# 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:

- XY: Represented as line with LNG and LAT dimensions.
- XYZ: Represented as line with LNG, LAT and LEVEL/ALTITUDE/ELEVATION as third dimension.
- XYM: Represented as line with LNG, LAT and CUSTOM1/CUSTOM2 as third dimension.
- XYZM: Not supported.

## 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 3.10.2, GDAL 3.4.2, 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...

### 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: 8.249901 ymin: 49.66116 xmax: 8.926888 ymax: 50.33096
    #> Geodetic CRS:  WGS 84
    #> LINESTRING (8.260591 49.66466, 8.258051 49.6643...
    #> LINESTRING (8.724747 50.12882, 8.722207 50.1285...
    #> LINESTRING (8.369015 49.77308, 8.366475 49.7728...
    #> LINESTRING (8.444729 49.8488, 8.442189 49.84853...
    #> LINESTRING (8.926888 50.33096, 8.924348 50.3306...

    (sfc_enc <- encode_sf(sfc))
    #> [1] "BFkkkvJ2styB1B7PzIhaxL7Y" "BFk5-xJ2ho1B1B7PzIhaxL7Y"
    #> [3] "BF4p5vJqyizB1B7PzIhaxL7Y" "BFgjowJyrxzB1B7PzIhaxL7Y"
    #> [5] "BFwomzJixv2B1B7PzIhaxL7Y"

    decode_sf(sfc_enc, crs = 4326)
    #> Simple feature collection with 5 features and 2 fields
    #> Geometry type: LINESTRING
    #> Dimension:     XY
    #> Bounding box:  xmin: 8.2499 ymin: 49.66116 xmax: 8.92689 ymax: 50.33096
    #> Geodetic CRS:  WGS 84
    #>   id   dim3                       geometry
    #> 1  1 ABSENT LINESTRING (8.26059 49.6646...
    #> 2  2 ABSENT LINESTRING (8.72475 50.1288...
    #> 3  3 ABSENT LINESTRING (8.36901 49.7730...
    #> 4  4 ABSENT LINESTRING (8.44473 49.8488...
    #> 5  5 ABSENT LINESTRING (8.92689 50.3309...

### 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: 8.249901 ymin: 49.66116 xmax: 8.926888 ymax: 50.33096
    #> Geodetic CRS:  WGS 84
    #>   name color                       geometry
    #> 1    A  blue LINESTRING (8.260591 49.664...
    #> 2    B  blue LINESTRING (8.724747 50.128...
    #> 3    C green LINESTRING (8.369015 49.773...
    #> 4    D  blue LINESTRING (8.444729 49.848...
    #> 5    E   red LINESTRING (8.926888 50.330...

    (sf_enc <- encode_sf(sf))
    #> [1] "BFkkkvJ2styB1B7PzIhaxL7Y" "BFk5-xJ2ho1B1B7PzIhaxL7Y"
    #> [3] "BF4p5vJqyizB1B7PzIhaxL7Y" "BFgjowJyrxzB1B7PzIhaxL7Y"
    #> [5] "BFwomzJixv2B1B7PzIhaxL7Y"

    decode_sf(sf_enc, crs = 4326)
    #> Simple feature collection with 5 features and 2 fields
    #> Geometry type: LINESTRING
    #> Dimension:     XY
    #> Bounding box:  xmin: 8.2499 ymin: 49.66116 xmax: 8.92689 ymax: 50.33096
    #> Geodetic CRS:  WGS 84
    #>   id   dim3                       geometry
    #> 1  1 ABSENT LINESTRING (8.26059 49.6646...
    #> 2  2 ABSENT LINESTRING (8.72475 50.1288...
    #> 3  3 ABSENT LINESTRING (8.36901 49.7730...
    #> 4  4 ABSENT LINESTRING (8.44473 49.8488...
    #> 5  5 ABSENT LINESTRING (8.92689 50.3309...

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

- CRAN: Simple Features for R
- Wikipedia: Simple Features
- OGC: Simple Feature Access