[Proj] proj4 string for perfect sphere
Demitri Muna
thatsanicehatyouhave at me.com
Wed Apr 18 11:50:22 EST 2012
Hi,
Just to tie up a few loose ends, comments below. I've decided that GIS code, for the moment, won't give me significantly more functionality for what I need than other options that don't require the conceptual overhead, but thanks to everyone for all the comments and help. It's nice to learn about something new. :)
On Apr 16, 2012, at 6:24 PM, Fischer, Robert P. (GISS-6110)[COLUMBIA UNIVERSITY] wrote:
> Have you tried creating temporary schemas/tables in PostgreSQL with q3c? Is that fast enough? It sounds like that would be by far the easiest solution for you.
q3c is very fast; I haven't needed to even consider this option. My initial aim was to come up with an alternate tool that didn't require the overhead of a PostgreSQL installation for smaller data sets. But yes, for larger data sets, it's the only tool I use.
>> Thanks very much for the extensive comments. The biggest thing I
>> learned about GIS this week (starting from knowing nothing about it
>> last week) is that calculations are primarily done on an XY plane
>> which are projections of a curved surface. I had naively assumed going
>> in that calculations and indices (e.g. spatialite) were optimized for
>> the surface of a sphere.
>
> Yes, that's what I surmised too. The good news is, the technique works quite well for local computations, and you often choose whatever projection you like. For example, area of a general closed curve on the surface of a sphere can be computed this way, using any old equal area projection centered near your polygon.
I see now how 2D projections are very beneficial for geographical concerns, but less so for astronomical. That said, I do see some potential uses for visualization, so I might be back at some point. :)
>> Thanks for the code. I've been using the law of cosines and wasn't
>> aware of the problem at small separations (though this seems to trade
>> that problem for one at large separations).
>
> Vincenty's formula solves that problem.
Very glad to have learned that!
>> I will also look at the other links you mentioned. Since all my source
>> points are in ra/dec (not 3D cartesian), there will be the computation
>> hit to convert them back and forth.
>
> Is this signficant for your problem?
For computing a correlation function with a data set of half a million point sources, yes. But for smaller data sets, no. (And as I mentioned above, I was looking for a tool for the latter.)
Cheers,
Demitri
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