topographic area calculations

[DON EBERT]

If you overlay the vector boundary line on the raster polygon, you'll see
that the proportion of the boundary cells that are on the inside and
outside of the vector boundary varies from cell to cell. By dividing the
area of the boundary cells by half, you are saying that, on average, half
of the cell is inside the vector boundary and half is on the outside. 

Another note about area calculations. Beware of small cell sizes. When
GRASS truncates r.mapcalc's area calculations to integer values, you can
lose a lot of area over a large polygon. You might consider exporting out
the percent slope values for each cell instead of its area value and do
the area calculations yourself using a spreadsheet or database. 

Don Ebert
Research Associate
National Biological Service
Cooperative Research Unit
University of Nevada - Las Vegas 

On Wed, 3 Apr 1996, Harini Nagendra wrote:

> This is really useful - I have been looking for a way of doing this for 
> quite some time. Just wanted one clarification however - why do you 
> divide the area occupied by the boundary cells by half?
> 
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