[GRASS-user] r.cva -f option: "calculate the visibility from rather than viewsheds of"; English translation requested ; -)

[Mark Lake]

Gary,
>
> Would someone elaborate on the difference between "visibility from"  
> and "viewshed of".

As the original author of r.cva I have to admit that the choice of  
terminology could perhaps have been clearer, but this choice is  
explained in the man page if you have that on your system.  Also  
check  the info. at http://www.ucl.ac.uk/~tcrnmar/GIS/r.cva.html

Normally, i.e. without choosing the -f flag / NOT checking the  
"Calculate visibility from ..." box, r.cva produces output in which  
each non-NULL cell represents a viewpoint and contains a count of the  
number of cells visible from that viewpoint.  Thus if you specify  
just one viewpoint, you should get output containing one cell  
effectively recording the size (in number of cells) of the viewshed  
of that viewpoint.

If you do set the -f flag / check the "Calcuclate visibility  
from ..." box, then r.cva produces rather different output in which  
each non-NULL cell represents a cell which is visible from one or  
more viewpoints and contains a count of the number of viewpoints from  
which it is visible.  Thus if you specify just one viewpoint, you  
should get output that is essentially the same as converting the  
output from r.los into binary form (i.e. you get a traditional  
viewshed map showing what area is visible from the selected viewpoint).

You can further change exactly what is being calculated by reversing  
the observer and target offsets.  This is because the fact that you  
can see A from B does not guarantee that you can see B from A, unless  
the height of the observer is equal to any target offset that may  
have been set.

These issues have been widely discussed in the archaeological  
literature.  If you have access to a library you should look at:

Conolly, J. and Lake, M. (2006).
           Geographical Information Systems in Archaeology.
           Cambridge Manuals in Archaeology. Cambridge University Press,
           Cambridge.
           Pages 225-233

Wheatley, D. and Gillings, M. (2002).
           Spatial Technology and Archaeology: The Archaeological
           Applications of GIS.
           Taylor & Francis, New York.
           Pages 201-214

Directly related methodological issues are also discussed in:

Fisher, P. F., Farrelly, C., Maddocks, A., and Ruggles, C. (1997).
           Spatial analysis of visible areas from the Bronze Age  
cairns of
           Mull.
           Journal of Archaeological Science, 24:581-592.

Lake, M. W., Woodman, P. E., and Mithen, S. J. (1998).
           Tailoring GIS software for archaeological applications: An
           example concerning viewshed analysis.
           Journal of Archaeological Science, 25:27-38.

I'm out of contact now for 10 days, so won't be able to answer  
further questions until after that.

Mark

--
Dr Mark Lake

Institute of Archaeology
University College London
31-34 Gordon Square
London.  WC1H 0PY

Tel: +44 (0)207 679 7495
Fax: +44 (0)207 383 2572