knitr::opts_chunk$set(collapse = TRUE)
This vignette describes a number of issues that did not come up in the previous vignettes, and that may or may not be categorized as "frequently asked questions". Readers are encouraged to provide entries for this vignette (as for the others).
EPSG stands for a maintained, well-understood registry of spatial
reference systems, maintained by the International Association
of Oil \& Gas Producers (IOGP). EPSG
stands for the authority,
e.g. EPSG:4326
stands for spatial reference system with ID 4326
as it is maintained by the EPSG authority. The website for the EPSG
registry is found at the epsg.org domain. Using 4326
instead of
EPSG:4326
is allowed (EPSG
is the default authority) but the
latter form, EPSG:4326
is better (less ambiguous).
OGC:CRS84
instead of EPSG:4326
?EPSG:4326 formally defines coordinate axes to be in the order
latitude-longitude, but practically all data sources and software
environments use longitude-latitude axis order. OGC:CRS84 is
equivalent to EPSG:4326 except that it defines coordinate axis
order longitude-latitude, removing this ambiguity so to speak.
See also st_axis_order()
sf
deal with secondary geometry columns?sf
objects can have more than one geometry list-column,
but always only one geometry column is considered active,
and returned by st_geometry()
. When there are multiple
geometry columns, the default print
methods reports which
one is active:
library(sf) demo(nc, ask = FALSE, echo = FALSE) nc$geom2 = st_centroid(st_geometry(nc)) print(nc, n = 2)
We can switch the active geometry by using st_geometry<-
or st_set_geometry()
, as in
plot(st_geometry(nc)) st_geometry(nc) <- "geom2" plot(st_geometry(nc))
st_simplify
preserve topology?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 st_simplify
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.
sf
objects?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 sf
object
is needed: should one call dplyr::select
(won't know it should search
in package sf
) or sf::select
(which doesn't exist)? Neither works.
One should in this case simply load sf
, e.g. by
library(sf)
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