---
title: "Using the tidyverse with terra objects: the tidyterra package"
subtitle: "JOSS paper"
tags:
  - R
  - CRAN
  - spatial
  - vector
  - raster
  - tidyverse
  - terra
  - ggplot2
author:
  - name: Diego Hernangómez
    orcid: 0000-0001-8457-4658
date: "2023-07-18"
description: Paper published in the Journal of Open Source Software.
bibliography: paper.bib
citation:
  type: article-journal
  container-title: Journal of Open Source Software
  volume: 8
  issue: 91
  page: 5751
  available-date: 2023
  doi: 10.21105/joss.05751
  url: https://doi.org/10.21105/joss.05751
  title: "Using the tidyverse with terra objects: the tidyterra package"
link-citations: true
vignette: >
  %\VignetteIndexEntry{Using the tidyverse with terra objects: the tidyterra package}
  %\VignetteEngine{quarto::html}
  %\VignetteEncoding{UTF-8}
---

<!-- tidyterra.qmd is generated from tidyterra.qmd.orig. Please edit that file -->



[![](https://joss.theoj.org/papers/10.21105/joss.05751/status.svg){fig-alt="DOI"}](https://doi.org/10.21105/joss.05751)

## Summary

**tidyterra** is an **R** [@r-project] package that lets users manipulate
`SpatRaster` and `SpatVector` objects provided by **terra** [@R-terra], using
verbs from packages in the **tidyverse** [@R-tidyverse], such as **dplyr**
[@R-dplyr], **tidyr** [@R-tidyr] or **tibble** [@R-tibble]. This makes spatial
data manipulation and analysis more approachable for users already familiar with
the **tidyverse**.

**tidyterra** also extends **ggplot2** [@R-ggplot2] by providing additional
geoms and stats,[^1] such as `geom_spatraster()` and `geom_spatvector()`, as
well as carefully chosen scales and color palettes for map production.

**tidyterra** can manipulate the following classes of **terra** objects:

1.  `SpatVector` objects, which represent vector data such as points, lines or
    polygon geometries.

2.  `SpatRaster` objects, which represent raster data in the form of a grid
    consisting of equally sized cells. Each cell can contain one or more values.

The first stable version of **tidyterra** was released on **CRAN** on April 24,
2022. Since then, it has been actively used by other packages, such as
**ebvcube** [@R-ebvcube], **biomod2** [@R-biomod2], **inlabru** [@R-inlabru],
**RCzechia** [@R-rczechia] and **sparrpowR** [@R-sparrpowr]. It has also been
cited in academic research and publications (@bahlburg2023, @moraga2023,
@Leonardi2023, @meister2023).

## Statement of need

The [**tidyverse**](https://tidyverse.org/) is a collection of **R** packages
that share an underlying design philosophy, grammar and data structures. The
packages within the tidyverse are widely used by **R** users for tidying,
transforming and plotting data.

The **tidyverse** is designed to work with tidy data (*"every column is a
variable, every row is an observation, every cell is a single value"*),
represented in the form of data frames or **tibbles**. However, it is possible
to extend the functionality of **tidyverse** packages to work with new **R**
object classes by registering the corresponding S3 methods [@wickham_s32019].
This means that `dplyr::mutate()` can be adapted to work with any object of
class `foo` by creating the corresponding S3 method `mutate.foo()`.

While other popular packages designed for spatial data handling, such as **sf**
[@R-sf] or **stars** [@R-stars], already provide integration with the
**tidyverse** as part of their infrastructure, **terra** objects lack this
integration natively. Although **terra** offers a wide range of functions for
transforming and plotting `SpatRaster` and `SpatVector` objects, some users who
are not familiar with this package may need extra time to learn that syntax.
This can make their first steps in spatial analysis more difficult.

The **tidyterra** package was developed to address this integration gap. By
providing the corresponding S3 methods, users can apply familiar syntax and
functions for rectangular data to objects provided by **terra**. This makes
spatial data analysis more approachable for users who are new to the field.

In addition, **tidyterra** offers functions for plotting **terra** objects using
the **ggplot2** syntax. Although packages like **rasterVis** [@R-rastervis] and
**ggspatial** [@R-ggspatial] already support plotting `SpatRaster` objects with
**ggplot2**, **tidyterra** functions provide additional support for advanced
mapping. This support includes faceted maps, contours and automatic conversion
of spatial layers to the same CRS[^2] through `ggplot2::coord_sf()`.
**tidyterra** also provides support for `SpatVector` objects, similar to the
native support of **sf** objects in **ggplot2**.

Finally, **tidyterra** provides a collection of color palettes specifically
designed for representing spatial phenomena [@whitebox]. It also implements the
cross-blended hypsometric tints described by @Patterson_Jenny_2011.

## A note on performance

The development philosophy of **tidyterra** is to adapt **terra** objects to
data frame-like structures while performing data transformations, which can
affect performance.

When manipulating large `SpatRaster` objects, such as objects with more than
10,000,000 data slots, use native **terra** syntax, which is designed for this
type of data. For plotting, the geoms resample `SpatRaster` objects with more
than 500,000 cells by default to speed up rendering, as `terra::plot()` does.
You can override this upper limit with the geom's `maxcell` argument.

When possible, each **tidyterra** help page references its equivalent **terra**
function.

## Example of use

**tidyterra** is available on
[**CRAN**](https://CRAN.R-project.org/package=tidyterra) and can be installed
easily from **R** with:


``` r
install.packages("tidyterra")
```

The latest development version is hosted on
[GitHub](https://github.com/dieghernan/tidyterra) and can be installed from
**R** with:


``` r
remotes::install_github("dieghernan/tidyterra")
```

The following example demonstrates how to manipulate a `SpatRaster` object with
**dplyr** syntax. It also shows how to plot a `SpatRaster` object with
**ggplot2** using `geom_spatraster()`:


``` r
library(tidyterra)
library(tidyverse) # Load all tidyverse packages at once.
library(scales) # Additional package for labels.

# Temperatures in Castile and Leon (selected months).
rastertemp <- terra::rast(system.file(
  "extdata/cyl_temp.tif",
  package = "tidyterra"
))

# Rename with the tidyverse.
rastertemp <- rastertemp |>
  rename(April = tavg_04, May = tavg_05, June = tavg_06)

# Plot with facets.
ggplot() +
  geom_spatraster(data = rastertemp) +
  facet_wrap(~lyr, ncol = 2) +
  scale_fill_whitebox_c(
    palette = "muted",
    labels = label_number(suffix = "º"),
    n.breaks = 12,
    guide = guide_legend(reverse = TRUE)
  ) +
  labs(
    fill = "",
    title = "Average temperature in Castile and Leon (Spain)",
    subtitle = "Months of April, May and June"
  )
```

<div class="figure">
<img src="./fig-ex1-1.png" alt="Faceted map with a multi-layer SpatRaster object." width="100%" />
<p class="caption">Faceted map with a multi-layer SpatRaster object.</p>
</div>

In the following example, we combine a common **dplyr** workflow (`mutate()` +
`select()`) and plot the result. In this case, the plot is a contour plot of the
original `SpatRaster` using `geom_spatraster_contour_filled()` and includes an
overlay of a `SpatVector` for reference:


``` r
# Compute the variation between April and June and apply a different palette.
incr_temp <- rastertemp |>
  mutate(var = June - April) |>
  select(Variation = var)

# Overlay a SpatVector.
cyl_vect <- terra::vect(system.file("extdata/cyl.gpkg", package = "tidyterra"))

# Contour map with overlay.
ggplot() +
  geom_spatraster_contour_filled(data = incr_temp) +
  geom_spatvector(data = cyl_vect, fill = NA) +
  scale_fill_whitebox_d(palette = "bl_yl_rd") +
  theme_grey() +
  labs(
    fill = "º Celsius",
    title = "Temperature variation in Castile and Leon (Spain)",
    subtitle = "Difference between April and June"
  )
```

<div class="figure">
<img src="./fig-ex2-1.png" alt="Contour map of temperature variation with a SpatVector overlay." width="100%" />
<p class="caption">Contour map of temperature variation with a SpatVector overlay.</p>
</div>

## Additional materials

The package includes extensive documentation available online at
<https://dieghernan.github.io/tidyterra/> including:

- Details on each function, including the equivalent **terra** function when
  available, for users who prefer to include those functions in their workflows.
- Working examples that use the functions and create plots.
- Additional articles and vignettes, including a complete demo of the color
  palettes included in the package (see
  [Palettes](https://dieghernan.github.io/tidyterra/articles/palettes.html)).

## Acknowledgements {.appendix}

I would like to thank [Robert J. Hijmans](https://github.com/rhijmans) for his
advice and support in adapting some of the methods and for the suggestions that
helped us improve the package features. I am also thankful to [Dewey
Dunnington](https://dewey.dunnington.ca/), Brent Thorne and the rest of the
contributors to the **ggspatial** package, which served as a key reference
during the initial stages of the development of **tidyterra**.

**tidyterra** also incorporates some pieces of code adapted from **ggplot2** for
computing contours, which relies on the package **isoband** [@R-isoband]
developed by [Claus O. Wilke](https://clauswilke.com/).

## References

[^1]: The term geoms refers to geometric objects and stats refers to statistical
    transformations, following **ggplot2** naming conventions.

[^2]: CRS stands for coordinate reference system.
