angle_correction {ggfields}

Calculate correction for angle in the plot coordinate system

Description

The angle of a vector may be distorted when your plot uses a different coordinate system than the one for which the angle is specified. If data is a simple feature object (sf), the angle will be corrected for the displayed coordinate reference system (crs). When the crs is missing, an aspect ratio of 1 is assumed. For any other data, the angle is corrected for the aspect ratio in the plot.

Usage

angle_correction(data, panel_params, coord)

Arguments

data

fortified data used in a geom_fields(). Should at least contain numeric columns x, y and angle.

panel_params

panel parameters as returned by GeomFields$setup_params()

coord

A coord object.

Details

This function is used by default by geom_fields(). For more details on why this correction is required and how to customize corrections please see vignette("angle_correction").

Value

A data.frame with an additional angle_correction column. The corrected angle is given by angle_correction + angle.

Author(s)

Pepijn de Vries

Examples

## Create a data.frame with some xy-coordinates and all angles pointing North (0 degrees)
d <-
  data.frame(
    x = seq(1, 2, 0.1),
    y = seq(50, 51, 0.1),
    angle = 0
  ) |>
  sf::st_as_sf(coords = c("x", "y"), crs = 4326, remove = FALSE)

## Create a mockup of ggplot params. Normally this is handled automatically by ggplot2
params_mockup <-
  c(
    ggplot2::ggplot() + geom_fields(),
    list(
      x_range = range(d$x),
      y_range = range(d$y),
      crs = sf::st_crs(4326),
      default_crs = 4326
    )
  )

## When plotting as lon-lat, the angle correction will be zero
angle_correction(d, params_mockup, ggplot2::coord_sf(default_crs = 4326))

## Transform to UTM zone 31N in meters
d2 <- d |> sf::st_transform(32631)

## Again get parameter mockup values
params_mockup2 <-
  c(
    ggplot2::ggplot() + geom_fields(),
      list(
        x_range = range(sf::st_coordinates(d2)[,1]),
        y_range = range(sf::st_coordinates(d2)[,1]),
        crs = sf::st_crs(32631),
        default_crs = 4326
      )
    )
## in UTM projection in this area (which is slightly tilted) the correction is
## larger than zero
angle_correction(d2, params_mockup2,
                 ggplot2::coord_sf(crs = 32631, default_crs = 4326))