DEMs

[Malcolm Williamson]

On Tue, 9 Dec 1997, Johnny Duke wrote:

> I would like to know if GRASS uses the inverse linear 
> algorithm for building DEMS from contours.
> 
> 
> Johnny Duke
> USDA/NRCS
> GIS TECH
> jduke at ar.nrcs.usda.gov
> 
Hi, Johnny
Looking at the r.surf.contour manual page, I would interpret it to be an 
inverse linear interpolation based purely on the distances to the nearest 
occurences of uphill and downhill contour lines. The paragraph below is 
what I'm basing my interpretation on.

    "r.surf.contour works in a similar way.  Initially, a vector
     map of the contour lines is made with the elevation of each
     line as its label (see v.digit).  When the program v.to.rast
     is run on the vector map, continuous "lines" of rasters
     containing the contour line values will be the input for
     r.surf.contour.  For each cell in the input map, either the
     cell is a contour line cell (which is given that value), or
     a flood fill is generated from that spot until the fill
     comes to two unique values.  The flood fill is not allowed
     to cross over the rasterized contour lines, thus ensuring
     that an uphill and downhill contour value will be the two
     values chosen. r.surf.contour interpolates from the uphill
     and downhill values by the true distance."

Regards,
	-Malcolm Williamson