Add implementation for geographic coordinates conversion

[santisoler]

Description of the desired feature

The geographic coordinates conversion have been implemented on Harmonica.
I don't remember exactly why we added the functions there and not on Verde.
Would it be Verde a better place for them?

[leouieda]

The geographic -> geocentric spherical conversion relies on the ellipsoid parameters, which is in Harmonica for normal gravity. We could do that here and ask for pyproj or cartopy ellipsoids but I'm not sure how well that would work.

[santisoler]

I don't think having repeated code would be a good choice. Giving second thoughts to this issue, I realize that even the geographic -> geocentric spherical conversion is a mere geometry operation on coordinates, it would rarely be used in Verde.

@leouieda if you agree we could close this issue.

[ahartikainen]

Is there a possibilty to use geographical coordinates in cKDTree calculations for distance?

Pyproj has implemented these.

[djhoese]

@ahartikainen Do you have a link to these calculations? In the pyresample project we've always converted lon/lats to xyz coordinates before putting them in a kdtree.

[ahartikainen]

Sorry, I meant that distance calculations can be done with pyproj, but I'm not sure how we could use those in KDTree structure.

[leouieda]

@ahartikainen not that I'm aware. I wondered if we could use great circle distances but it seems like there is no easy way to plug a custom distance function.

[leouieda]

@leouieda if you agree we could close this issue.

I agree. This is common in global gravity using tesseroids but probably not much else.