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I cannot speak for VisAD, however in McIDAS the algorithm for drawing
streamlines is the one described in: Monthly Weather Review: Vol.
105, No. 6, pp. 786–788.
The "density" value is used to determine in what grid "boxes"
streamlines can start. Highest density says that a streamline will
start in every grid box and be drawn to its termination (rules for
stopping drawing are described in the paper). Streamlines are always
started in the center of a grid box, thus no more than one will ever
be started in a box -- that does not, of course, prevent more than one
streamline from passing through a grid box.
Hope that helps.
tom
On 3/29/06, James Kelly <J.Kelly@xxxxxxxxxx> wrote:
> Hi,
>
> I've been looking at the quality of streamline analyses (of atmospheric
> wind fields) lately.
>
> I was experimenting with streamline displays in the McIDAS system, and
> thought (IMHO) that the streamline analyses looked better as the density
> of the streamlines increased.
>
> I then tried to increase the density of the streamlines in a visad
> display, and but wasn't able to generate the same density as the McIDAS
> display.
> Should I be able to do this? If so how?
>
> Tried changing "streamlineDensity" from the default of 1.0 to higher
> values of 10.0, 100.0, 1000.0 and 1000.0, but didn't notice much change
> beyond 100.0.
>
> Further details:
> 1) In McIDAS I did this:
> GRDDISP <modelname> FHOUR=0 PARAM=STREAML LEV=SFC LAT=-20 20 LON=-160
> -100 CINT=0 COLOR=15
>
> Image is attached as "mcidas_streamline.png"
>
> 2) In Visad/jython I did this:
>
> maps = (xMap, yMap, rngUMap, rngVMap)
>
> disp = subs.makeDisplay(maps)
> flowcontrol = rngUMap.getControl()
> flowcontrol.enableStreamlines(TRUE)
> strmDensity = flowcontrol.getStreamlineDensity()
> stepFactor = flowcontrol.getStepFactor()
> flowcontrol.setStreamlineDensity(10000.0)
> print " streamline density " , strmDensity, " stepFactor ", stepFactor
>
> drLAPSuv=subs.addData("windbarbs", uv, disp)
>
> Image is attached as "visad_streamline.png"
>
> Cheers,
>
> James
>
>
> --
> James Kelly
> Australian Bureau of Meteorology
>
>
>
>
>
--
Tom Whittaker
University of Wisconsin-Madison
SSEC/CIMSS
1225 W. Dayton Street
Madison, WI 53706 USA
ph: 608-262-2759
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