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This function was designed to find lines that are close to parallel and perpendicular to some pre-defined route. It can return results that are absolute (contain information on the direction of turn, i.e. + or - values for clockwise/anticlockwise), bidirectional (which mean values greater than +/- 90 are impossible).


angle_diff(l, angle, bidirectional = FALSE, absolute = TRUE)



A spatial lines object


an angle in degrees relative to North, with 90 being East and -90 being West. (direction of rotation is ignored).


Should the result be returned in a bidirectional format? Default is FALSE. If TRUE, the same line in the oposite direction would have the same bearing


If TRUE (the default) only positive values can be returned


Building on the convention used in in the bearing() function from the geosphere package and in many applications, North is definied as 0, East as 90 and West as -90.


lib_versions <- sf::sf_extSoftVersion()
#>           GEOS           GDAL         proj.4 GDAL_with_GEOS     USE_PROJ_H 
#>       "3.10.2"        "3.4.1"        "8.2.1"         "true"         "true" 
#>           PROJ 
#>        "8.2.1" 
# fails on some systems (with early versions of PROJ)
if (lib_versions[3] >= "6.3.1") {
  # Find all routes going North-South
  lines_sf <- od2line(od_data_sample, zones = zones_sf)
  angle_diff(lines_sf[2, ], angle = 0)
  angle_diff(lines_sf[2:3, ], angle = 0)
#> Creating centroids representing desire line start and end points.
#> Linking to GEOS 3.10.2, GDAL 3.4.1, PROJ 8.2.1; sf_use_s2() is TRUE
#> [1]  92.2313 105.0539