Spatial data and analysis in R using terra

Spatial analysis and the handling of spatial data have long been powerful capabilities in R. However, recently the dominant libraries providing these capabilities (including sp, raster, rgdal, ...) have been declared obsolete and were removed from current versions of R. New replacement packages include terra and sf. This move has advantages in that it removed a mix of incompatible tools, forcing standardization, and boosting the ability to use larger, modern datasets. It does mean, however, that there are a lot of tutorials and other documentation on the Internet or in books about spatial data analysis which is now also out of date.

This tutorial aims to help fix that situation. In the spirit of most of the tutorials on this site, it will use only open data, and while we focus on data centred on Carleton University, the same methods should work on similar data from anywhere else in the world. It was developed using R-4.3.1, and I am running it within the RStudio environment, which will show up in screenshots, and there are a few setup instructions given that are specific to RStudio. Besides that, the actual processing will be demonstrated using just console-based R commands, so they should work in any R environment.

This is written assuming that you have general computer / file management skills. We don't assume you already know how to use R, but we also don't go out of our way to explain it. A good general introduction is available at the RSpatial site (click here). We also assume that you already know some basics of spatial data, such as what is meant by vector and raster data, projections, and coordinate reference systems.

Get data

To start with, we need some data. First, we will use some Landsat imagery, from the open archive provided by USGS. To speed things up, I have searched for images free from cloud cover, and clipped an image surrounding Carleton University. This file contains that image in GeoTIFF format. Download that file into a folder of your choice.

Next, we will get some vector data from the City of Ottawa Open Data; there's a lot there you can explore, but to get started, we will download Parks and Greenspace data and Ottawa Neighbourhoods into the same folder you used above. Extract the contents of any .zip files.

Start R using RStudio and set environment

Launch RStudio on your computer (e.g. on a Windows computer, find it on the Start Menu; on a macOS computer, it's probably in your Applications folder). On the File menu, choose New Project. Choose the Existing Directory option, and then point it to the directory where you saved your data files, above. You should then have a blank RStudio project, set to use your data directory as the working directory for all work in R (you could use the getwd() command to double check this).

Try loading the Terra library (we will discuss more on what this does below) by typing the following at the prompt in the RStudio Console, and pressing Enter:

library("terra")

If you get an error stating that the library was not found, you will need to install it in the local library collection on your machine, using the following command:

install.packages("terra")

That should download and install the library, so that it will be available for you from then on if you return to this same computer.

About the Terra library

When you loaded the Terra library, it added a number of classes to the R environment that support spatial data, and functions to access, analyze, and visualize those data. The data class SpatVector holds vector data, and SpatRaster handles raster data (for more details, see RSpatial's documentation of SpatVector and SpatRaster). When I wrote that SpatRaster "handles" raster data I chose that verb carefully - an important advance in raster data handling compared to some older libraries is that the data do not have to all reside inside the R data library - R typically holds all of its active data objects in the computer's working memory. Raster grids can be very large, so loading all the raster files you are working on into the computer's active memory can quickly fill it up; instead, it is much more efficient to keep the raster data on your storage device (hard drive, network drive, etc.) and just access parts of the grid as needed. SpatRaster objects CAN have data stored within them, but in the majority of "real life" cases, most of the data stays in external files and the R data object holds metadata like the coordinate reference system being used, the bounding coordinates and resolution of the grid, etc.