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Name

velfitpatch - fit local linear velocity field

Synopsis

velfitpatch [parameter=value]

Description

velfitpatch fits a local linear velocity field (v = vc + ox * x + oy * y) to a small square region around each point in a velocity field. Output can be specified to be in linear gradients (ox,oy) or a radial,position angle (om,pa)

Parameters

The following parameters are recognized in any order if the keyword is also given:
in=
input velocity field. No default.
out=
basename for 3 output files (vc,ox,oy) or (vc,om,pa). See ode.
patch=
Patch (size of box will be 2*patch+1) [3]
scale=
Scale factor for gradients [1]
blank=
Blank Value in velocity field [0.0]
mode=
Output mode (xy derivates, vs. gp (gradient/positionangle) [xy]
tab=
If given, tabular output file name []

Examples

Here is an example of following the velocities along a set of (x,y) positions in a map: 
    velfitpatch in=mom1 out=fit1 mode=rt
    ccdtrace fit1.vc fit1.vctab xytrace.tab wcs=f cumul=t
    ccdtrace fit1.om fit1.omtab xytrace.tab
    ccdtrace fit1.pa fit1.patab xytrace.tab
    paste fit1.vc fit1.om fit1.pa | awk ’{print $4,$3,$7,$10}’ > fit1.vop
resulting in a file fit1.vop, with columns L,Vc,Omega,P.A, where L is the length along the trace. Watch out for units. See ccdtrace(1NEMO) for more info on units.

If a curve needs to be spline interpolated, one method can be to spline smooth the X and Y independantly: 

  ccdtrace fit-a.vc - curve.tab wcs=f cumul=t 
  # note it goes from 0 to 160 pixels (for example)
  # we are then resample along the curve from 0 to 160 pixels in steps
of 5 pixels
  ccdtrace fit-a.vc - curve.tab wcs=f cumul=t | tabspline - 4 1 0:160:5 | tabmath
- - delcol=1 > x.tab
  ccdtrace fit-a.vc - curve.tab wcs=f cumul=t | tabspline - 4 2 0:160:5 | tabmath
- - delcol=1 > y.tab
  paste x.tab y.tab > curve-xy.tab
  # now we have a smoother curve-xy.tab table for input to

See Also

rotcur(1NEMO) , ccdtrace(1NEMO) , image(5NEMO)

Files

Author

Peter Teuben

Update History


26-Jul-15    V0.1 Created    PJT

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