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snapmap - map a snapshot into a 2D image
snapmap in=snapshot
out=image [parameter=value]
snapmap "grids" two arbitrary bodytrans(3NEMO)
expressions (default: x, and y) of a snapshot into a 2D map, using another
bodytrans(3NEMO)
expression (default: m) as the map value. The result is
an image(5NEMO)
, with optional astronomical projection for direct comparision
with true astronomical images. There are a number of ways implemented to
"grid" the particles: mean value, with or without gaussian or cone weighting,
and forms of linear interpolation. These functions are also known as "kriging".
Use snapgrid(1NEMO)
instead to grid datacubes (e.g. X-Y-Z), or if you need
various styles of line of sight integrations. snapmap is more appropriate
to obtain well interpolated maps from irregularly sampled data where the
input mapping I(x,y) is still a smooth function, although good results
can also be ontained by averaging the values (the default mode) or using
a smoothing length svar=.
Parameters
The following parameters are recognized:
- 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. Note that multiple
times and/or multiple evar’s will produce multiple images in this file. [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. Note these
are the outer edges, not their cell centers. [default: -2:2].
- yrange=xb:xe
- Range in yvar to bin [default: -2:2].
- 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].
- evar=e1,e2,...
- Variable(s)
to denote the emissivity per particle. Multiple expressions, separated
by commas, can be given, each of them generating a separate image in a
single multi-image output file. [default: m].
- svar=smoothing
- Variable to denote
gaussian weighting size in X and Y. Note this is the gaussian sigma, not
the FWHM (FMHW = 2.355 * sigma). Only used if mode=mean.
- sfunc=smoothing-function
- The function/method to used when smoothing/interpolating. Options are gaussian
and cone. [Default: gaussian].
- nx=x-pixels
- Number of pixels along the X axis
of the image [default: 64].
- ny=y-pixels
- Number of pixels along the Y axis
of the image [default: 64].
- 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.
- mode=gridding-mode
- mean (can be
optionally with gaussian/cone weighting), linear interpolation [Default:
mean].
- stack=t|f
- Should all snapshots from the input file be stacked, or
write one image per selected (see times=) time? [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.
- emax=
- Normally gaussian smoothing gets cut off when the argument to the
exponent is larger than emax, to prevent roundoff error and/or contributions
from too far away. This equals sqrt(2*emax) times the sigma listed in svar=.
[Default: 10]
Combinations of large snapshots and large images may
run into memory problems since both the snapshot and the image(s) must
fit into memory.
Sky projections do not guarantee flux conservation, interpolation
is done in linear space.
This program was written to create maps
with more accurate interpolated velocity fields from models. However, in
the default mode it used the same mean pixel value as snapgrid(1NEMO)
:
snapmap snap101 ccd101 evar=-vz
snapgrid snap101 ccd102 evar=-vz mean=t
ccdmath ccd101,ccd102 - %1-%2 | ccdstat -
Min=0.000000 Max=0.000000
...
produces the same images ccd101 and ccd102 from the input snapshot snap101
When using multiple evar’s, the multiple images are not in a cube, but
separate images. If you want them in a single image cube, use snapmerge(1NEMO)
.
Here’s a simple example using NEMO’s pipes:
mkspiral - |\
snapmap - - m,x,y,vx,vy svar=0.05 |\
ccdmerge - spiral.ccd
snapgrid(1NEMO)
, snapifu(1NEMO)
, snapsmooth(1NEMO)
, snapccd(1NEMO)
,
ccdintpol(1NEMO)
, ccdmerge(1NEMO)
, ccdsky(1NEMO)
, image(5NEMO)
kiging programs
such as lmtgridder and gbtgridder take single dish pointings and create
a map from them.
Peter Teuben
src/nbody/image snapmap.c
20-jun-09 V1.0: created PJT
6-may-11 V1.3: implemented emax= and sfunc=cone PJT
8-may-11 V2.0: evar=m is now default PJT
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