Yes, there are some differences. Please look at where these differences
are located and if they are significantly changing the results. In flat
areas, there will be several possibilities about how water could flow.
This might also affect the flowpath further upstream. Both the original
version and the fast version have a certain degree of randomness on how
to find a path through flat terrain, but it's a different type
randomness, so to speak. The fast version can not exactly replicate the
results, but it is pretty close. If there are only slight shifts in the
flowpath and these differences can be justified by comparing them to the
input DEM, the results could be useful. I think of particular interest
are larger differences in flow accumulation, because they have a big
influence on other output like basins and flow direction.
Thanks for testing!
G. Allegri wrote:
A correction: as you can see from the stats, the differences are not
only (1,-1,7) as I was saying... Now I have to check how much these
differences affect the results. Anyway, above 99.5 % of values are
equal.
2008/7/29 G. Allegri <[EMAIL PROTECTED]>:
Ivan anticipated me for a bit.
GREAT Markus! I could crunch Sardinia in one shot. I haven't measured
the time but it was less then 2 minutes, while with r.watershed I got
stalled.
Analyzing the differences between the r.watershed and r.watershed.fast
for a narrower region, there was some subtle, sparse differences
(1,-1,7) meaning 45° differences:
The following are the differences values (from r.mapcalc) statistics
(number of pixels for values)
-7 76
-6 34
-5 30
-4 49
-3 43
-2 194
-1 1165
0 813894
1 1218
2 253
3 83
4 45
5 25
6 93
7 250
8 22
9 21
10 5
11 15
12 30
13 18
14 3
15 5
16 4
* 36917172
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