Coordinate System with Geographic Limits Automatically Transformed to a Projection

`coord_proj()` extends ggplot2::coord_sf() by allowing users to specify map limits (`xlim`, `ylim`) in geographic coordinates (longitude/latitude, WGS84). These limits are automatically transformed into the target projected CRS, ensuring that maps display the intended region correctly under any projection.

This wrapper is particularly useful because coord_sf() interprets `xlim` and `ylim` as *projected* coordinates. Passing longitude/latitude directly to coord_sf() results in incorrect map extents unless the CRS is WGS84. `coord_proj()` provides a safe, projection-aware workflow that requires no manual calculation of projected bounding boxes.

Usage

coord_proj(
  crs = NULL,
  xlim = NULL,
  ylim = NULL,
  expand = TRUE,
  default_crs = "EPSG:4326",
  ...
)

Arguments

crs

Character string specifying the output coordinate reference system (e.g., `"EPSG:4326"`, `"EPSG:3857"`, or a PROJ string such as `"+proj=aeqd +lat_0=35 +lon_0=105"`). Required.

xlim

Numeric vector of length 2. Longitude limits in degrees (WGS84).

ylim

Numeric vector of length 2. Latitude limits in degrees (WGS84).

expand

Logical. Passed to coord_sf(). Default: `TRUE`.

default_crs

Character. CRS of `xlim` and `ylim`. Default: `"EPSG:4326"`.

...

Additional arguments passed to coord_sf().

Value

A ggplot2::coord_sf object with automatically transformed limits.

See also

ggplot2::coord_sf, geom_world, geom_mapcn

Examples

# Example 1: China (AEQD projection) with geographic limits
china_proj <- "+proj=aeqd +lat_0=35 +lon_0=105 +ellps=WGS84 +units=m +no_defs"

ggplot() +
  geom_world(crs = china_proj) +
  coord_proj(
    crs = china_proj,
    xlim = c(60, 140),
    ylim = c(-10, 50)
  ) +
  theme_minimal()



# Example 2: South China Sea region using geom_mapcn + geom_boundary_cn
ggplot() +
  geom_mapcn(fill = "white") +
  geom_boundary_cn() +
  theme_bw() +
  coord_proj(
    crs = china_proj,
    expand = FALSE,
    xlim = c(105, 126),   # lon range
    ylim = c(2, 23)       # lat range
  )