position angle or bearing calculation

Hello,

Does astropy support calculation of the PA or position angle between two points on the
celestial sphere. I am using astropy.coordinates.angles.AngularSeparation to compute the angular separation.

Something like what slalib SLA_BEAR is what I am looking for:

http://star-www.rl.ac.uk/docs/sun67.htx/node26.html

Returns the bearing (position angle) of one point on a sphere seen from another

Thanks

@richardgmcmahon

@richardgmcmahon This is not implemented yet, but I think would be great to have!
Do you think you could implement this in Astropy in the coming weeks if we give pointers?

Implementing a function

position_angle(reference_coordinate, other_coordinate)

that takes two astropy.coordinates as input, converts other_coordinate to the same coordinate system as reference_coordinate if needed and then executes the appropriate formula (see e.g. here or here) is easy (and would already be useful).

But sometimes one needs to convert position angles from one coordinate system (say ICRS) to another (say Galactic) and for that I'm not sure what a good API would be. Off the top of my head one option would be an attribute on the coordinates base class that is None by default, but gets transformed if present:

from astropy.coordinates import ICRS, Galactic, Angle
coordinate = ICRS(0, 0, ('deg', 'deg')
coordinate.position_angle = Angle(42, 'deg')
print(coordinate.to(Galactic).position_angle)

But this might very well be a bad idea ... @eteq will know.

A while ago I had to do this transformation of position angles and came up with this bad algorithm of creating a second point eps away from the reference point ... there must be some way to do this with cartesian unit 3D vectors and rotation matrices, but I don't know any code that implements this.
In any case the NED coordinate calculator can transform position angles and can be used to generate reference results for unit tests.

OK, I have forked astropy and think that I can create a function angular_bearing based on astropy angular_separation and slalib sla_bear.f

Now, I am a bit confused since astropy 0.2x has an AngularSeparation class in angles.py which is not present in 0.3x

https://github.com/astropy/astropy/blob/v0.2.x/astropy/coordinates/angles.py
all = ['Angle', 'RA', 'Dec', 'AngularSeparation']

https://github.com/astropy/astropy/blob/v0.3.x/astropy/coordinates/angles.py
all = ['Angle', 'Latitude', 'Longitude']

the angular separation is in angles_utilities

https://github.com/astropy/astropy/blob/v0.3.x/astropy/coordinates/angle_utilities.py
def angular_separation(lon1, lat1, lon2, lat2):

I am easily confused and planned to implement an PositionAngle function based on AngularSeparation.

Comments appreciated.

@richardgmcmahon

@richardgmcmahon GitHub has a great full-text search for issues, pull requests and code.

In this case if you search for AngularSeparation you will find #1007 which shows you when it was removed and the dicsussion why:
https://github.com/astropy/astropy/search?q=angularseparation&ref=cmdform&type=Issues
(I'm not saying you have to read that ... I'm just mentioning it as an example how you can find out such things quickly.)

To answer your actual question: to compute an angular separation use SphericalCoordinatesBase.separation, i.e. take an astropy coordinates as input for your function and then call the separation method on it.

Thanks, that is very helpful. I will look at this on a weeks timescale.

@richardgmcmahon

@richardgmcmahon Thanks for doing this!

If you're not familiar with Astropy coding much, the hardest part for this PR is probably not implementing the formula for the position angle, but finding out how to add and run unit tests. There's quite a bit of documentation for this here, but you should also be able to copy & paste & edit existing tests.

Just so that you don't waste time, I think it makes sense to agree on the desired API before you implement this. After looking a bit at how that current implementation, I think you should follow the example of the SphericalCoordinatesBase.separation method calling an angular_separation function, i.e. you should implement a method and a function in this format:

SphericalCoordinatesBase.position_angle(self, other):
def position_angle(lon1, lat1, lon2, lat2):

@eteq Agreed?

@cdeil

@cdeil @richardgmcmahon - Note that this has been on the "todo" list since #1012 .

So I'm 👍 to this, in the form @cdeil mentioned in his last message: a method in the same way separation is implemented, called position_angle.

Also, note that there's currently a major re-write of the coordinates framework happening, which will lead to these methods being moved to a different class in the next astropy version. But It's fine to implement this now as you're planning, @richardgmcmahon , and one of us developers will take care of moving it to the right place down the road.

Thanks for your interest in doing this, @richardgmcmahon !

The SOFA fortran is here

http://www.iausofa.org/2013_1202_F/sofa/pas.html

Slalib is here:

https://github.com/richardgmcmahon/starlink/blob/master/libraries/sla/bear.f

Oh, if it's in SOFA then it must be in ERFA (which we include in Astropy), so maybe we then just need a wrapper to it?

@astrofrog

@astrofrog - I took a look at this and it's actually really trivial in SOFA/ERFA - three lines of C code. Probably better to just implement it in python as a method of SkyCoord.

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position angle or bearing calculation #2311

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position angle or bearing calculation #2311

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cdeil commented on Apr 13, 2014

richardgmcmahon commented on Apr 13, 2014

richardgmcmahon commented on Apr 14, 2014

cdeil commented on Apr 14, 2014

richardgmcmahon commented on Apr 14, 2014

cdeil commented on Apr 14, 2014

eteq commented on Apr 14, 2014

richardgmcmahon commented on May 7, 2014

astrofrog commented on May 7, 2014

eteq commented on May 14, 2014

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