[R-sig-Geo] [FORGED] Create a Spatial Weight Matrix based on road distance

[Rolf Turner]

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 ....)

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?

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.

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.

You need to get *your* thoughts clear; make some definitions and 
specifications, and decide what you really want or need.

It seems that you are expecting R to magically do your thinking for you; 
it won't!

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>) 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>>)
>     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>>>)
>      >     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