[R-sig-Geo] sub-selecting spatial data by distance from a different point(set)

[Sean O'Riordain]

Thank you very much Roger and Paul.

Kind regards,
Sean


On Sat, Nov 14, 2009 at 3:31 PM, Roger Bivand <Roger.Bivand at nhh.no> wrote:
> On Sat, 14 Nov 2009, Paul Hiemstra wrote:
>
>> Hi Sean,
>>
>> Take a look at the spDistsN1 function to calculate the distances from a
>> single point to all other points. You can then make the subset based on the
>> distance. What could also be an options is to build a polygon that has the
>> radius around the point you are taking the distance from, and then use
>> overlay to find which points are in the radius polygon. This might be faster
>> when using a large dataset. I'm not sure how nto make the polygon, but it
>> should be possible. But you could first take a look at spDistsN1.
>
> In fact spDistsN1() in a loop over the coordinates of B runs (for me) faster
> than building and searching a quadtree in A, and (to machine precision) gets
> the same results. So allocate an empty list, and step through:
>
> dn <- vector(mode="list", length=nrow(B))
> for (i in 1:nrow(B)) {
>  dn[[i]] <- which(spDists(A, B[i,]) <= threshold)
> }
>
> where A is a 2M by 2 matrix and B is a 500 by 2 matrix. dn will be a list of
> 500 vectors of point IDs in A. For me on a laptop (with A 2M points from
> runif() and B 500 points from runif()), this search took under 5 minutes,
> with # neighbours between 7000 and 16000.
>
> Roger
>
>>
>> cheers,
>> Paul
>>
>> Sean O'Riordain wrote:
>>>
>>> Good afternoon,
>>>
>>> Apologies if this seems a basic question but I'm quite new to spatial
>>> analysis (1 week now!) and I'm reading and learning as fast as I can.
>>> I've read the Spatial Data task view, "Analysing spatial point
>>> patterns in R" by Bradley and I've ordered "Applied Spatial Data
>>> Analysis with R" - but it hasn't arrived yet...  I've looked and
>>> searched - but I'm guessing that I'm not using the right buzzwords...
>>>
>>> I have a data.frame (call it A) of about 2 million X,Y points each
>>> with info with various characteristics, numeric and factor.  I have a
>>> separate much smaller data.frame (call it B) of say 500 rows of X,Y
>>> and I'd like to be able to pull a subset of A, so that for each B[i,]
>>> I will get all rows from A that are within a given straight line
>>> distance (i.e. within a short 'as the crow flies' radius).
>>>
>>> Could somebody please give me some starting hints please?
>>>
>>> Thanks in advance,
>>> Sean O'Riordain
>>>
>>> _______________________________________________
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>>> R-sig-Geo at stat.math.ethz.ch
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>>>
>>
>>
>>
>
> --
> Roger Bivand
> Economic Geography Section, Department of Economics, Norwegian School of
> Economics and Business Administration, Helleveien 30, N-5045 Bergen,
> Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
> e-mail: Roger.Bivand at nhh.no
>
>