[GRASSLIST:590] Understanding why r.flow gives me bad results.

Tue Jul 1 11:55:59 EDT 2003

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Hi.

I'm trying to compute a flowline path with r.flow so as to create a
vector drainage pattern. However I'm not getting good results as r.flow
is giving me a vector file in which the the flowline are very short and
do not connect in the valleys. The valleys actually seem to have no
flowlines while only very steep slopes do have flowlines. 
My DEM does have flat areas (r.stats reports 500.000/2.800.000 zero 
degree cells) so I presume it's this that stops the flowlines.

I must say though, that my DEM was built using only contour lines and no
quoted points. I presume this is the cause of my 500.000/2.800.000 zero
cells .....?

I built my DEM using r.surf.contour on a 7,5 meter square cell
resolution for an area of 11,5 Km x 14 Km (total of 1800x1550 celled
grid). Do you think my resolution is too low to get a valid flowline
raster map? I read in the r.flow man page that centimeter resolution
should be used. But if I use a centimeter resolution my PC would take
1000 light years to compute my DEM .... :-)
I'm sure I missing something but I'm coming up to Gis technologies only
recently so ...... not good at it yet.

Bye

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