[R-sig-Geo] [FORGED] Create a Spatial Weight Matrix based on road distance
Rolf Turner
r@turner @end|ng |rom @uck|@nd@@c@nz
Sun Jun 23 10:17:21 CEST 2019
On 23/06/19 6:01 PM, Rolando Valdez wrote:
> I apologize for the lack of clarity.
>
> Let me try again:
>
> The SWM captures the spatial structure among territories. In the case of
> a matrix based on distance, you define a distance-threshold, say 50 km,
> and every territory under that distance is considered as neighbor, in
> the matrix, those territories considered neighbors take the value 1, and
> 0 otherwise (territories beyond 50 km). This is what 'dnearneigh'
> function does.
>
> Then, I want to define a distance-threshold, say 50 km by road (not
> euclidean) and every territory under that distance (by road) be
> considered as neighbor.
You still have not defined what you mean by *distance* between
territories (regions, counties). Distance between *points* is well
defined; distance between territories is not. You have to specify what
you mean by such a distance. This could be the minimum distance between
points in the regions (which is not of course a metric), distance
between centroids of the territories, Hausdorff distance, or something
else. This applies whether you are talking about the distance between
points being Euclidean distance or road distance or some other metric.
Thresholding that distance (e.g. at 50 km.) is then a trivial matter.
I have tried my best to get you to clarify what you mean, and my efforts
seem to be in vain. Since Juan Pablo thinks that I am "bullying you"
(which mystifies me completely) I guess I'll give up. And to respond to
Juan Pablo's hope, nothing whatever is "bothering" me.
cheers,
Rolf
>
> El sáb., 22 de jun. de 2019 a la(s) 21:15, Rolf Turner
> (r.turner using auckland.ac.nz <mailto:r.turner using auckland.ac.nz>) escribió:
>
> On 23/06/19 3:30 PM, Rolando Valdez wrote:
> > Sorry again.
> >
> > A Spatial Weight Matrix (swm) is an object used in spatial
> econometrics
> > to characterize the spatial structure among territories. It is an
> > element nxn where n is the number of territorial units (counties,
> > districts, states, cities, regions) in the sample and it could be
> based
> > on contiguity or distance. Usually, you can create a swm based on
> > distance using 'dnearneigh' from spdep and then convert to a listw
> > through 'nb2listw'. The problem is that the matrix that you generate
> > trough 'dnearneigh'computes the euclidean distance among
> centroids of
> > polygons. This is where I spot my issue, I need to compute the
> swm using
> > the road distance instead of euclidean distance computed through
> > 'dnearneigh'. I do have a shapefile with poligons (counties) and
> another
> > shapefile with lines (roads).
>
> OK. It's getting a *bit* clearer .... You are interested in "road
> distances" between counties. I'm still not entirely sure what this
> means. Is it the *minimum* distance by road from one county to
> another?
> In which case, if two counties are contiguous (adjacent) and there is a
> road crossing the border between the two, is the distance between the
> counties equal to zero? (This doesn't seem like it would be
> satisfactory ....)
>
> Yes, actually it is possible that two counties were connected by more
> than one road, however it's not a big deal. If I define a distance of 50
> km, it doesn't matter how many times two counties are connected, I just
> need that they are at 50 km trough, at least, one road.
>
> If this is not the case, then what *is* the case? Perhaps you want
> distances between the *centroids* of the counties. What then do you
> mean by road distance when the centroids do not lie on a road?
>
>
> This is a big challenge, I'm still working on it.
>
>
> You apparently need to deal with counties in which there are no
> roads at
> all. To handle this you have to define what *you* mean by the distance
> by road from county A to county B when there are no roads at all in
> county B. Perhaps infinity would be the appropriate distance, but *I*
> don't know; you have to make the call.
>
>
> If two counties are not connected through a road, they could not be
> neighbors. In this case, it would correspond to a value 0 in the matrix.
>
>
> Previously you indicated that you needed to know (pairwise) road
> distances between specified points in a given set, and I showed you how
> to obtain those using pairdist(), from spatstat. Now it seems that you
> want something rather different, and it's still not clear what.
>
> In a sense is the same, but you said so properly, We have different
> research fields.
>
> You need to get *your* thoughts clear; make some definitions and
> specifications, and decide what you really want or need.
>
>
> I got it.
>
>
> It seems that you are expecting R to magically do your thinking for
> you;
> it won't!
>
> No, I'm not expecting that.
>
> cheers,
>
> Rolf
>
> > El sáb., 22 de jun. de 2019 a la(s) 20:00, Rolf Turner
> > (r.turner using auckland.ac.nz <mailto:r.turner using auckland.ac.nz>
> <mailto:r.turner using auckland.ac.nz <mailto:r.turner using auckland.ac.nz>>)
> escribió:
> >
> >
> > On 23/06/19 2:38 PM, Rolando Valdez wrote:
> >
> > > I am sorry, I was not clear enough. My goal is to calculate a
> > spatial
> > > weight matrix (nxn) across counties but, instead of euclidean
> > distance,
> > > to use road distance.
> >
> > I'm afraid I still don't understand. To put it mildly. You
> presumably
> > have a clear idea of what you are trying to, but those of us
> who are
> > not
> > involved in your research have no such idea. We (or at least I)
> > haven't
> > a clue as to what you are talking about.
> >
> > What do you mean by "spatial weight"? What are these weights
> used for?
> > What is n? How are the counties involved? Is n the number of
> > counties?
> > Are you interested in the road distance (minimum road distance?)
> > between
> > pairs of counties?
> >
> > Please explain *clearly* and do not expect those who are
> trying to help
> > you to be mind-readers!!!
> >
> > cheers,
> >
> > Rolf
> >
> > >
> > > El sáb., 22 de jun. de 2019 a la(s) 19:28, Rolf Turner
> > > (r.turner using auckland.ac.nz <mailto:r.turner using auckland.ac.nz>
> <mailto:r.turner using auckland.ac.nz <mailto:r.turner using auckland.ac.nz>>
> > <mailto:r.turner using auckland.ac.nz
> <mailto:r.turner using auckland.ac.nz> <mailto:r.turner using auckland.ac.nz
> <mailto:r.turner using auckland.ac.nz>>>)
> > escribió:
> > >
> > >
> > > On 23/06/19 1:17 PM, Rolando Valdez wrote:
> > >
> > > > Thank you for your answer.
> > > >
> > > > I have a shapefile with, say, counties, and I got
> another
> > > shapefile with
> > > > the roads. ¿What if a county does not intersect any
> road?
> > >
> > > I am sorry, but it is not at all clear to me just what the
> > problem is.
> > > How do the counties come into the picture? You said you
> > wanted to get
> > > the road distance between points on the roads. What
> have the
> > counties
> > > got to do with this?
> > >
> > > Can you perhaps provide a reproducible example?
> > >
> > > cheers,
> > >
> > > Rolf
> > >
> > > >
> > > > El jue., 20 de jun. de 2019 a la(s) 19:08, Rolf Turner
> > > > (r.turner using auckland.ac.nz
> <mailto:r.turner using auckland.ac.nz> <mailto:r.turner using auckland.ac.nz
> <mailto:r.turner using auckland.ac.nz>>
> > <mailto:r.turner using auckland.ac.nz
> <mailto:r.turner using auckland.ac.nz> <mailto:r.turner using auckland.ac.nz
> <mailto:r.turner using auckland.ac.nz>>>
> > > <mailto:r.turner using auckland.ac.nz
> <mailto:r.turner using auckland.ac.nz>
> > <mailto:r.turner using auckland.ac.nz
> <mailto:r.turner using auckland.ac.nz>> <mailto:r.turner using auckland.ac.nz
> <mailto:r.turner using auckland.ac.nz>
> > <mailto:r.turner using auckland.ac.nz
> <mailto:r.turner using auckland.ac.nz>>>>)
> > > escribió:
> > > >
> > > >
> > > > On 21/06/19 12:26 PM, Rolando Valdez wrote:
> > > >
> > > > > Dear community,
> > > > >
> > > > > Is there any way to create a spatial weight
> matrix
> > based
> > > on road
> > > > distance?
> > > > > I am trying to use the road distance between
> two points
> > > instead of
> > > > > euclidean distance.
> > > > >
> > > > > I've seen that there is a package named
> osrm. Can
> > anyone give
> > > > some advice?
> > > >
> > > > I don't know anything about "osrm".
> Calculating "road
> > distances"
> > > > can be
> > > > done in the spatstat package reasonably easily,
> if you
> > take
> > > the trouble
> > > > to represent your collection of roads as a "linnet"
> > object.
> > > >
> > > > Given that you have done so, suppose that your
> linnet
> > object
> > > is "L" and
> > > > that you have vectors "x" and "y" specifying the
> > points on L
> > > (i.e. on
> > > > your roads) between which you want to know the
> distances.
> > > >
> > > > Do:
> > > >
> > > > X <- lpp(data.frame(x=x,y=y),L)
> > > > dMat <- pairdist(X)
> > > >
> > > > The object "dMat" is a (symmetric) square matrix;
> > dMat[i,j]
> > > is the
> > > > distance between point i and point j. (Of
> course the
> > > diagonal entries
> > > > are all 0.)
> > > >
> > > > If your collection of roads is specified by
> means of a
> > shapefile,
> > > > vignette("shapefiles") will tell you how to
> turn this
> > > collection into a
> > > > "psp" ("planar segment pattern") object; the
> function
> > (method)
> > > > as.linnet.psp() can then be used to turn the "psp"
> > object into a
> > > > "linnet" object.
> > > >
> > > > HTH
More information about the R-sig-Geo
mailing list