CODEWITHSUNDEEP
r
Maria123
Maria123

Reputation: 13

How to count how many roads intersect polygons?

I'm working with Polygons and Lines, now I want to count how many Lines intersect each Polygon.

I can intersect them, but I don't understand how to count the intersections.

inter <- intersect(district_sp, osm_maj_roads)

district_sp is the spatial polygon. osm_maj_roads is a spatial line object.

This way I get a complete new spaial polygon, but all I really want is a vector with the number of intersections.

Upvotes: 0

Views: 786

Answers (2)

Stewart Macdonald
Stewart Macdonald

Reputation: 2132

Do you really get a new spatial object when you use st_intersects? It should return a sparse index list. st_intersection should return a new spatial object.

library(sf)

somePolygons <- st_transform(st_read('/temp/GIS/test/polygons.shp'), 3577)
someLines <- st_transform(st_read('/temp/GIS/test/linestrings.shp'), 3577)

i <- st_intersects(someLines, somePolygons)
print(i)
Sparse geometry binary predicate list of length 4, where the predicate was 'intersects'
 1: (empty)
 2: 2
 3: 1, 3
 4: 1, 2, 3

# Get a vector with the number of polygons each line intersects with:
print(sapply(i, length))
[1] 0 1 2 3

enter image description here

Upvotes: 1

Maria123
Maria123

Reputation: 13

I don't understand why nobdy answered, I had a hard time finding what I needed to do. In the end I used the aggregate funtion. roads_agg <- aggregate(x = osm_roads["osm_id"], by = polygon_shp, FUN = length) this gives me a vector with the rows being the like the polygons and the value in them the number of intersections. Hope this helps somebody ;)

Upvotes: 0

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