"Extracting a 2D surface from a 3D field" solved!

[Gerard Eldin <Gerard.Eldin@xxxxxxx>]

An answer to my help request (see below) was provided by Steve Hankin (as
often!). Here it is, as it may be of interest to others:

The trick is to build a 3D field from the 2D field of depths, using the 3D
Z coordinate matrix:

let Z0 = Z[g=temp] - my_2D_field_of_Z_values

This field will cross zero wherever the Z axis matches the z(x,y) value
of the 2D field.

Then the @WEQ transformation can be used to get an integration kernel to
retrieve temperatures on the 2D surface:

	temp_kernel = temp * z0[z=@weq:0]
	temp_on_2D_surf = temp_kernel[z=@sum]

Here's my original question:
> 
> I have a 3D (x,y,z) field of oceanic temperatures, temp. I want to select
> values of temp on some surface, which is neither a constant depth nor an
> isotherm: that surface is a  2D (x,y) array of depth values, independant
> from temp. I simply can't find a way to do that.
> 
>


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