Datum and axis breaks

[dieghernan]

From an email of Ken Nussear @knussear:

Hi - thanks for the great package. I have a question for modifying the number of axis ticks - I'm getting output like seen in your vignette - where the x-axis overwrites itself. (Also I notice my labels are in lat long, when my data were UTM). Is there a way to reduce the number of ticks? I tried using scale_x_continuous with lower n.breaks values - but the plot gives an error.
Thanks

Ken

[dieghernan]

My take on this:

Modifying the labels to avoid conversion to lon/lat

This can ve avoided by using coord_sf(datum) option. Just use the original datum defined on the CRS of the SpatRaster file coord_sf(datum = terra::crs(volcano2)):

library(tidyterra)
library(terra)
library(ggplot2)

f_volcano <- system.file("extdata/volcano2.tif", package = "tidyterra")
volcano2 <- rast(f_volcano)

p <- ggplot() +
  geom_spatraster(data = volcano2) +
  geom_spatraster_contour(data = volcano2, breaks = seq(80, 200, 5)) +
  scale_fill_whitebox_c() +
  labs(fill = "elevation")

p

# Keep datum
p +
  coord_sf(datum = terra::crs(volcano2))

Modifying the number of axis ticks

This is interesting. It depends on what datum is in use by ggplot2. If no datum is provided, ggplot defaults to WGS84, so my approach is:

• Project the SpatRaster to EPSG:4326 (datum WGS84) and get the extent.
• Create a sequence of breaks between the values of the extent (xmin xmax in this case)
• Use scale_x_continuous(breaks)

# Change axis ticks
# 2 situations: Datum by default WGS84 (lon,lat)

ext_lonlat <- as.double(ext(project(volcano2, "EPSG:4326"))@ptr$vector)
x_breaks_lonlat <- round(seq(ext_lonlat[1], ext_lonlat[2], length.out = 3), 3)


p +
  scale_x_continuous(breaks = x_breaks_lonlat)

If we want the axis to be in the original datum of the SpatRaster, the approach is similar but the projection step is not needed:

# Axis on original datum
ext <- as.double(terra::ext(volcano2)@ptr$vector)[1:2]
x_breaks <- seq(ext[1], ext[2], length.out = 3)


p +
  coord_sf(datum = terra::crs(volcano2)) +
  scale_x_continuous(breaks = x_breaks)

^{Created on 2022-09-21 by the reprex package (v2.0.1)}