I guess I sent a copy of this to Rich, but forgot to CC the list
Roger Miller
"Roger S. Miller" wrote:
Rich Shepard wrote:
> While I'm on the topic of terrain analysis, let me ask this of the
> community: why does it appear that so many modules overlap in function? For
> example, r.slope.aspect, r.drain, r.flow, r.watershed (and others, most
> likely) all have what appears to me to be either the same functionality (in
> part), or they are so complementary that they should be aggregated into a
> single module.I wasn't around GRASS at the time the programs were written, so I'm
probably not the best person to answer this. But I will anyway. The
various modules originated at different times and some from different
authors. I suspect that they have never been integrated into a single
module mostly because noone has ever simultaneously had both the need
and the time to integrate them.
Hello all,
I think some of these modules (r.watershed, r.fill.dir, r.drain) use older
methods of
flowtracing based on so-called D8 algorithm, i.e. use only limited 8 directions
of
flow with zig-zag flow pattern. Also, the flowaccumulation is computed using a
single
flow direction (flow is assigned only to 1 downhill cell).
The second group of flowrouting modules contains r.flow and r.flowmd. They use a
D-infinity algorithm, i.e. consider all possible flow directions and therefore
produce
smooth flowlines.
The flowaccumulation maps are produced using a single flow direction model in
r.flow
and multiple flow direction algorithm in r.flowmd.
Helena sent me a very interesting link to TerraFlow project at Duke University
(http://www.cs.duke.edu/geo*/terraflow/). Those interested in this topic might
find a
lot of useful information here including comparisons with r.watershed and
ArcInfo.
Jaro
On Thu, 19 Jul 2001, Jaro Hofierka wrote:
I think some of these modules (r.watershed, r.fill.dir, r.drain) use older
methods of flowtracing based on so-called D8 algorithm, i.e. use only
limited 8 directions of flow with zig-zag flow pattern. Also, the
flowaccumulation is computed using a single flow direction (flow is
assigned only to 1 downhill cell).The second group of flowrouting modules contains r.flow and r.flowmd. They
use a D-infinity algorithm, i.e. consider all possible flow directions and
therefore produce smooth flowlines. The flowaccumulation maps are produced
using a single flow direction model in r.flow and multiple flow direction
algorithm in r.flowmd.Helena sent me a very interesting link to TerraFlow project at Duke
University (http://www.cs.duke.edu/geo*/terraflow/). Those interested in
this topic might find a lot of useful information here including
comparisons with r.watershed and ArcInfo.
Jaro,
Thank you for the insight. I suspected something like this was the case.
What I've learned so far from the terrain analysis book is that the D8
(discrete, 8 adjacent cells) algorithm is not nearly as representative as
others -- particularly DEMON. The book cite's Helena's efforts at
implementing this method in one of the GRASS modules (I forget which one it
is right now).
I greatly appreciate this information, and I need to look into the modules
more closely so that I can understand and document just how GRASS is doing
its thing.
Rich
Dr. Richard B. Shepard, President
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