[GRASS-user] r.walk usage

[Joshua Quesenberry]

Good Morning Everyone,

 

I'm looking for some help with the r.walk tool. I'm finding some tutorials
and man pages with examples but having trouble applying what I've found. I'm
trying to create mobility models for use with lost person analytics.

 

To start off, I'm running QGIS 2.18.13 64-bit on a laptop with Windows 10
64-bit. I've posted a similar question on the QGIS Dev Listserv and they
recommended I re-post here.

 

Q1: Does it matter what format the input layers are in? Should they all be
UTM since the result is in meters and seconds? Or is the tool smart enough
to mix and match WGS84 and UTM layers? And along the same lines, does my DEM
need to have cells in meters? Most of what I get comes native in WGS84
(lat/lon) with altitudes in feet, so if I don't need to add extra steps
converting to UTM and cells to meters that's a good thing.

 

For the friction layer, the example on the man page appears to use
landclass96 and only has 7 classifications. the NLCD 2011 land
classification file I have has 30-40 classifications ranging from 11 to 95,
this was initially hard for me to understand but then I stumbled upon the
man page for r.reclass and saw that the NLCD data was being generalized into
the major sub-groups so I did the same by first converting the large set of
classifications into the 7 groups and then I created a friction layer using
the example for r.walk.

 

Q2: Now with everything in UTM and my DEM's cells being in meters I'm trying
to run r.walk and the algorithm is never able to finish I assume because I'm
running out of HD space. Is it normal for an r.walk over a DEM covering an
area of 10,500 acres to use up 12GB of HD space and want more? That's using
up every bit of space I have left, so it's crashing out. I tried setting the
maximum cumulative cost to 10 and 1 also, but the algorithm keeps running
for a long time making these really large files. My understanding from the
man page is that the values calculated are in seconds, so when I set maximum
cumulative cost to 1 or 10 seconds that should be really quick, right? Maybe
I'm missing something.

 

Here's the command as run from within QGIS: r.walk
elevation="tmp151153693667" friction="tmp151153693668"
start_points="tmp15115369366310" walk_coeff="0.72,6.0,1.9998,-1.9998"
lambda="1" slope_factor="-0.2125" max_cost="1" null_cost="0" memory="2048"
output=output8468f646ce7b4b838f8c319a88ad3d73
outdir=outdir8468f646ce7b4b838f8c319a88ad3d73 --overwrite 

 

Thanks,

 

Josh Q

 

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