[ferret_users] Color-filled vector script

["William S. Kessler" <William.S.Kessler@xxxxxxxx>]

Dear Ferreters -

Several versions of scripts to plot color-filled vectors have turned  
up on this list over the years, written originally by Ned Cokelet and  
modified by myself. This message gives yet another one, but I think  
this one is really the best way to do it. I have attached an example  
plot showing velocity and salinity on an isopycnal in the western  
equatorial Pacific from an OGCM. The advantage of the script is in  
visualizing multiple variables in a single plot, and seeing the  
relationship among them. The vectors are scaled so the area of the  
vector (not the length) scales by its magnitude, which I think is  
appropriate for such filled vectors.

Billy K

There are comments at the top of the  attached script  
(poly_vectors_scaled_arrows_area.jnl), but it might be useful to see  
the commands used to make the above plot.

There were 7 steps to using the script, shown schematically as follows:

 1) find u, v, and salt on isopycnals
define axis/z=23:28:.5 axden
let u_on_sigma=zaxreplace(u,sigma,z[gz=axden])
let v_on_sigma=zaxreplace(v,sigma,z[gz=axden])
let s_on_sigma=zaxreplace(salt,sigma,z[gz=axden])

 2) draw a basemap (essential; the vector script must be an overlay)
     In this step it is also essential to use the correct aspect  
ratio for the plot,
     or else the vectors will not be correctly oriented
set win/asp=.45     ! for 32 deg lon by 10 deg lat
shade/set/nolab/nokey/x=128e:160e/y=8s:2n/hli=128e:160e:5/vli=8s:2n:2/ 
lev=(0,10000,10000,-3)/pal=gray rose[d=etopo02]
ppl yfor,(i5,''lat'')
ppl shade

 3) define the positions (x_vec,y_vec) and values (u_comp,v_comp) of  
the vectors
     note that x,y_vec are defined to include values on the complete  
(x,y) grid: x+0*y, 0*x+y)
let x_vec = x[gx=u_on_sigma,x=128e:160e] + 0*y[gy=u_on_sigma,y=8s:2n]
let y_vec = 0*x[gx=u_on_sigma,x=128e:160e] + y[gy=u_on_sigma,y=8s:2n]
let u_comp = u_on_sigma[z=25,x=128e:160e,y=8s:2n]
let v_comp = v_on_sigma[z=25,x=128e:160e,y=8s:2n]

 4)  call the filled vector script (choosing scale length 100)
go poly_vectors_scaled_arrows_area.jnl x_vec y_vec u_comp v_comp 100  
"arrow"

5) define the value of the variable to be filled (using ysequence to  
make it a list along the y-axis)
let saltseq = ysequence(s_on_sigma[z=25,x=128e:160e,y=8s:2n])

6) use POLYGON to actually draw the vectors
    (plt_x_arrow, plt_y_arrow are the outputs of the filled vector  
script in step 4)
polygon/over/nolab/key/lev=(34.5,35.7,.1,1)/pal=default/line  
plt_x_arrow, plt_y_arrow, saltseq

7) make a scale vector using the same script and call to POLYGON
    in this case there is only a single value for each and they are  
just written out using { } notation
let x_vec={141}; let y_vec={-5}; let u_comp={100}; let v_comp={0};  
let mag={1}
go poly_vectors_scaled_arrows_area.jnl x_vec y_vec u_comp v_comp 100  
"arrow"
polygon/over/nolab/nokey/lev=(0,1000,1000)/pal=red/LINE plt_x_arrow,  
plt_y_arrow, mag
label 140 -6.5 -1 0 .18 @dr100 cm s^-^1

Attachment: poly_vectors_scaled_arrows_area.jnl
Description: Binary data