Re: [ferret_users] Plotting Satellite Wind Vectors

["E. D. Cokelet" <Edward.D.Cokelet@xxxxxxxx> (Ned)]

Dear Steve,

I was never happy with the way plot_vectors.jnl worked, but it should not be as bad as what your graphic shows.  However on map projections it is just plain wrong.  Try it near the poles on a polar projection and you'll see.   So I wrote the mp_poly_vectors.jnl script a couple of years ago.  Ansley generalized it a bit, and it's available as a public Ferret script.  Run the poly_vec_demo.jnl script to see it in action.  Use mp_poly_vectors.jnl for map-projected vectors.  

P.S.  I would caution you though, that there are still some problems.  The way most of the Ferret mp_xxx commands (e.g. mp_line.jnl)  are written, they redefine the mapping between (lat,lon) values in one's present data set, and the x_page, y_page parameters that define the mapping from (lat,lon) to the page for each and every new data set you plot.  That's not desirable.  If you draw a large map of coastlines, say, over a large region and then want to add data to it that are defined by its axes in a much smaller region, the mapping is redefined through Ferret's delayed evaluation philosophy.  That's not correct.  What you really want is to set up that mapping once and for all for your entire plot, say when you plot the coastline, and then leave it unchanged when you add other things like labels, graticules, vectors, etc.  I get around this by defining my own mp_line.jnl, etc. scripts that have the 

let/quiet mp_central_meridian = `mp_central_meridian`
let/quiet mp_standard_parallel = `mp_standard_parallel`

lines commented out.  So what you want is mp_central_meridian and mp_standard_parallel defined once for the projection you desire, and never redefined by some other mp_xxx command.  

------------------------------------------------------------------------------------------
Edward D. (Ned) Cokelet, Ph.D.	Edward.D.Cokelet@noaa.gov
Oceanographer
NOAA/PMEL					off:  (206) 526-6820
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