ggmapcn (version 0.3.0)

coord_proj: 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",
  ...
)

Value

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

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()`.

See Also

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

Examples

library(ggplot2)

# \donttest{
# 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()
# }