**snapgrid in=**snapshot **out=**image [parameter=value]

The X and Y
coordinates of the datacube can only be regularly gridded in histogram
fashion (for spatial XY-smoothing see *ccdsmooth(1NEMO)*
, for interpolating
see also *snapmap(1NEMO)*
), however the Z coordinate has the property that
it can take moments in this variable, pick a number of planes or planes
with smoothing.

The output image is written in standard *image(5NEMO)*
format,
and can be accessed by various other programs for smoothing, display etc.

Although *snapgrid(1NEMO)*
can grid datacubes (e.g. X-Y-Z), *snapgridsmooth(1NEMO)*
is probably easier to use, since it does not integrate along any lines
of sight, whereas this program is more suited for taking moments along
the 3rd axis.

For images that require more accurate interpolation, instead
of this histogramming approach, use *snapmap(1NEMO)*
.

For a related program
that creates images (and can create movies as well), see *uns_2dplot(1NEMO)*
.

As of V6.0 the units in 3D cubes will be particle density. This ensures that
programs such as *ccdmom(1NEMO)*
and *ccdstat(1NEMO)*
give the proper answer
in their summed emission.

In the **yt** package such images are called *phase
plot*, where a 2D grid in field1 (**xvar=**) and field2 (**yvar=**) is computed,
with some statistic (**mom=**) on field3 (**evar=**).

**in=***in-file*- input file, must be in
*snapshot(5NEMO)*format. Multiple snapshots can be stacked uses the**times=**keyword: see**stack=**below. [no default]. **out=***out-file*- output file, will be in
*image(5NEMO)*format [no default]. **times=***t1:t2,t3,..*- Selection of the times of snapshots to be selected
for gridding. For
**stack=t**all snapshots will be co-added into one image, however selecting**stack=f**or selecting multiple**evar**’s one can request multiple output images. [Default:**all**]. **xrange=***xb:xe*- Range in
**xvar**to bin, the coordinates are allowed to decrease as well as increase. [default:**-2:2**].

**yrange=***xb:xe*- Range in
**yvar**to bin [default:**-2:2**]. **zrange=***xb:xe*- Range in
**zvar**to bin, or take moments of [default:**-infinity:infinity**]. **xvar=***x-expression*- The value of
*x-expression*is gridded along the X axis. [default:**x**]. **yvar=***y-expression*- The value of
*y-expression*is gridded along the Y axis. [default:**y**]. **zvar=***z-expression*- The value of
*z-expression*is gridded along the Z axis (**nz**>1), or moments taken off (**nz=1**). [default:**-vz**]. **evar=***emissivity*- Variable to denote emissivity
per particle. You can select more than 1 expression, in which case different
images will be written out (only in
**stack=f**mode) [default:**m**]. **tvar=***tau*- Variable to denote the optical depth of a particle. [Default: 0]
**dvar=***depth*- Variable to denote the line of sight. [Default: z]
**svar=***smoothing*- Variable to denote gaussian smoothing Note this is the gaussian sigma, not the FWHM (FMHW = 2.355 * sigma).
**nx=***x-pixels*- Number of pixels along the X axis
of the cube [default:
**64**]. **ny=***y-pixels*- Number of pixels along the Y axis
of the cube [default:
**64**]. **nz=***z-pixels*- Number of pixels along the Z axis
of the cube. If one pixel is choosen, moments can be taken (see below),
else a simple gridding is used. [default:
**1**]. **xlab=***x-label*- Text used to label
the X-axis. By default the
**xvar**expression is used. It may be useful in certain astronomical environment to label the axis with recognized labels like RA---TAN, DEC--SIN, GLON etc. **ylab=***y-label*- Same for the Y-axis.
**zlab=***z-label*- Same for the Z-axis.
**moment=***number*- Order of the Z-gridding. Most commonly choosen
are:
**0**(total intensity),**1**(velocity zvar weighted intensity) and**2**(velocity square weighted intensity), where ’intensity’ should really be read as surface density per square unit length. Special values of**-1**and**-2**can be used to directly compute the*evar*weighted mean and the dispersion from the mean.**-3**and**-4**are used to compute the gaussian-hermitian h3 and h4 moments (see e.g. van der Marel & Franx, 1993) [default:**0**]. **mean=t|f**- Should the
*emission*in a cell be averaged? This also controls the units of the gridding. For**mean=f**(the default) a*surface-density*is computed (emission per square length), whereas for**mean=t**the average per

pixel (or voxel) is computed of the units of emission. Another way of looking at this,**mean=t**is for interpolating maps (see also*snapmap(1NEMO)*), where as**mean=f**is for splatting information as if this was observed. [Default:**f**]. **stack=t|f**- Should all snapshots from the input file be stacked, or write
one image per selected (see
**times=**) time? [default:**f**]. **integrate=t|f**- If
selected, instead of summing points along the
*zvar*, they are sorted and integrated along*dvar*. This is appropriate when emission represents something like a density, instead of a mass, and a total column density is needed. ** This option can only compute 2D moment=0 maps and also cannot handle stacked snapshots yet ** [default:**f**]. **proj=**- If a valid projection type
(SIN, TAN, ARC, NCP, GLS, MER, AIT) but see also
*wcs(1NEMO)*, the input coordinates are interpreted in angular degrees, and griddes with the appropriate sky projection. Default: no sky projection.

Note that the moment maps must be smoothed
*before* they can be combined to the proper velocity and dispersion maps.

% snapgrid in=nbody.dat out=map0 moment=0 % snapgrid in=nbody.dat out=map1 moment=1 % snapgrid in=nbody.dat out=map2 moment=2 % ccdsmooth in=map0 out=map00 gauss=0.1 % ccdsmooth in=map1 out=map11 gauss=0.1 % ccdsmooth in=map2 out=map22 gauss=0.1 % mv map00 int % ccdmath in=int,map11 out=vel fie=%2/%1 % ccdmath in=int,vel,map22 out=sig fie="sqrt(%3/%1-%2*%2)" % rm map11 map22Alternatively, with the option of using negative

% snapgrid in=nbody.dat out=int moment=0 % snapgrid in=nbody.dat out=vel moment=-1 % snapgrid in=nbody.dat out=sig moment=-2Consider now the situation where a coordinate is regularly sampled, with N values between A and B. In order to grid these, one would normally use a range=A-dx/2:B+dx/2, where dx=(B-A)/(N-1). One can also make a grid with N cells with emission, and K blank cells between each valued cell (K would be typically small, perhaps 1 or 2). With NK=(K+1)N-K and dx=(B-A)/(NK-1), a range=A-dx/2:B+dx/2 is used. If this is done in both the X and Y dimension, the program

Here is an example of making a gridded map of ungridded
data. Both unweighted, and weighted. Suppose the snapshot has the weights
stored in the *Aux* field, and we use these as weights (i.e. sum(mass*Aux)/sum(Aux)
would be the quantity of interest). The unweighted average uses the **mean=t**
key:

snapgrid ... out=map0 evar=m mean=tbut the weighted average computes the two maps seperately and uses

snapgrid ... out=map1 evar=’m*aux’ snapgrid ... out=map2 evar=’aux’ ccdmath in=map1,map2 out=map3 fie="ifeq(%2,0,0,%1/%2)"with an additional safeguard to set cells to 0 if no emission with found in them.

% snapgrid ...

When channel maps are
produced (**moment=0**), the data are not normalized w.r.t. the convolving velocity
beam. For a rectangular beam (**vrange=vmin:vmax**) the data should formally
be divided by **(vmax-vmin)**, for a gaussian beam (**vrange=vmean,vsig**) by **vsig*sqrt(2*pi)**.
Also remember that a gaussian beam has **FWHM = 2.355*sigma**.

Although *snapscale(1NEMO)*
can also be used, after a snapshot has been gridded into a map/cube, *ccdsky(1NEMO)*
can optionally be used to rescale a cube in astronomical units (degrees
and m/s) such that exported FITS files can be compared directly with model
generated FITS files.

Sky projections do not guarantee flux conservation.

http://www.iinet.com.au/~watson/nngridr.html

http://www.ncarg.ucar.edu//ngmath/natgrid/nnhome.html (based on NNGRIDR)

http://tdc-www.harvard.edu/software/skypic/

http://dipastro.pd.astro.it/~cosmo/Splotch/

src/nbody/imagesnapgrid.c

19-jan-89V1.0: CreatedPJT 12-mar-89V1.1: added emisitivity evarPJT 2-nov-90V2.0: allow stacked snapshotsPJT 21-oct-91V3.0: moment -1,-2 implementedPJT 12-jun-92V3.1: added times=PJT 18-jul-92V3.2: fixed bug when moment<0 and stacked snapshotsPJT 30-jul-93V4.0: allow multiple evar’s - default is now stack=fPJT 18-jun-98V4.4: added xlab/ylab/zlab and allow range[0]>range[1]PJT 8-may-04V5.0: added proj= to optionallaly allow sky projectionsPJT 7-feb-06V5.1: added integrate=t to deal with 3D density pointsPJT 2-mar-11V5.3: moment -3,-4 implementedPJT 18-may-12V5.4: added smoothing in VZ (szvar) 14-feb-13V6.0: units changed on a cube (now xyz-density instead of xy-surface brightness)PJT