Spatial Query (Selection) with Apply

Thanks, Robin

It worked perfectly, although I transformed the data from sf to
SpatialDataFrame objects with:

zones <- as(zones, "Spatial")
nodes <- as(nodes, "Spatial")

The next step I will follow is to create Distance Matrix like this:

coord <- tmp2[[1]]@coords
dist1 <- apply(X = coord, MARGIN = 1, FUN = function(x) spDistsN1(coord,
x, longlat = T))

obviously, the task is a distance matrix for every selection. ?What would
you recommend, the aggregate function as you talked before or something
like a nested apply?

Regards,
Ariel



2018-08-27 19:32 GMT-03:00 Robin Lovelace <rob00x at gmail.com>:

Hi Ariel,

It helps when asking for help with code to produce a reproducible
example. To understand your input data I've created nodes and zones based
on the spData data nz_height and nz_elev. Based on the assumption you want
to find all the nodes in each zone I think the direct answer to your
question is something like the following:

tmp2 = lapply(s, function(x) {
  nodes[zones[x, ], ]
})

The longer answer is that aggregate(), st_join() +
aggregate()/summarize() may provide quicker solutions, depending on what
you want to do with the points after grouping them by which zone they fall
in.
Note: I've used sf objects based on the explanation here
https://geocompr.robinlovelace.net/spatial-operations.html#spatial-vec
which may not work with sp data but the aggregate code should work roughly
the same:

# install.packages("sf")
# install.packages("spData")
library(spData)
library(sf)
#> Linking to GEOS 3.6.2, GDAL 2.2.3, proj.4 4.9.3
zones = nz
nodes = nz_height
tmp = list()
s = 1:nrow(nz)

for(i in s) {
  tmp[[i]] = nodes[nz[i, ], ]
}

# understand what's going on with plots (not shown)
# plot(st_geometry(nz))
# plot(st_geometry(tmp[[i]]), add = TRUE, col = "red")
# plot(nz[i, ], col = "green", add = TRUE)

tmp2 = lapply(s, function(x) {
  nz_height[nz[x, ], ]
})

identical(tmp, tmp2)
#> [1] TRUE

Created on 2018-08-27 by the [reprex package](http://reprex.tidyverse.org)
(v0.2.0).

I've also pasted the reprex into the geocompr github tracker so the plots
can be seen and the code formatted: https://github.com/Robinlovela
ce/geocompr/issues/294

Hope this helps,

Robin


On Mon, Aug 27, 2018 at 9:53 PM, Ariel Fuentesdi <
ariel.fuentesdi at usach.cl> wrote:

Hi,

I want to do multiple selections of a point shapefile based on polygons
on
other layers, I can do this in a for loop, but I desire to do this in a
function of the apply family.

I named the point shapefile "nodes" and the polygons shapefile "zones".

This is what I did:

tmp <- list()
for (i in 1:nrow(zones at data)) {
    tmp[[i]] <- nodes[subset(zones, ESTUDIO == i),]
    tmp
  }

But I have no clue how to change it to the apply family, can you provide
an example of this?

Thanks in advance.

Regards,
Ariel Fuentes

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