Advanced scale bar and compass annotations

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1. Setup

This vignette demonstrates how to use the advanced customization options available in annotation_scalebar() and annotation_compass() from the ggmapcn package.

We will use the standard North Carolina shapefile provided by the sf package as our base map.

library(ggplot2)
library(sf)
#> Linking to GEOS 3.11.2, GDAL 3.8.2, PROJ 9.3.1; sf_use_s2() is TRUE
library(ggmapcn)

# Load example data
nc <- sf::st_read(system.file("shape/nc.shp", package = "sf"), quiet = TRUE)

# Create a base map object
base_geo <- ggplot() +
  geom_sf(data = nc, fill = "grey90", color = "grey40") +
  theme_minimal()

base_geo

A base map of North Carolina with grey fill and minimal theme.

2. Scale bars in a projected CRS

Scale bars are most accurate when used with a projected Coordinate Reference System (CRS). Here, we project the map to EPSG:32617 (UTM Zone 17N).

You can control the position using location (e.g., "bl" for bottom-left, "tr" for top-right) and adjust the relative width using width_hint.

base_proj <- base_geo +
  coord_sf(crs = 32617) +
  theme(axis.title = element_blank())

# Default scale bar at bottom-left
base_proj +
  annotation_scalebar(location = "bl")

Two maps of North Carolina. The first shows a scale bar in the bottom left. The second shows a wider scale bar in the top right.


# Top-right scale bar with custom width hint
base_proj +
  annotation_scalebar(
    location   = "tr",
    width_hint = 0.5
  )

Two maps of North Carolina. The first shows a scale bar in the bottom left. The second shows a wider scale bar in the top right.

3. Scale bar styles

annotation_scalebar() supports multiple styles: "segment", "ticks", and "bar". You can also customize colors and text visibility.

# 1. Segment style
p_segment <- base_proj +
  annotation_scalebar(
    location = "bl",
    style    = "segment",
    label_show = "all"
  )
p_segment

A map showing the 'segment' style scale bar.

# 2. Ticks style
p_ticks <- base_proj +
  annotation_scalebar(
    location = "br",
    style    = "ticks"
  )
p_ticks

A map showing the 'ticks' style scale bar.

# 3. Bar style with custom colors
p_bar <- base_proj +
  annotation_scalebar(
    location = "tl",
    style    = "bar",
    bar_cols = c("black", "white")
  )
p_bar

A map showing the 'bar' style scale bar with black and white blocks.

You can also set a fixed distance width (e.g., 100km) and change the line colors:

base_proj +
  annotation_scalebar(
    location     = "bl",
    fixed_width  = 100000,   # 100 km in meters
    display_unit = "km",
    line_col     = "red"
  )

A map with a red scale bar fixed to 100km length.

4. Geographic CRS: Approx distance vs. Degrees

When plotting unprojected data (Latitude/Longitude, EPSG:4326), accurate distance calculation is difficult.

geographic_mode = "approx_m": Attempts to calculate distances in meters (approximation).
geographic_mode = "degrees": Shows the scale in decimal degrees.

# Approximate meters
base_geo +
  coord_sf(crs = 4326) +
  annotation_scalebar(
    location        = "bl",
    geographic_mode = "approx_m"
  )
#> Warning: Scale bar is approximate in geographic CRS (degrees). Distances vary
#> with latitude. For accuracy, use a projected CRS, or set `geographic_mode =
#> "degrees"`.

Two maps comparing scale bars. One shows distance in kilometers, the other in degrees.


# Degrees
base_geo +
  coord_sf(crs = 4326) +
  annotation_scalebar(
    location        = "bl",
    geographic_mode = "degrees"
  )

Two maps comparing scale bars. One shows distance in kilometers, the other in degrees.

5. Basic compass (Grid North)

Add a north arrow using annotation_compass(). You can adjust the size and padding using grid::unit().

# Default classic arrow
base_geo +
  annotation_compass(
    location = "tl",
    style    = north_arrow_classic()
  )

Two maps showing basic north arrows. One is standard size, the other is larger with padding.


# Custom size and padding
base_geo +
  annotation_compass(
    location = "tl",
    height   = grid::unit(0.8, "cm"),
    width    = grid::unit(0.8, "cm"),
    pad_x    = grid::unit(0.3, "cm"),
    pad_y    = grid::unit(0.3, "cm")
  )

Two maps showing basic north arrows. One is standard size, the other is larger with padding.

6. True North vs. Grid North

In many projections (like Lambert Conformal Conic), “Grid North” (straight up on the page) is different from “True North” (the direction to the North Pole).

Use which_north = "true" to automatically calculate the convergence angle and rotate the compass.

# Define a Lambert Conformal Conic projection
base_lcc <- base_geo +
  coord_sf(crs = "+proj=lcc +lon_0=-100 +lat_1=33 +lat_2=45") +
  theme(axis.title = element_blank())

# True North (automatically rotated)
base_lcc +
  annotation_compass(
    which_north = "true",
    location    = "br",
    style       = compass_rose_simple()
  )

Two maps demonstrating north arrows. The first shows a rotated compass rose pointing to True North. The second shows a manually rotated Grid North arrow.


# Grid North with manual rotation
base_lcc +
  annotation_compass(
    which_north = "grid",
    rotation    = 30,
    location    = "tr",
    style       = north_arrow_solid()
  )

Two maps demonstrating north arrows. The first shows a rotated compass rose pointing to True North. The second shows a manually rotated Grid North arrow.

7. Compass styles

ggmapcn provides various styles, including the unique compass_sinan() (referencing the ancient Chinese compass).

p_classic <- base_proj +
  annotation_compass(
    location = "tl",
    style    = north_arrow_classic()
  )

p_rose <- base_proj +
  annotation_compass(
    location = "br",
    style    = compass_rose_classic()
  )

p_sinan <- base_proj +
  annotation_compass(
    location = "tl",
    style    = compass_sinan()
  )

p_classic

Three maps showing different compass styles: Classic, Rose, and Sinan (Si Nan).

p_rose

Three maps showing different compass styles: Classic, Rose, and Sinan (Si Nan).

p_sinan

Three maps showing different compass styles: Classic, Rose, and Sinan (Si Nan).

8. Combining scale bar and compass

Finally, you can layer both annotations onto a single map.

base_proj +
  annotation_scalebar(
    location   = "bl",
    style      = "ticks",
    width_hint = 0.3
  ) +
  annotation_compass(
    location = "tl",
    style    = compass_rose_circle()
  )

A final map combining a ticks-style scale bar at the bottom left and a compass rose at the top left.

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They may not be fully stable and should be used with caution. We make no claims about them.
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