[GRASS-user] r.watershed speed-up
giohappy at gmail.com
Fri Aug 1 05:54:26 EDT 2008
With the "fast" drainage versions I could reproduce water outlet
basins with unsignificant differences. Apart the time saving :-)
2008/8/1 G. Allegri <giohappy at gmail.com>:
> Great. Things get better and faster!
> I've tried on a not so big region. But it was enough:
> 989861 cells (172286 null cells under MASK, where I have the see)
> It has taken about 55 seconds (I don't have time to set up a
> profiling), where I asked for drain and visual outputs creation.
> The stats are (calculated in OO):
> DIFF-VALUES N.PIXELS PERCENTAGE
> -7 56 0,0068
> -6 30 0,0037
> -5 32 0,0039
> -4 49 0,0060
> -3 38 0,0046
> -2 180 0,0220
> -1 914 0,1118
> 0 814650 99,6422
> 1 910 0,1113
> 2 229 0,0280
> 3 79 0,0097
> 4 42 0,0051
> 5 21 0,0026
> 6 70 0,0086
> 7 152 0,0186
> 8 22 0,0027
> 9 21 0,0026
> 10 5 0,0006
> 11 15 0,0018
> 12 30 0,0037
> 13 18 0,0022
> 14 3 0,0004
> 15 5 0,0006
> 16 4 0,0005
> Things has run better then with the previous version, as >99.6% of
> values are equal to r.watershed.
> Thanks, a lot, for r.watershed.fast! :-)
> 2008/8/1 Markus Metz <markus_metz at gmx.de>:
>> Hello list,
>> there is now a new version of r.watershed.fast where results are even more
>> similar to r.watershed. They are still not 100% identical to r.watershed,
>> but I can't get it more similar. But it comes with a further speed increase.
>> I repeated the test of Moritz with the same commands on GRASS 6.4.svn,
>> results are below.
>> Moritz Lennert wrote:
>>> First test in North Carolina demo data set:
>>> g.region rast=elevation
>>> time r.watershed elevation=elevation at PERMANENT accumulation=old_accum
>>> drainage=old_dir basin=old_sheds stream=old_streams thresh=500
>>> real 19m2.744s
>>> user 18m41.318s
>>> sys 0m1.884s
>>> time r.watershed.fast elevation=elevation at PERMANENT
>>> accumulation=fast_accum drainage=fast_dir basin=fast_sheds
>>> stream=fast_streams thresh=500
>>> real 0m18.034s
>>> user 0m17.833s
>>> sys 0m0.196s
>> Absolute times are not really comparable between systems, but relative
>> differences in time should be similar. The following numbers are calculated
>> with real time. In the test Moritz did, r.watershed took 63x as long as
>> r.watershed.fast, i.e. r.watershed.fast needed only 1.6% of the time of
>> New version: r.watershed took 127x as long as r.watershed.fast, i.e.
>> r.watershed.fast needed only 0.8% of the time of r.watershed.
>>> Of the 2025000 cells in the map, 1991218 show the same direction, i.e.
>>> 98%. Those which have different directions are overwhelmingly low slope
>> New version: 2004480 cells, i.e. 99% of all cells show the same flow
>>> 1833907 cells have the same accumulation value, i.e. 90%, but I guess this
>>> is to be expected.
>> New version: 1921510 cells, i.e. 95% of all cells show the same accumulation
>> The idea is that a faster r.watershed can also be used for massive grids,
>> where GRASS users frequently gave up using r.watershed because it would have
>> taken hours or even days. I resampled "elevation" in the North Carolina demo
>> data set from 10m to 3m with r.resamp.rst using default values (after the
>> GRASS book Section 5.3.3, paragraph "Regularized spline with tension (RST)
>> interpolation") to generate a fairly large map and ran the same test on the
>> resampled map.
>> cells in region : 22,500,000
>> The results:
>> r.watershed took 5459x as long as r.watershed.fast, i.e. r.watershed.fast
>> needed only 0.02% of the time of r.watershed (here 10h2m55s vs. 1m7s, 10
>> hours versus 1 minute...).
>> Flow direction differences:
>> 22288539 cells, i.e. 99% of all cells show the same flow direction.
>> Flow accumulation differences:
>> 20963653 cells, i.e. 93% of all cells show the same accumulation value.
>> Memory usage of r.watershed and r.watershed.fast: maximum of about 940MB
>> I don't understand why memory usage increases after <SECTION 1a: Initiating
>> Memory> is completed.
>> Assuming that there is no longer a time constraint but only a memory
>> constraint (although <SECTION 4: Watershed Determination> can take some time
>> on large maps with a large threshold value), the upper region sizes that
>> r.watershed.fast can process in RAM would be *roughly* for
>> 1GB RAM: 14,000,000 cells
>> 2GB RAM: 38,000,000 cells
>> 4GB RAM: 86,000,000 cells
>> 8GB RAM: 181,000,000 cells
>> after putting 400MB aside for the system and other open applications.
>> Estimate based on Linux 64bit.
>> If you want to repeat and analyse the tests with the North Carolina demo
>> data set, the new r.watershed.fast is here
>> and the test script is below.
>> test script:
>> g.region rast=elevation
>> time r.watershed elevation=elevation at PERMANENT accumulation=nc_accum_old
>> drainage=nc_dir_old basin=nc_sheds_old stream=nc_streams_old thresh=500
>> time r.watershed.fast elevation=elevation at PERMANENT
>> accumulation=nc_accum_fast drainage=nc_dir_fast basin=nc_sheds_fast
>> stream=nc_streams_fast thresh=500
>> r.mapcalc nc_dir_dif='if(("nc_dir_old" - "nc_dir_fast" != 0),1,0)'
>> r.mapcalc nc_accum_dif='if(("nc_accum_old" - "nc_accum_fast" != 0),1,0)'
>> r.stats -c input=nc_dir_dif at PERMANENT
>> r.stats -c input=nc_accum_dif at PERMANENT
>> r.resamp.rst input=elevation at PERMANENT ew_res=3 ns_res=3 elev=elevation_rst
>> overlap=3 zmult=1.0 tension=40.
>> g.region rast=elevation_rst
>> time r.watershed elevation=elevation_rst at PERMANENT
>> accumulation=nc_rst_accum_old drainage=nc_rst_dir_old basin=nc_rst_sheds_old
>> stream=nc_rst_streams_old thresh=500
>> time r.watershed.fast elevation=elevation_rst at PERMANENT
>> accumulation=nc_rst_accum_fast drainage=nc_rst_dir_fast
>> basin=nc_rst_sheds_fast stream=nc_rst_streams_fast thresh=500
>> r.mapcalc nc_rst_dir_dif='if(("nc_rst_dir_old" - "nc_rst_dir_fast" !=
>> r.mapcalc nc_rst_accum_dif='if(("nc_rst_accum_old" - "nc_rst_accum_fast" !=
>> r.stats -c input=nc_rst_dir_dif at PERMANENT
>> r.stats -c input=nc_rst_accum_dif at PERMANENT
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