finding contours in a matrix

Hi,
I have matrix of sea bottom depths that I am plotting in R with the function
'image'.
I am particularly interested in the 200m depth contour (I'm using 'distance
from this feature' as a covariate in a model) and would like to extract the
data at evenly 
spaced points along it.  I can easily superimpose a line at 200m using the
function 'contour'.  What I
want to know is: can I actually get the data used to plot this line? If I
just select the relevant data depth[depth==200] I
get a very few observations, but if I say select depth[depth > 180 & depth <
220] I get a "band" of 
unevenly space data along the contour which I don't want either.
Regards,
Doug Beare
Fisheries Research Services,
Marine Laboratory,
Victoria Road,
Torry,
Aberdeen, UK.
Tel. 44 (0) 1224 295314