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Source: vignettes/flexpolyline.Rmd

The flexpolyline R package provides a binding to the C++ implementation of the flexible polyline encoding by HERE. The flexible polyline encoding is a lossy compressed representation of a list of coordinate pairs or coordinate triples.

The encoding is achieved by:

  1. Reducing the decimal digits of each value;
  2. encoding only the offset from the previous point;
  3. using variable length for each coordinate delta; and
  4. using 64 URL-safe characters to display the result. The felxible polyline encoding is a variant of the Encoded Polyline Algorithm Format by Google.

Note:

  • Decoding gives reliable results up to a precision of 7 digits. The tests are also limited to this range.
  • The order of the coordinates (lng, lat) does not correspond to the original C++ implementation (lat, lng). This enables simple conversion to sf objects, without reordering the columns.
  • The encoding is lossy, this means the encoding process could reduce the precision of your data.

Installation

You can install the released version of flexpolyline from CRAN with:

install.packages("flexpolyline")

Install the development version from GitHub with:

remotes::install_github("munterfi/flexpolyline")

C++ binding

Encoding and decoding in R is straight forward by using encode() and decode(). These functions are binding to the flexpolyline C++ implementation and have similar arguments and return values as their counterparts (hf::encode_polyline and hf::decode_polyline):

library(flexpolyline)

(line <- matrix(
  c(
    8.69821, 50.10228, 10,
    8.69567, 50.10201, 20,
    8.69150, 50.10063, 30,
    8.68752, 50.09878, 40
  ),
  ncol = 3, byrow = TRUE
))
#>         [,1]     [,2] [,3]
#> [1,] 8.69821 50.10228   10
#> [2,] 8.69567 50.10201   20
#> [3,] 8.69150 50.10063   30
#> [4,] 8.68752 50.09878   40

(encoded <- encode(line))
#> [1] "B1Voz5xJ67i1Bgkh9B1B7Pgkh9BzIhagkh9BxL7Ygkh9B"

(decoded <- decode(encoded))
#>          LNG      LAT ELEVATION
#> [1,] 8.69821 50.10228        10
#> [2,] 8.69567 50.10201        20
#> [3,] 8.69150 50.10063        30
#> [4,] 8.68752 50.09878        40

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. The functions encode_sf() and decode_sf() provide an interface that support the encoding of sf objects:

(sfg <- sf::st_linestring(line, dim = "XYZ"))
#> LINESTRING Z (8.69821 50.10228 10, 8.69567 50.10201 20, 8.6915 50.10063 30, 8.68752 50.09878 40)

(encoded <- encode_sf(sfg))
#> [1] "B1Voz5xJ67i1Bgkh9B1B7Pgkh9BzIhagkh9BxL7Ygkh9B"

(decoded <- decode_sf(encoded))
#> 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 zmax: 40
#> CRS:           NA
#>   id      dim3                       geometry
#> 1  1 ELEVATION LINESTRING Z (8.69821 50.10...

References