In ocean-satellite-tools/basics: Some very basic raster manipulations

This example assumes that you have a sample raster. You can load the sample one or go to the download data vignette to download your own. sample_raster is a demo SST raster and data frame.

Load the sample data.

data("sample_raster", package="basics")
df <- sample_raster$df
ras <- sample_raster$raster
lons <- sample_raster$lons
lats <- sample_raster$lats

Load the needed packages for plotting.

require(raster)
require(ggplot2)

Download a coastline

There are a variety of places you can get a coastline.

You can download via raster.

coast <- raster::getData("GADM", country = "USA", level = 1)
wa_or_coast <- subset(usashp, NAME_1 %in% c("Washington", "Oregon"))

Or you could get it from rnaturalearth which is quite a bit faster. With scale=50, the coastline has some detail. You could pass in scale of 110 or 10.

coast <- rnaturalearth::ne_coastline(scale = 50, returnclass = "sp")

Crop and plot

I'll use rnaturalearth. The coast just downloaded is for the whole world. We'll want to crop that down to our region. Note I need to use library(raster) so that I have access to the plot methods for spatial objects.

library(raster)
wa_or_coast <- raster::crop(coast, raster::extent(lons[1], lons[2], lats[1], lats[2]))
plot(wa_or_coast)

Add on raster with base R

plot(ras)
plot(wa_or_coast, add=TRUE)

We can plot with ggplot2 also.

require(ggplot2)
# Plot
gg <- ggplot(df) +
  geom_raster(aes(lon, lat, fill = sst)) +
  scale_fill_gradient2(midpoint = mean(df$sst, na.rm = TRUE),
                       low = "blue",
                       mid = "white",
                       high = "red") +
  labs(x = NULL,
       y = NULL,
       fill = "Celcius",
       title = "Sea Surface Temperature (SST)")

The way that ggplot2 works is to run fortify() on the SpatialLines object to create a data frame. Then we use geom_path() to plot that. But if you look at the coast, you see lots of islands. We need to tell geom_path() that there are these groups of paths in the data frame.

gg <- gg + 
  geom_path(data=wa_or_coast,  aes(x=long,y=lat, grouping=id), size=1, na.rm=TRUE)
gg

cmocean palette

Let's use the cmocean package to use it's thermal palette.

library(cmocean)
gg + scale_fill_cmocean(alpha=1) + theme_bw()

We can use this for our raster plot too.

plot(ras, col = cmocean("thermal")(100))

Add a line parallel to the coastline

coast110 <- rnaturalearth::ne_coastline(scale = 110, returnclass = "sp")
coast110 <- raster::crop(coast110, raster::extent(lons[1], lons[2], lats[1], lats[2]))
offcoast <- raster::shift(coast110, dx=-2)

plot(ras)
plot(wa_or_coast, add=TRUE)
plot(offcoast, add=TRUE, col="red")

The default raster plot is a bit deformed since it is long-lat on the x and y axis. We can see what it would look like in a different projection.

newcrs <- "+proj=wintri +lon_0=-125 +lat_1=46 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs"
ras_win <- projectRaster(ras, crs=newcrs, over=T)
plot(ras_win)
plot(spTransform(wa_or_coast, newcrs), add=TRUE)
title(stringr::str_sub(newcrs, 1, 10))