Manipulating Elevation Data from the USGS National Map

Question

I am trying out @tylermorganwall Rayshader R library. Some instructions are given here by tylermw8 to get elevation data from the USGS National Map.

I am interested in obtaining data around the Golden Gate Bridge. The data I download from from USGS National Map (n38w123) is, I believe, a one degree square tile which is 60 miles N/S and about 53 miles W/E. That's too much data for playing around with his rayshader library.

The bounding square of interest is:

(37.88, -122.56) (37.88, -122.41)

(37.77, -122.56) (37.77, -122.41)

How do I 'cut' this elevation data out of the large download from USGS? Is this where the library(sf) comes in handy?

Answer 1

Yes, it does look like the USGS National Map page is down. I think creating a simple feature dataframe and using that with raster::crop is what I was looking for.

library(raster)
library(sf)

# Bay area data: long_min = -123.0056, long_max = -121.99944, lat_min = 36.99944, lat_max = 38.00056
sf_bay_data = raster::raster("data/goldengate/n38w123/imgn38w123_13.img")

# Create Simple Feature Geometry (sfg) Polygon around GG Bridge
gg_sfg <- st_polygon(list(cbind(c(-122.56, -122.56, -122.41, -122.41, -122.56), c(37.88, 37.77, 37.77, 37.88, 37.88))))

# Add projection info by creating an sfc object
gg_sfc <- st_sfc(gg_sfg, crs = "+init=epsg:4326")

# Create simple feature dataframe
gg_sf <- st_sf(gg_sfc)

# Crop SF Bay data to zoom in around GG Bridge
gg_data <- raster::crop(sf_bay_data, gg_sf)