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I'm working with Brazilian deforestation data - specifically two sf objects:

  • A point shapefile on environmental fines (centroid of land plots that have been fined for deforestation).
  • A polygon of (unioned) municipalities.

My aim is to calculate the nearest distance from each point to the municipal boundary line. I can do this using st_distance, however I would like the distance to be negative for points that are within the municipality polygon (as in picture below). Could anyone please advise?

deforest <- st_read("Environmental_Fines.shp")
munic <- st_read("Municipalities_Union.shp")

Municipalities

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Outine solution:

  1. Cast the polygons to lines using st_cast and compute distances of pounts to lines (otherwise you get zero for the distance of points inside polygons).

  2. Use st_intersects on the points/polygons to test if point is inside polygon, and negate the distance for those that are.

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  • Thanks very much! I've just added a column to my Deforestation df with st_intersects, with value 0/1 - however it's been added in list format (integer(0) or 1). Do you know how this can be converted to an object within the df instead?
    – cstock
    May 1, 2022 at 20:45
  • st_intersects returns a list because a point could be in several polys. If you are sure this isn't the case (maybe you only have one poly) then you can compute a "is it in a poly?" with lengths(st_intersection(....)) > 0 - note use of lengths (not length) which returns zero for integer(0) when there's no intersection. Use that to make a vector to put in your data frame.
    – Spacedman
    May 2, 2022 at 12:03

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