Check if lat/lon in polygon (R)-CodePudding

I have a lat/lon combination and want to check whether the point is inside a polygon (sp::Polygon class)

Consider this example:

UKJ32 <- sp::Polygon(cbind(c(-1.477037449999955, -1.366895449999959, -1.365159449999965, -1.477037449999955),
                           c(50.923958250000027, 50.94686525000003, 50.880069750000018, 50.923958250000027))) %>%
   list() %>% 
   sp::Polygons(ID="UKJ32 - Southampton")

I would now like to test whether the points in df are in this polygon (and if so, return the Polygon ID).

tibble(lon = c(-1.4, 10), lat = c(50.9, 10))

Can someone tell me how I get to the result

tibble(lon = c(-1.4, 10), lat = c(50.9, 10), polyg_ID = 'UKJ32')

CodePudding user response：

If you wish to stick to sp, there is a point.in.polygon() function in sp package:

UKJ32 <- sp::Polygon(cbind(c(-1.477037449999955, -1.366895449999959, -1.365159449999965, -1.477037449999955),
                           c(50.923958250000027, 50.94686525000003, 50.880069750000018, 50.923958250000027))) |>
  list() |>
  sp::Polygons(ID="UKJ32 - Southampton")

a <- tibble::tibble(lon = c(-1.4, 10), lat = c(50.9, 10))

sp::point.in.polygon(a$lon, a$lat, UKJ32@Polygons[[1]]@coords[,1], UKJ32@Polygons[[1]]@coords[,2])
#> [1] 1 0

^{Created on 2022-10-16 with reprex v2.0.2}

CodePudding user response：

The {sp} package is by now somewhat dated - after having lived a long & fruitful life - and most of current action happens in context of its successor, the {sf} package.

Assigning some kind of a polygon feature - either an id or a metric - to a points dataset is a frequent use case. It at present often done via a sf::st_join() call. For an example in action consider this earlier answer https://stackoverflow.com/a/64704624/7756889

I suggest that you try to move your workflow to the more current {sf} package; you will find it easier to keep up with recent development.

And even if this were not possible for whatever reason - use sp::Polygons() with utmost caution. I carries no information about coordinate reference system - which is a fancy way of saying it has no way of interpreting the coordinate numbers. Are they decimal degrees, or meters? Could be feet or fathoms for all that I know.

Strictly speaking you should not be allowed to proceed with a point-in-polygon operation calculation without this information.