On Tue, Dec 17, 2013 at 3:57 PM, Roman Fleysher < roman.fleys...@einstein.yu.edu> wrote:
> > Since coordinate system is spherical, how do you tell that RA=23:59 and > RA=00:01 are next to each other using usual comparisons? I don't; usual comparisons won't work so I do two comparisons: I am usually looking for stars within a convex field of view (FOV), typically a frustum with a rectangular footprint, so I determine if and where RA=0=360 crosses that footprint, and break the FOV into two pieces, from 0<=RA<=loRA and hiRA<=RA<=360, so loRA becomes hira in one search and hiRA become lora in the other. There are only three cases: zero, one or two crossings. Zero crossings means I can do everything in one SELECT; one crossing means either one of the poles is in the FOV and I search RA=0 to 360; DEC=someDEC to +/-90, or the FOV touches RA=0(=360) from one side or the other, which reduces to the zero case; two crossings means the poles are not in the FOV and I can do two searches as mentioned above, from 0 up to someLowRA and from 360 down to someHighRA. There are some edge cases but that is basically it. I actually handle "two or more crossings" cases the same as two cases, even though I don't think "more" can happen with a convex FOV footprint. For any edge (segment of the great circle between two vertices) of the FOV that crosses RA=0, which is easily determined since I have the vertices in XYZ coordinates, I insert a vertex in the edge at the crossing, and then recurse with subsets of vertices split across RA=0. _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
