[yt-users] Issues with code_length versus physical lengths for positions in x, y, z space

[Devin Silvia]

Hi all,

I've been trying to do some data analysis of an Enzo dataset to
extract information about volumes contained with the virial radius of
rockstar identified halos.

As a test, I've identified the most massive halo and created sphere at
that halo position using a bit of code that looks like this:

sp = data_ds.sphere([most_massive.quantities['particle_position_x'],
                     most_massive.quantities['particle_position_y'],
                     most_massive.quantities['particle_position_z']],
                    most_massive.quantities['virial_radius'])

Then, when I actually try to use the sphere to analyze data or make
plots, it seems like there is something screwy going on with the
position information, which you can see if you look at the
calculations done here:

http://paste.yt-project.org/show/5522/

When you look at the "center" based on a calculation of the positions
with respect to the domain, you get a different center than the one
that just comes of sp.center in "code_length".  Specifically, the
"code_length" center values all seem to be off from the actual
position of the halo by a factor of 1.4:

In [98]: sp.center.in_units("kpc")/data_ds.domain_width.in_units("kpc")
/ sp.center.d
Out[98]: YTArray([ 1.40845077,  1.40845077,  1.40845077]) (dimensionless)

I'm not sure where this factor is coming from, but it seems to be
buried in some sort of unit conversion step somewhere.

Thanks,
Devin

-- 
Devin W. Silvia
NSF Astronomy and Astrophysics Postdoctoral Fellow
Department of Physics and Astronomy
Michigan State University
www.devinsilvia.com
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