coord_proj: Coordinate System with Geographic Limits Automatically...

In ggmapcn: Customizable China and Global Map Visualizations

Coordinate System with Geographic Limits Automatically Transformed to a Projection

Description

'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.

Usage

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

Arguments

crs	Character string or object specifying the output coordinate reference system (e.g., '"EPSG:3857"', '"+proj=robin"', or an 'sf::crs' object). **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 [ggplot2::coord_sf()]. Default is 'TRUE'.
default_crs	Character or object. The CRS of the input 'xlim' and 'ylim'. Default is '"EPSG:4326"' (WGS84).
...	Additional arguments passed to [ggplot2::coord_sf()].

Details

This wrapper is particularly useful because [ggplot2::coord_sf()] interprets 'xlim' and 'ylim' as *projected* coordinates (in the units of the target CRS). Passing longitude/latitude directly to 'coord_sf()' results in incorrect map extents unless the output CRS is also WGS84.

'coord_proj()' provides a safe, projection-aware workflow that calculates the bounding box in WGS84, transforms it to the target CRS, and passes the new limits to 'coord_sf()'.

Value

A 'CoordSf' object (specifically a result of 'coord_sf()') with automatically transformed limits.

See Also

* [ggplot2::coord_sf()] for the underlying function. * [geom_world()] for the basemap layer.

Examples

library(ggplot2)


# 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: Zooming into a specific region
# Even though the map is projected (Robinson), we specify limits in Lat/Lon
crs_robin <- "+proj=robin +lon_0=0 +datum=WGS84"

ggplot() +
  geom_world(crs = crs_robin) +
  coord_proj(
    crs = crs_robin,
    xlim = c(-20, 50), # Focus on Africa/Europe
    ylim = c(-40, 40)
  ) +
  theme_minimal()

ggmapcn documentation built on Nov. 23, 2025, 9:06 a.m.