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Masking fields by topography

Dear Ferreters,

One of the things I've always had trouble doing with ferret is
masking a field to be plotted as a section (ie, lat or lon vs
depth) by bottom topography. Say I want to plot a section of
sigma-0. I read in my data and generate the plot. However, I
then want to overlay a plot of the bathymetry.

How does one do this? 

It is straight forward to simply draw a line plot of depth vs
latitude say over the sigma-0 field. There is a problem though;
that is, the line plot simply places a line over the field 
without covering up (in a uniform color) the underlying colored
field. In essence if there was a means to do "filled curves"
this would solve the problem. One would say something like:

yes? plot/vs/over/yfilled/nolab/line=1 lat,zb

where the yfilled would instruct to fill the curve down to the
"y-axis" (defined by the grid) in the color specified by "line".
Alternatively, if one could mask the field by the topography
with something like:

yes? let masked_field = if (field_depth lt zb) then field else missing

This would allow one to contour the field with the lower bound
masked off by the topographic height. One could then plot the
topography line to make it stand out. This approach limits the 
topography "color" to be white though which is not entirely 

Any suggestions?


David Oxilia			Tel: (541) 737-2368
COAS:Oregon State University	Fax: (541) 737-2064
Ocean Admin. Bldg. 104		WWW: http://www.oce.orst.edu/po
Corvallis, OR 97331-5503	Email: oxilia@kundun.oce.orst.edu

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