# coord_map: Map projections In ggplot2: Create Elegant Data Visualisations Using the Grammar of Graphics

## Description

`coord_map` projects a portion of the earth, which is approximately spherical, onto a flat 2D plane using any projection defined by the `mapproj` package. Map projections do not, in general, preserve straight lines, so this requires considerable computation. `coord_quickmap` is a quick approximation that does preserve straight lines. It works best for smaller areas closer to the equator.

## Usage

 ```1 2 3 4``` ```coord_map(projection = "mercator", ..., parameters = NULL, orientation = NULL, xlim = NULL, ylim = NULL) coord_quickmap(xlim = NULL, ylim = NULL, expand = TRUE) ```

## Arguments

 `projection` projection to use, see `mapproject` for list `..., parameters` Other arguments passed on to `mapproject`. Use `...` for named parameters to the projection, and `parameters` for unnamed parameters. `...` is ignored if the `parameters` argument is present. `orientation` projection orientation, which defaults to `c(90, 0, mean(range(x)))`. This is not optimal for many projections, so you will have to supply your own. See `mapproject` for more information. `xlim, ylim` Manually specific x/y limits (in degrees of longitude/latitude) `expand` If `TRUE`, the default, adds a small expansion factor to the limits to ensure that data and axes don't overlap. If `FALSE`, limits are taken exactly from the data or `xlim`/`ylim`.

## Details

In general, map projections must account for the fact that the actual length (in km) of one degree of longitude varies between the equator and the pole. Near the equator, the ratio between the lengths of one degree of latitude and one degree of longitude is approximately 1. Near the pole, it is tends towards infinity because the length of one degree of longitude tends towards 0. For regions that span only a few degrees and are not too close to the poles, setting the aspect ratio of the plot to the appropriate lat/lon ratio approximates the usual mercator projection. This is what `coord_quickmap` does, and is much faster (particularly for complex plots like `geom_tile`) at the expense of correctness.

## Examples

 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53``` ```if (require("maps")) { nz <- map_data("nz") # Prepare a map of NZ nzmap <- ggplot(nz, aes(x = long, y = lat, group = group)) + geom_polygon(fill = "white", colour = "black") # Plot it in cartesian coordinates nzmap # With correct mercator projection nzmap + coord_map() # With the aspect ratio approximation nzmap + coord_quickmap() # Other projections nzmap + coord_map("cylindrical") nzmap + coord_map("azequalarea", orientation = c(-36.92, 174.6, 0)) nzmap + coord_map("lambert", parameters = c(-37, -44)) states <- map_data("state") usamap <- ggplot(states, aes(long, lat, group = group)) + geom_polygon(fill = "white", colour = "black") # Use cartesian coordinates usamap # With mercator projection usamap + coord_map() usamap + coord_quickmap() # See ?mapproject for coordinate systems and their parameters usamap + coord_map("gilbert") usamap + coord_map("lagrange") # For most projections, you'll need to set the orientation yourself # as the automatic selection done by mapproject is not available to # ggplot usamap + coord_map("orthographic") usamap + coord_map("stereographic") usamap + coord_map("conic", lat0 = 30) usamap + coord_map("bonne", lat0 = 50) # World map, using geom_path instead of geom_polygon world <- map_data("world") worldmap <- ggplot(world, aes(x = long, y = lat, group = group)) + geom_path() + scale_y_continuous(breaks = (-2:2) * 30) + scale_x_continuous(breaks = (-4:4) * 45) # Orthographic projection with default orientation (looking down at North pole) worldmap + coord_map("ortho") # Looking up up at South Pole worldmap + coord_map("ortho", orientation = c(-90, 0, 0)) # Centered on New York (currently has issues with closing polygons) worldmap + coord_map("ortho", orientation = c(41, -74, 0)) } ```

### Example output

```Loading required package: maps
```

ggplot2 documentation built on May 30, 2017, 2:36 a.m.