albers_fwd: Albers Equal Area projection

In geographiclib: Access to 'GeographicLib'

Albers Equal Area projection

Description

Convert geographic coordinates to/from Albers Equal Area conic projection. This is an equal-area projection commonly used for thematic maps of regions with greater east-west extent.

Usage

albers_fwd(
  x,
  lon0,
  stdlat = NULL,
  stdlat1 = NULL,
  stdlat2 = NULL,
  k0 = 1,
  k1 = 1
)

albers_rev(
  x,
  y,
  lon0,
  stdlat = NULL,
  stdlat1 = NULL,
  stdlat2 = NULL,
  k0 = 1,
  k1 = 1
)

Arguments

x	For forward conversion: a two-column matrix or data frame of coordinates (longitude, latitude) in decimal degrees. For reverse conversion: numeric vector of x (easting) coordinates in meters.
lon0	Central meridian in decimal degrees. Can be a vector to specify different central meridians for each point.
stdlat	Standard parallel for single standard parallel projections (e.g., Lambert Cylindrical Equal Area when stdlat = 0).
stdlat1, stdlat2	First and second standard parallels in decimal degrees for two standard parallel projections.
k0	Scale factor at the standard parallel(s). Default is 1.
k1	Scale factor at the first standard parallel for two standard parallel projections. Default is 1.
y	Numeric vector of y (northing) coordinates in meters (reverse only).

Details

The Albers Equal Area conic projection preserves area, making it ideal for:

• Thematic/choropleth maps where area comparison matters

• Continental-scale maps (e.g., USGS maps of CONUS)

• Statistical mapping and analysis

Common configurations:

• CONUS: stdlat1 = 29.5, stdlat2 = 45.5, lon0 = -96

• Australia: stdlat1 = -18, stdlat2 = -36, lon0 = 132

• Europe: stdlat1 = 43, stdlat2 = 62, lon0 = 10

Special cases:

• When stdlat1 = -stdlat2, the projection becomes Lambert Cylindrical Equal Area

• When stdlat1 = stdlat2 = 0, it becomes the cylindrical equal-area projection

The lon0 parameter is vectorized, allowing different central meridians for each point.

Value

Data frame with columns:

• For forward conversion:

  • x: Easting in meters

  • y: Northing in meters

  • convergence: Grid convergence in degrees

  • scale: Scale factor at the point

  • lon, lat: Input coordinates (echoed)

  • lon0: Central meridian (echoed)

• For reverse conversion:

  • lon: Longitude in decimal degrees

  • lat: Latitude in decimal degrees

  • convergence: Grid convergence in degrees

  • scale: Scale factor at the point

  • x, y: Input coordinates (echoed)

  • lon0: Central meridian (echoed)

See Also

lcc_fwd() for Lambert Conformal Conic (conformal, not equal-area).

Examples

# CONUS Albers Equal Area
pts <- cbind(lon = c(-122, -74, -90), lat = c(37, 41, 30))
albers_fwd(pts, lon0 = -96, stdlat1 = 29.5, stdlat2 = 45.5)

# Australia
aus <- cbind(lon = c(151.2, 115.9, 153.0), lat = c(-33.9, -32.0, -27.5))
albers_fwd(aus, lon0 = 132, stdlat1 = -18, stdlat2 = -36)

# Antarctic projection
ant <- cbind(lon = c(166.67, 77.97, -43.53), lat = c(-77.85, -67.60, -60.72))
albers_fwd(ant, lon0 = 0, stdlat1 = -72, stdlat2 = -60)

# Round-trip conversion
fwd <- albers_fwd(pts, lon0 = -96, stdlat1 = 29.5, stdlat2 = 45.5)
albers_rev(fwd$x, fwd$y, lon0 = -96, stdlat1 = 29.5, stdlat2 = 45.5)

# Single standard parallel (cylindrical-like)
albers_fwd(pts, lon0 = -96, stdlat = 37)

geographiclib documentation built on March 4, 2026, 9:07 a.m.