These functions compute measurements for geometries. The input geometries may be given as a single raw vector of WKB, a list of WKB raw vectors, or a character vector containing one or more WKT strings.
Usage
g_area(geom, quiet = FALSE)
g_centroid(geom, quiet = FALSE)
g_distance(geom, other_geom, quiet = FALSE)
g_length(geom, quiet = FALSE)
g_geodesic_area(geom, srs, traditional_gis_order = TRUE, quiet = FALSE)
g_geodesic_length(geom, srs, traditional_gis_order = TRUE, quiet = FALSE)Arguments
- geom
Either a raw vector of WKB or list of raw vectors, or a character vector containing one or more WKT strings.
- quiet
Logical value,
TRUEto suppress warnings. Defaults toFALSE.- other_geom
Either a raw vector of WKB or list of raw vectors, or a character vector containing one or more WKT strings. Must contain the same number of geometries as
geom.- srs
Character string specifying the spatial reference system for
geom. May be in WKT format or any of the formats supported bysrs_to_wkt().- traditional_gis_order
Logical value,
TRUEto use traditional GIS order of axis mapping (the default) orFALSEto use authority compliant axis order. By default, inputgeomvertices are assumed to be in longitude/latitude order ifsrsis a geographic coordinate system. This can be overridden by settingtraditional_gis_order = FALSE.
Details
These functions use the GEOS library via GDAL headers.
g_area() computes the area for a Polygon or MultiPolygon. Undefined
for all other geometry types (returns zero). Returns a numeric vector,
having length equal to the number of input geometries, containing
computed area or '0' if undefined.
g_centroid() returns a numeric vector of length 2 containing the centroid
(X, Y), or a two-column numeric matrix (X, Y) with number of rows equal to
the number of input geometries.
The GDAL documentation states "This method relates to the SFCOM
ISurface::get_Centroid() method however the current implementation based
on GEOS can operate on other geometry types such as multipoint, linestring,
geometrycollection such as multipolygons. OGC SF SQL 1.1 defines the
operation for surfaces (polygons). SQL/MM-Part 3 defines the operation for
surfaces and multisurfaces (multipolygons)."
g_distance() returns the distance between two geometries or -1 if an
error occurs. Returns the shortest distance between the two geometries.
The distance is expressed into the same unit as the coordinates of the
geometries. Returns a numeric vector, having length equal to the number of
input geometry pairs, containing computed distance or '-1' if an error
occurs.
g_length() computes the length for LineString or MultiCurve objects.
Undefined for all other geometry types (returns zero). Returns a numeric
vector, having length equal to the number of input geometries, containing
computed length or '0' if undefined.
g_geodesic_area() computes geometry area, considered as a surface on the
underlying ellipsoid of the SRS attached to the geometry. The returned area
will always be in square meters, and assumes that polygon edges describe
geodesic lines on the ellipsoid. If the geometry SRS is not a geographic
one, geometries are reprojected to the underlying geographic SRS.
By default, input geometry vertices are assumed to be in longitude/latitude
order if using a geographic coordinate system. This can be overridden with
the traditional_gis_order argument.
Returns the area in square meters, or NA in case of error (unsupported
geometry type, no SRS attached, etc.)
Requires GDAL >= 3.9.
g_geodesic_length() computes the length of the curve, considered as a
geodesic line on the underlying ellipsoid of the SRS attached to the
geometry. The returned length will always be in meters. If the geometry SRS
is not a geographic one, geometries are reprojected to the underlying
geographic SRS.
By default, input geometry vertices are assumed to be in longitude/latitude
order if using a geographic coordinate system. This can be overridden with
the traditional_gis_order argument.
Returns the length in meters, or NA in case of error (unsupported geometry
type, no SRS attached, etc.)
Requires GDAL >= 3.10.
Note
For g_distance(), geom and other_geom must contain the same number of
geometries (i.e., operates pair-wise on the inputs with no recycling), and
are assumed to be in the same coordinate reference system.
Geometry validity is not checked. In case you are unsure of the validity
of the input geometries, call g_is_valid() before, otherwise the result
might be wrong.
Examples
g_area("POLYGON ((0 0, 10 10, 10 0, 0 0))")
#> [1] 50
g_centroid("POLYGON ((0 0, 10 10, 10 0, 0 0))")
#> x y
#> 6.666667 3.333333
g_distance("POINT (0 0)", "POINT (5 12)")
#> [1] 13
g_length("LINESTRING (0 0, 3 4)")
#> [1] 5
f <- system.file("extdata/ynp_fires_1984_2022.gpkg", package = "gdalraster")
lyr <- new(GDALVector, f, "mtbs_perims")
# read all features into a data frame
feat_set <- lyr$fetch(-1)
head(feat_set)
#> OGR feature set
#> FID event_id incid_name incid_type map_id burn_bnd_ac
#> 1 1 WY4413411069519870807 POLECAT Wildfire 10015934 1093
#> 2 2 WY4509511033019880815 HELLROARING Wildfire 10014146 77598
#> 3 3 MT4507511010219880619 STORM CREEK Wildfire 10014147 116027
#> 4 4 WY4491210990219880711 CLOVER Wildland Fire Use 10014202 34035
#> 5 5 WY4463411003119880709 MIST Wildfire 10014203 4376
#> 6 6 WY4499211096519880625 FAN Wildland Fire Use 10014215 20422
#> burn_bnd_lat burn_bnd_lon ig_date ig_year
#> 1 44.132 -110.696 1987-08-07 1987
#> 2 45.089 -110.328 1988-08-15 1988
#> 3 45.146 -110.013 1988-06-19 1988
#> 4 44.717 -110.034 1988-07-11 1988
#> 5 44.633 -110.028 1988-07-09 1988
#> 6 44.994 -110.976 1988-06-25 1988
#> geom
#> 1 WKB MULTIPOLYGON: raw 01 06 00 00 ...
#> 2 WKB MULTIPOLYGON: raw 01 06 00 00 ...
#> 3 WKB MULTIPOLYGON: raw 01 06 00 00 ...
#> 4 WKB MULTIPOLYGON: raw 01 06 00 00 ...
#> 5 WKB MULTIPOLYGON: raw 01 06 00 00 ...
#> 6 WKB MULTIPOLYGON: raw 01 06 00 00 ...
g_area(feat_set$geom) |> head()
#> [1] 3626011 219210971 261779718 137157336 17666635 82653959
g_centroid(feat_set$geom) |> head()
#> x y
#> [1,] 504185.6 -11945.72
#> [2,] 534834.7 86727.91
#> [3,] 553856.8 89236.52
#> [4,] 556496.3 51930.36
#> [5,] 557768.3 42471.93
#> [6,] 483922.9 82676.71
lyr$close()