[R-sig-Geo] dnearneigh() from spdep: Points with the exact same location are not considered neighbours.

[Maël Le Noc]

Dear Roger,
Thank you for your answer, (And sorry for the HTML posting).

The issue persists if I specify "GE" for the lower bound, but only when
the parameter latlong is set to TRUE (see example below).


Regarding the nature of my data, it is a series of record of Jews
arrested during the Holocaust in Italy. Those are point data, and some
people have been arrested at the same place and at the same time (hence
my problem). I am trying to assess spatial autocorrelation for a binary
attribute (whether they survived the Holocaust or not), and I plan to
use a Join-count method, for which I need a spatial weight matrix. Is
using Join-count on such a dataset wrong ?

Best



Code:

library(data.table)
library(spdep)
pointstable <- data.table(XCoord=c(13.667029,13.667029,13.667028),
YCoord=c(42.772396,42.772396,42.772396))
print(pointstable)
coords <-cbind(pointstable$XCoord, pointstable$YCoord)
nbLocal<- dnearneigh(coords, d1=0, d2=25, longlat = TRUE, bound =
c("GE", "LE"))
summary(nbLocal)
nbLocal<- dnearneigh(coords, d1=0, d2=25, longlat = FALSE, bound =
c("GE", "LE"))
summary(nbLocal)


Output:
> print(pointstable)
     XCoord  YCoord
1: 13.66703 42.7724
2: 13.66703 42.7724
3: 13.66703 42.7724

> nbLocal<- dnearneigh(coords, d1=0, d2=25, longlat = TRUE, bound =
c("GE", "LE"))
> summary(nbLocal)
Neighbour list object:
Number of regions: 3
Number of nonzero links: 4
Percentage nonzero weights: 44.44444
Average number of links: 1.333333
Link number distribution:

1 2
2 1
2 least connected regions:
1 2 with 1 link
1 most connected region:
3 with 2 links

> nbLocal<- dnearneigh(coords, d1=0, d2=25, longlat = FALSE, bound =
c("GE", "LE"))
> summary(nbLocal)
Neighbour list object:
Number of regions: 3
Number of nonzero links: 6
Percentage nonzero weights: 66.66667
Average number of links: 2
Link number distribution:

2
3
3 least connected regions:
1 2 3 with 2 links
3 most connected regions:
1 2 3 with 2 links



On 12/04/2017 02:27, Roger Bivand wrote:
> Do not post HTML-mail, only plain text. Your example is not reproducible
> because you used HTML-mail.
> 
> Please read the help file, the bounds are described as being between
> lower (greater than) and upper (less than or equal to) bounds. Since the
> distance between identical points is strictly zero, they are not
> neighbours because the distance must be > d1 and <= d2. If d1 is < 0, it
> is reset to 0, as it is assumed that a negative lower bound is a user
> error (and it would break the underlying compiled code).
> 
> In any case, no reasonable cross-sectional spatial process has
> duplicated point (nugget) observations in situations in which spatial
> weights would be used (spatio-temporal panels will have, but then time
> differs).
> 
> Hope this clarifies,
> 
> Roger
> 
> On Wed, 12 Apr 2017, Maël Le Noc via R-sig-Geo wrote:
> 
>> Dear List
>>
>> As I was working on a project, I realized that when I use dnearneigh
>> from spdep, two (or more) points that have the exact same coordinates
>> are not considered neighbours and thus are not linked (even when the
>> lower bound is put to 0 or even to -1). See below for an example.
>> (However this does not happen if the parameter longlat is set to false)
>>
>> Does the function behave the same way for you? Am I missing something?
>> Is this an expected behavior? And if so, if there a way to change that ?
>>
>> In the example below, points 1 and 2 are not connected to each other/are
>> not neighbours (as you can see since the both have only one link, to 3),
>> even though they have the exact same coordinates (and are thus less than
>> 25km apart), while point 3 is connected to both point 1 and 2.
>> If I want to assess autocorrelation using, for instance joincount.test,
>> this is then an issue...
>>
>>> /library(data.table) />/library(spdep) />/pointstable <-
>>> data.table(XCoord=c(13.667029,13.667029,13.667028),
>>> /YCoord=c(42.772396,42.772396,42.772396))
>>> /print(pointstable) /     XCoord  YCoord
>> 1: 13.667029 42.772396
>> 2: 13.667029 42.772396
>> 3: 13.667028 42.772396
>>> /coords <-cbind(pointstable$XCoord, pointstable$YCoord) />/nbLocal<-
>>> dnearneigh(coords, d1=0, d2=25, longlat = TRUE) />/nbLocal<-
>>> dnearneigh(coords, d1=-1, d2=25, longlat = TRUE) #both lines /produce
>>> the same output
>>> /summary(nbLocal) /Neighbour list object:
>> Number of regions: 3
>> Number of nonzero links: 4
>> Percentage nonzero weights: 44.44444
>> Average number of links: 1.333333
>> Link number distribution:
>>
>> 1 2
>> 2 1
>> 2 least connected regions:
>> 1 2 with 1 link
>> 1 most connected region:
>> 3 with 2 links
>>> //
>> Thanks
>> Maël
>>
>>
>>     [[alternative HTML version deleted]]
>>
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