Re: [ferret_users] Finding the area a certain distance/number of grid cells from a boundary

[Russell Fiedler <russell.fiedler@xxxxxxxx>]

Rob,

I think constructing a mask using the FILL_XY() function is what you want.

You have the array thick

let ocean_mask = if ( thick eq 0 ) then 1
let shelf_mask = if ( thick gt 0 ) then 1
let extended_ocean_mask = fill_xy(ocean_mask,shelf_mask,2)       ! All shelf points within 2 of the ocean are now also filled with 1

let shelf_near_ocean =  if ( thick gt 0 ) then extended_ocean_mask  !  Only want shelf points not ocean

shade thick
shade/ov/pat=tiny_grid/pal=black shelf_near_ocean   ! Check

Multiplying by the area of the cells (1600 in your case) and summing should do the trick

let area = 1600 * shelf_near_ocean[i=@sum,j=@sum]

This could also be done using the @SHF operator but it's rather messy.

Russ



On Sunday 20 February 2011 04:12, Rob Briggs wrote:
> Hello, 
> 
> I have a data set that is ice shelf thickness of Antarctica. I want to
> find the total area of shelf that is bounded between the shelf edge and
> say 100km inland. 
> 
> The elements of the dataset are either: 
> a) some value for iceshelf thickness 
> b) 0 for ocean 
> c) NaN for not ice shelf and not ocean. 
> 
> See attached pdf. Thus there is a way to identify the ice-shelf edge,
> i.e. boundary between ocean and ice shelf. 
> 
> An equivalent example, using a tweaked etopo (except my dataset is on a
> polar stereo grid) would be, find the area of land that <500m elevation
> and is within 100km of the ocean/land interface. 
> 
> yes? use etopo60
> yes? let r1 = if rose gt 0 then rose else 0
> yes? let r2 = if r1 lt 500 then r1
> yes? shade r2
> 
> As my data is on a polar stereo grid, it doesn't even need to be a 100km
> segment; each grid cell is know dimension e.g. 40km x 40km so if I could
> identify the first 2 or 3 grid cells from the shelf edge grid cell that
> would suffice.
> 
> Regards,
> 
> Rob 
> 
> -----------------------------------------------
> Ph.D candidate Glacial Systems Dynamics
> 
> C4043C
> 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/
>