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).
What is this EPSG code all about?
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).
Why should we use 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()
How does 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)
## Linking to GEOS 3.12.1, GDAL 3.8.4, PROJ 9.4.0; sf_use_s2() is TRUE
demo(nc, ask = FALSE, echo = FALSE)
nc$geom2 = st_centroid(st_geometry(nc))
print(nc, n = 2)
## Simple feature collection with 100 features and 14 fields
## Attribute-geometry relationships: aggregate (8), identity (6), NA's (1)
## Active geometry column: geom
## Geometry type: MULTIPOLYGON
## Dimension: XY
## Bounding box: xmin: -84.32385 ymin: 33.88199 xmax: -75.45698 ymax: 36.58965
## Geodetic CRS: NAD27
## First 2 features:
## AREA PERIMETER CNTY_ CNTY_ID NAME FIPS FIPSNO CRESS_ID BIR74 SID74
## 1 0.114 1.442 1825 1825 Ashe 37009 37009 5 1091 1
## 2 0.061 1.231 1827 1827 Alleghany 37005 37005 3 487 0
## NWBIR74 BIR79 SID79 NWBIR79 geom
## 1 10 1364 0 19 MULTIPOLYGON (((-81.47276 3...
## 2 10 542 3 12 MULTIPOLYGON (((-81.23989 3...
## geom2
## 1 POINT (-81.49823 36.4314)
## 2 POINT (-81.12513 36.49111)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))
Does 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.
Why do my dplyr verbs not work for 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