Paul,
Does mapserver have the capability to do the math in the mapfile
(e.g. via RFC 64)? If not, it's another good reason to load the raster
as a point table in a database. And the font/anno layer method has the
added feature of automatically decluttering the map using the
labelcache. I may have to write a loader script....
Best Regards,
Brent Fraser
On 9/27/2011 12:03 PM, Paul Ramsey wrote:
Once you have vectors, the classic trick I've seen for doing the barbs
is to do math to map the windspeed into an appropriate char() value
and store the barbs as TTF font entries. So you only end up with one
class, and the barb is an appropriately rotated label.
P.
On Tue, Sep 27, 2011 at 10:31 AM, Brent Fraser<[email protected]> 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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