[R-sig-Geo] calculate "regional" slope

chris english englishchristophera at gmail.com
Sat Feb 20 08:49:50 CET 2016

There's mention of grass not working on osx 10.11 and a work around

I was generally using PCRaster when I was doing watershed/flood
re-creations to handle the watershed delineations and the like and don't
have an opinion on Grass.

Yeah, that transect was maybe a silly idea but it generalizes surface to
line, sort of like Mauana volcano is a cake you've cut in half. And the
line is outline.

On Feb 20, 2016 2:09 AM, "Dominik Schneider" <Dominik.Schneider at colorado.edu>

> Chris R,
> I'd be happy to use grass7 but I can't get it to run on my mac. osx
> 10.11.3. I have a working qgis from kyngchaos, installed pandoc and cairo
> on top of that, and disabled sip but the grass7.app just hangs when I try
> to open it.
> Chris E,
> I will try to clarify. Rather than thinking about "side of a mountain",
> think about "side of a mountain **range**".  The point of calculating a
> regional slope is that if I am on the west side of a continental divide but
> on the east side of mountain, the function still returns a value indicating
> west-facing.  so maybe it's easier to think in meters.   lets assume my DEM
> is a 500m grid. the slope calculation would give a local value at 500m.
> This local slope might be east facing, but I am interested in the overall
> slope across 10km to indicate which way e.g. the watershed is draining.
> What do you mean with "transect along the z. I.e. roll your dem on it's
> side."?
> On Fri, Feb 19, 2016 at 2:50 PM, chris english <
> englishchristophera at gmail.com> wrote:
>> Dominik,
>> Sorry, I'm still trying to understand your 0.05 to 1.5 degree part of
>> your problem.
>> Otherwise, I think you are limited to 8 neighbors as this reflects the
>> documentation as I read it.
>> Even Roualt perhaps would be up in arms; but, there's nothing saying you
>> can't do a 16 vs 8 neighbor. You'd have to examine the impacts thereafter,
>> but basically you'd be amending some gdal (probably a line or two of code)
>> for your purposes.
>> There are a bunch of things to consider, theoretical and practical; but,
>> why 16 better than 8. And more importantly as you relax, as a matter of
>> rings (in this case), would your analytical result be better? Or
>> potentially have any remainder meaning at all, I.e. I don't know my
>> neighbor's neighbor's neighbor (does that get us out to 16?).
>> And so generalizing beyond some given point might yield not much on a knn
>> influence/likeness basis.
>> I think we're first better off dialing back in on what you mean by
>> regional or the 0.5 to 1.5 resolution and then neighborhood size (4, 8,16?).
>> Of course another approach to this "what side of the mountain am I on" is
>> to transect along the z. I.e. roll your dem on it's side.
>> Anyway, clarify the 0.5/1.5 so I don't go too far astray.
>> Chris
>> Thanks for the suggestion Chris.  I'm familiar with gdaldem, which
>> raster::terrain is based on, to compute slope from a dem. I now realize
>> that my example isnt a good one because neighbors=8 would achieve what I
>> described. However I actually want some flexibility such that I can
>> specifiy neighbors=16 so that it uses the next "ring" of cells.
>> I played around with focal() with weight argument =
>> matrix(rep(c(1,0,0,0,1),5),byrow=T) but couldn't figure out how to solve
>> for a directional slope.
>> On Fri, Feb 19, 2016 at 4:09 AM, chris english <
>> englishchristophera at gmail.com> wrote:
>>> Dominik,
>>> r <- raster(nrows=22, ncols=20, xmn=-58, xmx=-48, ymn=-33, ymx=-22)
>>>  vals <- sample.int(1e3,440)
>>> r[ ] <- vals
>>> #raster::terrain
>>> terr_r <- terrain(r, opt='slope', unit='degrees', neighbors=8)
>>> Ah, but it appears you want up sampling to 1.5 degrees rather than 0.5
>>> deg.
>>> so maybe spatial.tools::projectRaster_rigorous then raster:terrain.
>>> I'm inclined to end that last so maybe with a question mark. Sorry for an
>>> essentially inconclusive response but I was happy to find terrain in any
>>> case.
>>> Chris
>>> On Fri, Feb 19, 2016 at 2:59 AM, Dominik Schneider <
>>> Dominik.Schneider at colorado.edu> wrote:
>>>> I need to calculate slope at different scales. In the case below, r is a
>>>> 0.5deg resolution raster and I want the slope for 1.5 deg centered on
>>>> each
>>>> of those 0.5 deg pixels. I'm trying to estimate which side of mountain
>>>> range each pixel is on. So the resulting raster would have the same
>>>> number
>>>> of pixels as r. The edges can be NA.
>>>> any suggestions would be appreciated. Thanks
>>>> r <- raster(nrows=22, ncols=20, xmn=-58, xmx=-48, ymn=-33, ymx=-22)
>>>> setValues(r,rnorm(440))
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