Follow up: [R] surfaces and digital terrain model

Renaud Lancelot lancelot at
Fri May 30 22:36:15 CEST 2003

Yesterday, I posted the following:

>>I have computed a digital terrain model from a set of points (x, y, z)
>>using the function interp() in package akima. I want to predict flooded
>>surfaces given target values of z. I can display the flooded surfaces
>>with contour() or image(), but I don't know how to get the polygons
>>delimiting the surfaces. Did anyone write a function for this purpose ?

Many thanks to Roger Bivand, Paul Murrel, Deepayan Sarkar, Barry 
Rowlingson and Thomas W Blackwell for their replies and their help. Paul 
Murrel provided me with a function "clines", kindly ported to Windows by 
Duncan Murdoch. This function does exactly what I need, i.e. it returns 
a list of polygons corresponding to target value(s) of z.

I wrote a function to compute (hopefully !) what I want, i.e. predicted 
flooded surfaces given target values of z (managing the cases of several 
independent watered surfaces, possibly with islands). Provided that Paul 
Murrel agrees to share his function, I will be happy to send it to 
anyone wishing to use and improve it (and debug it ;-) ).

Best regards and thanks again,


Dr Renaud Lancelot, vétérinaire
CIRAD, Département Elevage et Médecine Vétérinaire (CIRAD-Emvt)
Programme Productions Animales

ISRA-LNERV                      tel    +221 832 49 02
BP 2057 Dakar-Hann              fax    +221 821 18 79 (CIRAD)
Senegal                         e-mail renaud.lancelot at

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