In afsc-gap-products/akgfmaps: Alaska Groundfish and Ecosystem Survey Area Mapping

knitr::opts_chunk$set(echo = TRUE)

Get base mapping layers

The get_base_layers() function retrieves shapefiles and other map features that are typically used to make plots of RACE survey areas. The function transforms layers to a user-specified projection based on a coordinate reference system. Alternatively, a default coordinate reference system can be assigned by the function, based on the region.

Retrieve layers with projection automatically selected

This chunk of code retrieves layers for the EBS shelf survey area and automatically assigns a projection (Albers Equal Area with standard latitudes at 55 °N and 60 °N, cetner latitude 57.5 °N, center longitude at 170 °W ...this is relevant below). Layers are returned in a list.

library(akgfmaps)

SEBS <- akgfmaps::get_base_layers(select.region = "bs.south", set.crs = "auto")
class(SEBS)
names(SEBS)

The list returned by get_base_layers contains: akland = Alaska land (class sf). survey.area = Survey area boundary (class sf). bathymetry = Bathymetric contours. EBS shelf default: 50 m, 100 m, 200 m (class sf). place.labels = A data frame with labels and plotting locations (class data.frame). graticule = Graticule (class sf). crs = The coordinate reference system for the layers (class crs). plot.boundary = Suggested plot boundary (class data.frame). lon.breaks = Latitude breaks corresponding with graticule. *lat.breaks = Longitude breaks corresponding with graticule.

Make a map

Here is a map that uses all of the layers except the plot labels.

(SEBS_plot <- ggplot() +
  geom_sf(data = SEBS$akland) +
  geom_sf(data = SEBS$bathymetry) +
  geom_sf(data = SEBS$survey.area, fill = NA) +
  geom_sf(data = SEBS$graticule, color = "grey70", alpha = 0.5) +
  coord_sf(xlim = SEBS$plot.boundary$x, 
           ylim = SEBS$plot.boundary$y) +
  scale_x_continuous(name = "Longitude", 
                     breaks = SEBS$lon.breaks) + 
  scale_y_continuous(name = "Latitude", 
                     breaks = SEBS$lat.breaks) + 
  theme_bw())

Adding your own layers

Although the map shows the x-axis and y-axis as latitude and longitude with degrees, coordinates for all of the layers are in meters.

Therefore, adding text to the map at 58 N, 165 W using coordinates x = -165, y = 58 results in a label centered 165 m (longitudinally) and 58 m (latitudinally) away from the center of the projection.

my_label <- data.frame(x = -165, y = 58, label = "Thar be\ndragons")

SEBS_plot + geom_text(data = my_label, 
                      mapping = aes(x = x, 
                                    y = y, 
                                    label = label), 
                      color = "red")

The akgfmaps package includes a function to convert data frame coordinates to the projection without changing the data frame type.

my_label <- data.frame(x = -165, y = 58, label = "Thar be\ndragons") |> 
  akgfmaps::transform_data_frame_crs(in.crs = "+proj=longlat", out.crs = SEBS$crs)

SEBS_plot + geom_text(data = my_label, 
                      mapping = aes(x = x, 
                                    y = y, 
                                    label = label), 
                      color = "red")

Add labels

Formatted placename labels can be added directly to the plot using akgfmaps::add_map_labels(), although it is suggested to add labels manually to avoid formatting issues.

(SEBS_plot |> add_map_labels(region = "bs.south", new.places = SEBS$place.labels))

afsc-gap-products/akgfmaps documentation built on April 14, 2025, 7:13 p.m.