[yt-users] TotalQuantity vs. summing manually

[John ZuHone]

I think it depends on what you are trying to measure. If you're concerned with grid quantities maybe it makes more sense to use the dark matter density. It is certainly more accurate in a localized sense, since it is essentially the DM mass interpolated. 

However, since you're looking at a total quantity, the larger the region is the less the difference between the two will matter.

Neither quantity is really "better", if you ask me. It's more important just to be aware of what the subtle difference is between them.

In any case, the difference in the total quantity is about a few percent it looks like. Are you attempting to quantify something with that level of sensitivity?

John ZuHone
Laboratory for High-Energy Astrophysics
NASA/Goddard Space Flight Center
8800 Greenbelt Rd., Mail Code 662
Greenbelt, MD 20771
(w) 301-286-2531
(m) 781-708-5004
john.zuhone at nasa.gov
jzuhone at gmail.com

> On Dec 20, 2013, at 9:36 PM, Geoffrey So <gsiisg at gmail.com> wrote:
> 
> So in that case, would using TotalQuantity to sum up particle information inside 3D container be a bad practice?
> 
> From
> G.S.
> 
> 
>> On Fri, Dec 20, 2013 at 6:20 PM, John ZuHone <jzuhone at gmail.com> wrote:
>> Geoffrey,
>> 
>> It seems that what is going on here is that whereas the particles in the container are simply the ones contained within the container's cells, the dark matter density is smeared onto the grid using the cloud-in-cell algorithm, meaning that for particles on the edge of their container some of their mass has been smeared onto cells just on the outside, which is a thing that Enzo does.
>> 
>> So it's not really an apples-to-apples comparison.
>> 
>> John ZuHone
>> Laboratory for High-Energy Astrophysics
>> NASA/Goddard Space Flight Center
>> 8800 Greenbelt Rd., Mail Code 662
>> Greenbelt, MD 20771
>> (w) 301-286-2531
>> (m) 781-708-5004
>> john.zuhone at nasa.gov
>> jzuhone at gmail.com
>> 
>> > On Dec 20, 2013, at 9:02 PM, Geoffrey So <gsiisg at gmail.com> wrote:
>> >
>> > Hi all,
>> >
>> > I found that using a Enzo dataset I was getting slightly different numbers when
>> > 1) using the list of dark matter particles selected by creation_time < 0.0
>> >
>> > In [97]: sph_dm = sph['creation_time'] < 0.0
>> > In [98]: print "%12.12e" % (sph['ParticleMassMsun'][sph_dm]).sum()
>> > 1.211311468567e+11
>> >
>> > 2) compared to summing the dark matter particles inside a 3D container with TotalQuantity
>> >
>> > In [101]: print "%12.12e" % (sph.quantities['TotalQuantity']('Dark_Matter_Density')[0]*vol/Msun)
>> > 1.188937185993e+11
>> >
>> > I'm wondering if the field and particles are handled differently when being counted as inside or outside the 3D container?
>> >
>> > From
>> > G.S.
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