Simple feature support

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:

Encoding and decoding

Simple feature geometry (sfg)

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 3.9.0, GDAL 3.2.1, PROJ 7.2.1

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...

Simple feature geometry list-column (sfc)

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: 7.711615 ymin: 49.12288 xmax: 9.690838 ymax: 51.09491
#> Geodetic CRS:  WGS 84
#> LINESTRING (7.722305 49.12638, 7.719765 49.1261...
#> LINESTRING (8.540193 49.94426, 8.537653 49.9439...
#> LINESTRING (8.589667 49.99374, 8.587127 49.9934...
#> LINESTRING (9.555304 50.95937, 9.552764 50.9591...
#> LINESTRING (9.690838 51.09491, 9.688298 51.0946...

(sfc_enc <- encode_sf(sfc))
#> [1] "BF8_6rJuokvB1B7PzIhaxL7Y" "BF036wJmgk0B1B7PzIhaxL7Y"
#> [3] "BF8skxJu1t0B1B7PzIhaxL7Y" "BFigh3J0oq6B1B7PzIhaxL7Y"
#> [5] "BFmv73J43k7B1B7PzIhaxL7Y"

decode_sf(sfc_enc, crs = 4326)
#> Simple feature collection with 5 features and 2 fields
#> Geometry type: LINESTRING
#> Dimension:     XY
#> Bounding box:  xmin: 7.71162 ymin: 49.12288 xmax: 9.69084 ymax: 51.09491
#> Geodetic CRS:  WGS 84
#>   id   dim3                       geometry
#> 1  1 ABSENT LINESTRING (7.72231 49.1263...
#> 2  2 ABSENT LINESTRING (8.54019 49.9442...
#> 3  3 ABSENT LINESTRING (8.58967 49.9937...
#> 4  4 ABSENT LINESTRING (9.5553 50.95937...
#> 5  5 ABSENT LINESTRING (9.69084 51.0949...

Simple feature (sf)

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: 7.711615 ymin: 49.12288 xmax: 9.690838 ymax: 51.09491
#> Geodetic CRS:  WGS 84
#>   name color                       geometry
#> 1    A  blue LINESTRING (7.722305 49.126...
#> 2    B   red LINESTRING (8.540193 49.944...
#> 3    C green LINESTRING (8.589667 49.993...
#> 4    D green LINESTRING (9.555304 50.959...
#> 5    E green LINESTRING (9.690838 51.094...

(sf_enc <- encode_sf(sf))
#> [1] "BF8_6rJuokvB1B7PzIhaxL7Y" "BF036wJmgk0B1B7PzIhaxL7Y"
#> [3] "BF8skxJu1t0B1B7PzIhaxL7Y" "BFigh3J0oq6B1B7PzIhaxL7Y"
#> [5] "BFmv73J43k7B1B7PzIhaxL7Y"

decode_sf(sf_enc, crs = 4326)
#> Simple feature collection with 5 features and 2 fields
#> Geometry type: LINESTRING
#> Dimension:     XY
#> Bounding box:  xmin: 7.71162 ymin: 49.12288 xmax: 9.69084 ymax: 51.09491
#> Geodetic CRS:  WGS 84
#>   id   dim3                       geometry
#> 1  1 ABSENT LINESTRING (7.72231 49.1263...
#> 2  2 ABSENT LINESTRING (8.54019 49.9442...
#> 3  3 ABSENT LINESTRING (8.58967 49.9937...
#> 4  4 ABSENT LINESTRING (9.5553 50.95937...
#> 5  5 ABSENT LINESTRING (9.69084 51.0949...

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_.

References