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[ferret_users] Plotting a (irregular) transect of steepest descent


I'd like to plot a transect that follows, for example, the flowline of
the steepest descent of elevation. I was looking at the demos for using
DDC and flowline, but they didn't really get me to my objective. All I
can think of would be something as follows, which would be most unferret
like, and I'm sure there might be an easier way .

For example, looking at Greenland 

use etopo20
set region/X=80W:10W/Y=57:85
LET r1 if ROSE lt 0 then 0 else ROSE
shade R1; go land

! get array of diffs
LET xdiff = (-1)*R1[X=@DDC] ! negative so I can visualise downwards
LET ydiff = (-1)*R1[Y=@DDC]

! plot array of vectors 
VECTOR/over xdiff,ydiff

I'd then like to pick a point, say max elevation, or a certain lat/lon
and get the elevation transect down the path of steepest descent.

I could so as follows:  
for points around point of interest 
	find the max vector around this point
	record lat/lon/elevation of the max vector point
	jump to that point and set it as teh point of interest
repeat loop until no more vectors to check 

load and plot file of elevation points 

But I don't know how to get to the vectors as data, not as a plot.  

Further question; By creating the transect I will have lost my regular
grid, how could I plot the data with the lat and lon labels at each
index point (or every 2 or 4 or 5)?


Rob Briggs

Department of Physics and Physical Oceanography
Memorial University of Newfoundland, St. John's
NL, A1B 3X7, Canada

Phone   : 001 (709) 864-2407
Email   : rdbriggs@xxxxxx
Website : http://www.physics.mun.ca/~rdbriggs/

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