knitr::opts_chunk$set(collapse = TRUE)
This vignette describes a number of issues that did not come up in the previous vignettes, and that may or may not be categorized as "frequently asked questions". Readers are encouraged to provide entries for this vignette (as for the others).
EPSG stands for a maintained, well-understood set of spatial
reference systems, maintained by the International Association of
Oil \& Gass Producers (IOGP).
From R, we can get access to the EPSG
dataset programmatically (as a
see also the first vignette.
sfdeal with secondary geometry columns?
sf objects can have more than one geometry list-column,
but always only one geometry column is considered active,
and returned by
st_geometry. When there are multiple
geometry columns, the default
library(sf) demo(nc, ask = FALSE, echo = FALSE) nc$geom2 = st_centroid(st_geometry(nc)) print(nc, n = 2)
We can switch the active geometry by using
st_set_geometry, as in
plot(st_geometry(nc)) st_geometry(nc) <- "geom2" plot(st_geometry(nc))
st_simplify is a topology-preserving function, but does this on the
level of individual feature geometries. That means, simply said, that
after applying it, a polygon will still be a polygon. However when
two features have a longer shared boundary, applying
to the object does not guarantee that in the resulting object these
two polygons still have the same boundary in common, since the
simplification is done independently, for each feature geometry.
They do! However, many developers like to write scripts that never
load packages but address all functions by the
sf:: prefix, as in
i = sf::st_intersects(sf1, sf2)
This works up to the moment that a
dplyr generic like
select for an
is needed: should one call
dplyr::select (won't know it should search
sf::select (which doesn't exist)? Neither works.
One should in this case simply load
sf, e.g. by
Most (but not all) of the geometry calculating routines used by
sf come from the GEOS library. This library considers coordinates in a two-dimensional, flat, Euclidian space. For longitude latitude data, this is not the case. A simple example is a polygon enclosing the North pole, which should include the pole:
polygon = st_sfc(st_polygon(list(rbind(c(0,80), c(120,80), c(240,80), c(0,80)))), crs = 4326) pole = st_sfc(st_point(c(0,90)), crs = 4326) st_intersects(polygon, pole)
which gives a wrong result (no intersection).
Similar to the above, centroids are computed assuming flat, 2D space:
where the centroid should have been the pole.
This message indicates that
sf assumes a distance value is given in degrees. To avoid this message, pass a value with the right units:
pt = st_sfc(st_point(c(0,0)), crs = 4326) buf = st_buffer(polygon, 1) buf = st_buffer(polygon, units::set_units(1, degree))
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