A common way to deal with spatial data in R is the sf package, which is built on the concept of simple features. According to the the sf package vignette, simple features are “…a formal standard (ISO 19125-1:2004) that describes how objects in the real world can be represented in computers, with emphasis on the spatial geometry of these objects. It also describes how such objects can be stored in and retrieved from databases, and which geometrical operations should be defined for them.”
The most common geometry types of simple features are: POINT, LINESTRING, POLYGON, MULTIPOINT, MULTILINESTRING, MULTIPOLYGON. All geometry types are based on POINTs. This package supports the encoding and decoding of the geometry types POINT, LINESTRING and POLYGON.
There are four possible dimension combinations of geometries in the
sf package. In the flexpolyine package the first three
dimension combinations are supported and represented as follows:
If no value for third_dim is provided in
encode_sf(), the dimensions of the sf geometry are
considered. In case of "XYZ" the third dimension is set to
"ELEVATION" and thereby recognized again as
"XYZ" by decode_sf() during decoding:
library(flexpolyline)
library(sf)
#> Linking to GEOS 3.10.2, GDAL 3.4.1, PROJ 8.2.1; sf_use_s2() is TRUE
coords <- matrix(
c(
8.69821, 50.10228, 10.11111,
8.69567, 50.10201, 20.22222,
8.69150, 50.10063, 30.33333,
8.68752, 50.09878, 40.44444
),
ncol = 3, byrow = TRUE
)
(sfg_z <- st_linestring(coords, dim = "XYZ"))
#> LINESTRING Z (8.69821 50.10228 10.11111, 8.69567 50.10201 20.22222, 8.6915 50.10063 30.33333, 8.68752 50.09878 40.44444)
(sfg_enc_z <- encode_sf(sfg_z))
#> [1] "B1Voz5xJ67i1Bu629B1B7Pu629BzIhau629BxL7Yu629B"
decode_sf(sfg_enc_z)
#> Simple feature collection with 1 feature and 2 fields
#> Geometry type: LINESTRING
#> Dimension: XYZ
#> Bounding box: xmin: 8.68752 ymin: 50.09878 xmax: 8.69821 ymax: 50.10228
#> z_range: zmin: 10.11111 zmax: 40.44444
#> CRS: NA
#> id dim3 geometry
#> 1 1 ELEVATION LINESTRING Z (8.69821 50.10...In case of "XYM" the third dimension is set to
"CUSTOM1" and again set to "XYM" in
decoding:
(sfg_m <- st_linestring(coords, dim = "XYM"))
#> LINESTRING M (8.69821 50.10228 10.11111, 8.69567 50.10201 20.22222, 8.6915 50.10063 30.33333, 8.68752 50.09878 40.44444)
(sfg_enc_m <- encode_sf(sfg_m))
#> [1] "BlXoz5xJ67i1Bu629B1B7Pu629BzIhau629BxL7Yu629B"
decode_sf(sfg_enc_m)
#> Simple feature collection with 1 feature and 2 fields
#> Geometry type: LINESTRING
#> Dimension: XYM
#> Bounding box: xmin: 8.68752 ymin: 50.09878 xmax: 8.69821 ymax: 50.10228
#> m_range: mmin: 10.11111 mmax: 40.44444
#> CRS: NA
#> id dim3 geometry
#> 1 1 CUSTOM1 LINESTRING M (8.69821 50.10...The geometry column in simple feature data sets is called simple
feature geometry list-column. The sfc object has a Coordinate Reference
System (CRS) assigned. In the case of longitude and latitude data this
is most often WGS84 (EPSG: 4326). Objects of type sfc are
supported as input in encoding:
(sfc <- st_as_sfc(
lapply(seq(1, 5), function(x) {
st_linestring(coords[, 1:2] + runif(1, -1, 1), dim = "XY")
}),
crs = 4326
))
#> Geometry set for 5 features
#> Geometry type: LINESTRING
#> Dimension: XY
#> Bounding box: xmin: 7.887789 ymin: 49.29905 xmax: 9.664401 ymax: 51.06847
#> Geodetic CRS: WGS 84
#> LINESTRING (7.898479 49.30255, 7.895939 49.3022...
#> LINESTRING (8.727055 50.13113, 8.724515 50.1308...
#> LINESTRING (8.645169 50.04924, 8.642629 50.0489...
#> LINESTRING (8.226536 49.63061, 8.223996 49.6303...
#> LINESTRING (9.664401 51.06847, 9.661861 51.0682...
(sfc_enc <- encode_sf(sfc))
#> [1] "BF-s9sJw1mwB1B7PzIhaxL7Y" "BFyn_xJkwo1B1B7PzIhaxL7Y"
#> [3] "BF4nvxJqw40B1B7PzIhaxL7Y" "BFqv9uJ83myB1B7PzIhaxL7Y"
#> [5] "BF-p23Jwy_6B1B7PzIhaxL7Y"
decode_sf(sfc_enc, crs = 4326)
#> Simple feature collection with 5 features and 2 fields
#> Geometry type: LINESTRING
#> Dimension: XY
#> Bounding box: xmin: 7.88779 ymin: 49.29905 xmax: 9.6644 ymax: 51.06847
#> Geodetic CRS: WGS 84
#> id dim3 geometry
#> 1 1 ABSENT LINESTRING (7.89848 49.3025...
#> 2 2 ABSENT LINESTRING (8.72706 50.1311...
#> 3 3 ABSENT LINESTRING (8.64517 50.0492...
#> 4 4 ABSENT LINESTRING (8.22654 49.6306...
#> 5 5 ABSENT LINESTRING (9.6644 51.06847...A simple feature is a geometry that is connected with further
characteristics (other than it’s coordinates), which in practice means a
data.frame (also data.table or
tibble) with an sfc column. The
flexpolyline package only takes care of the coordinates
of the LINESTRINGs in the sf object, other columns and the CRS
information are not encoded and stored in the string:
(sf <- st_as_sf(
data.frame(
name = c("A", "B", "C", "D", "E"),
color = sample(c("red", "green", "blue"), 5, replace = TRUE),
geometry = sfc
)
))
#> Simple feature collection with 5 features and 2 fields
#> Geometry type: LINESTRING
#> Dimension: XY
#> Bounding box: xmin: 7.887789 ymin: 49.29905 xmax: 9.664401 ymax: 51.06847
#> Geodetic CRS: WGS 84
#> name color geometry
#> 1 A red LINESTRING (7.898479 49.302...
#> 2 B red LINESTRING (8.727055 50.131...
#> 3 C blue LINESTRING (8.645169 50.049...
#> 4 D green LINESTRING (8.226536 49.630...
#> 5 E red LINESTRING (9.664401 51.068...
(sf_enc <- encode_sf(sf))
#> [1] "BF-s9sJw1mwB1B7PzIhaxL7Y" "BFyn_xJkwo1B1B7PzIhaxL7Y"
#> [3] "BF4nvxJqw40B1B7PzIhaxL7Y" "BFqv9uJ83myB1B7PzIhaxL7Y"
#> [5] "BF-p23Jwy_6B1B7PzIhaxL7Y"
decode_sf(sf_enc, crs = 4326)
#> Simple feature collection with 5 features and 2 fields
#> Geometry type: LINESTRING
#> Dimension: XY
#> Bounding box: xmin: 7.88779 ymin: 49.29905 xmax: 9.6644 ymax: 51.06847
#> Geodetic CRS: WGS 84
#> id dim3 geometry
#> 1 1 ABSENT LINESTRING (7.89848 49.3025...
#> 2 2 ABSENT LINESTRING (8.72706 50.1311...
#> 3 3 ABSENT LINESTRING (8.64517 50.0492...
#> 4 4 ABSENT LINESTRING (8.22654 49.6306...
#> 5 5 ABSENT LINESTRING (9.6644 51.06847...Note: The columns "name" and
"color" were not encoded and are missing after decoding.
The CRS has to be provided in the decoding by the crs
argument. Otherwise it is set to sf::NA_crs_.