Identifying phenomenon trends

[Steve Franklin]

The field with which we are using GRASS is to analyze and digitize
coastlines and then measure the amount of coastal erosion we can 
recognize from year to year (we were fortunate enough to acquire 
air photos of this region spanning over 25 years, in a rapidly changing
coastline, with rates of recession exceeding 2-4 meters in spots).
   In any case, the digitization of the coastline can be a tedious process,
which to me seems like it could be automated, through image enhancement
to pick out the cliff face of the shoreline. The shoreline is reasonably
pronounced by dark hues on one side, and the other side is complete beach
(light hues). This differentiation between the two contrasts should be easily
distinguishable to software packages equipped to handle it.
   I am hoping that any of the grass users have found a way to achieve this,
perhaps through GRASS, or a secondary software package. I would be eager
and appreciative to hear any suggestions you might have on the matter.
All answers can be mailed to : 
     franklin at ug.cs.dal.ca
   thank you in advance for your time.

Steve

p.s: the image I"m dealing with is much like the following:

XXXXXXXXXXXXXXXXiiiii
XXXXXXXXXXXXXiiiiiiii
XXXXXXXXXXXXXXiiiiiii
XXXXXXXXXXXXXXXiiiiiii
XXXXXXXXXXXXXXXiiiiiii
XXXXXXXXXXXXXXiiiiiiiii
XXXXXXXXXXXXiiiiiiiiiii
XXXXXXXXXXXXiiiiiiiiii
XXXXXXXXXXXXiiiiiiiiiiii
XXXXXXXXXXXXXiiiiiiiii
XXXXXXXXXXXXXXiiiiiiiii
XXXXXXXXXXXXXXXiiiiiiii
              ^
(dark tundra) | (sandy beaches) 
              |    
              ---coastline

Other regions are distinctly light hued on either side of the coastline. 

I humbly ask for your forgiveness if I have approached the forum with an
inappropriate question. However, I would like to avoid re-inventing the
wheel, and am hoping someone has worked in a similar direction as that which
I have depicted.