> vincenty< parameters::ellipsoid_wgs84_2004<RadiusType> >
> s(parameters::ellipsoid_wgs84_2004<RadiusType>());
> vincenty< parameters::ellipsoid_latest<RadiusType> >
> s(parameters::ellipsoid_latest<RadiusType>());
>
> FYI, all names of namespaces, structs and functions are examples.
>
> For sphere we'd already have a small problem with this. However this could
> be implemented in a backward compatible way. But then it could be done this
> way in the future, for both coordinate systems.
>
> 4. 2 and 3 together:
>
> vincenty< parameters::ellipsoid<RadiusType> > s(parameters::ellipsoid<RadiusType>(major_radius(a),
> minor_radius(b)));
>
> Do you have any thoughts or requests regarding the above?
>
Thanks for a wonderful Geometry library and keep improving it!
Kris
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