Hi Leela,
This is not necessarily a better solution to your problem than
use of
transformations like @loc or @weq, but it illustrates how using
Ferret as a
digitizer, coupled with a Great Circle Distance calculator can be
useful in a
variety of situations. Apologies if this is already known to you
but it may
serve a wider audience.
First: Check that Ferret's "where" command is working for you:
go ptest
where
When you move the cursor over the plot and click. Ferret should
report the
"mouse" coordinates in user units. This is the key to digitizing
and a
variety of other tasks one could imagine.
Next: Add the attached scripts to your directories (I would
typically put the
.jnl ones in my "ferret" directory and the shell script
get_vertices [with
chmod +x get_vertices] in "bin")
Demo: Here is an illustration. What is the length of the 25C
isotherm of
Levitus SST between Australia and South America?
use levitus_climatology ; let sst=temp[k=1]
fill/x=140:310/y=-30:10 sst ; cont/o/lev=(25)/x=140:310/y=-30:10
sst
go polydef ! prompts user to click on desired points,
terminating by
! clicking offchart to the left. When this is
done
the coords of
! the point are loaded as a file with
variables VX,VY
sho data
2> ./vertices.xy (default)
name title I J
K L
VX VX ... ...
1:76 ...
VY VY ... ...
1:76 ...
! As seen in the attached graphic I took 76 points to delineate the
isotherm.
! In this case, where VX,VY are lon,lat values, what is needed is
to sum the
great
! circle distances along the curve. One way to do this is as follows:
go greatcircle ! definitions for great circle calculations
let lon1=vx ; let lat1=vy ; let lon2=vx[k=@shf] ; let
lat2=vy[k=@shf]
list gckm[k=1:75@sum] ! sum up the GC contributions, giving the
answer
(in kilometers)
18033.
Note that you sum over one less than the number of vertices so as
not to run
off the end. I did the above twice, because I forgot to save the
plot, and
the first time got 18042. This suggests the method is good enough
for a
certain class of problem. Perhaps a better demo would have been to
use the
hints in "greatcircle.jnl" to plot a portion of a great circle,
then digitize
it and compare the result with the known answer, but this one at
least
illustrates how it can deal with complex paths.
Hope it is of use,
Mick
----------------
On 5/19/11 11:06 AM, Leela Frankcombe wrote:
Dear Ferreters,
I've come across a question which is simple in theory but in
practice I'm
having a little trouble. So I'm wondering if anyone else has found a
solution.
What I would like to do is calculate the path length of an ocean
current.
To define the path of the current I pick a particular sea surface
height
contour, now I would like to be able to calculate the length of that
contour. I've been trying to use @loc or @weq to select the points
at which
SSH reaches the value that I've chosen but they only find the first
instance along a given latitude or longitude so they miss parts
where the
current meanders (and also sometimes pick up eddies). Is there a
way to use
@loc or @weq to find every instance of a particular value? Or has
someone
got a better solution?
Thanks!
Leela.
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Leela Frankcombe
Post-doctoral researcher
Institute for Marine and Atmospheric research Utrecht
Utrecht University
The Netherlands
www.phys.uu.nl/~frankcmb
l.m.frankcombe@xxxxx
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