[mapserver-users] Rendering weather data

[Bob Basques]

Could the barb class be built up from primitives, stem anddifferent flags on the tail, based on the numbers passed in.  Basically you would be writing the barb symbol out for each label location needed, instead of sampling from a list of many?  Just use a max/min thresholding to decide when the symbol has something added to it or not.  Still a bit laborious to setup, but I think it gets you there. 

bobb 





>>> Brent Fraser <bfraser at geoanalytic.com> wrote:


     At FOSS4G I was struck by the number of presentations involving
weather data and time series display.  Maybe I noticed because I have a
need to render wind velocity in KML, and while I haven't settled on
mapserver to do the rendering I've been wondering how to render wind
barb symbols (see attached graphics) efficiently.

   One problem is the source data: the magnitude is in one raster file
(GRIB format, float64)  and the direction in another.  GDAL's OGR
library will read the GRIB files, but I don't know of any way to use two
rasters to define a vector layer.  I think there is some work going on
with respect to displaying vector field data in mapserver, so maybe this
will be solved by that work.  Otherwise pre-processing the two rasters
into a point datasource containing lon,lat,speed,direction attributes
may be the way to go.

   The other issue of wind barb symbol rendering may have to be solved
by hand constructing a barb for 0 knots, 5 knots, etc (50-ish symbols?)
, and having 50-ish corresponding classes (this could be a performance
problem for mapserver).  But I'm open to better ideas...  SLD?  HTTP/CGI
request?, something like one class of:

   CLASS
     STYLE
       GEOMTRANSFORM "start"
       COLOR 0 255 0
       SIZE 15.0
       SYMBOL "http://localhost/symbolgen?[speed]"
       ANGLE [direction]
     END
   END

   but likely the cost of getting the symbol would be greater that
creating 50 separate class objects.

And for those interested in mapping and graphing weather data, have a
look at http://www.ncl.ucar.edu/Applications/
especially http://www.ncl.ucar.edu/Applications/barb.shtml

--
Best Regards,
Brent Fraser

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