---
title: "Package overview"
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documentclass: article
bibliography: references.bib
csl: reference-style.csl
vignette: >
  %\VignetteIndexEntry{Package overview}
  %\VignetteEngine{knitr::rmarkdown_notangle}
---

# 1. Introduction

_envar_ is an _R_ package that enables the download of a wide range of environmental variables and their processing. In this tutorial we explore all the potential uses of the package, to give an idea of the breadth of uses and of the potential challenges and shortcuts. First we illustrate the five ways in which it is possible to define a study area: 1) global data, 2) a country or continent is specified, 3) an ecological boundary is specified, 4) a polygon shapefile is loaded, 5) a point shapefile is loaded, 6) a point data.frame is loaded, 7) a buffer is specified in addition to another element, or 8) a species range is created. Then, we illustrate how to project to a different reference system, aggregate the results to a different resolution, how to check for correlation and extrapolation, how to set NAs consistently across raster layers, and how to optionally save the results locally.

# 2. Load the package

First we load _envar_. When executing this command, another package (`dplyr`) will be automatically loaded to ensure the full functionality of the package. However, other packages are often required for plotting and further analyses and we also load them here for later use.


``` r
require(envar)
require(terra)
require(sf)
```

# 3. Set study area

Below, we illustrate a set of R commands that can be used to download variables over a specified study area. The customization of the study area occurs through the modification of arguments of the `par_set()` function.

## 3.1. Global data

### 3.1.1. Single layer 

We here show an example of download of a single variable from a single source, in this case the 1 km land cover based on ESA [@loparrino2025]. The variable is downloaded and maintains its original extent. If not aggregated, the original grid resolution of any source is always 30 arcseconds (0.00833°, or ~ 1 km) at the equator.


``` r
processed_singlelayer_lc <- par_set() %>% 
  melc(vars=c("ice")) 
```

The print in the console is omitted in this tutorial to avoid output overflow. When run locally, the print in the console displays the citation(s) and doi(s) for the downloaded source(s) to make sure that users cite the proper source(s) when redacting manuscripts or using data for public presentations. Then, a progress bar is shown for each variable that is being downloaded and success messages are issued when each variable has been correctly processed. The output is a SpatRaster object from the _terra_ package, which can be plotted and further processed.


``` r
plot(processed_singlelayer_lc)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-5-1.png" alt="plot of chunk unnamed-chunk-5"  />
<p class="caption">plot of chunk unnamed-chunk-5</p>
</div>

### 3.1.2. Multiple layers with different extents

When multiple layers from different sources are downloaded together, the output automatically crops all layers to the smallest shared extent. If more than two sources are reported, the code updates step-by-step the shared extent across all layers.


``` r
processed_bilayer_global <- par_set() %>%
  chelsa(vars = c("pr"), months = 12, year = 2015) %>%
  melc(vars = c("ice"))
```


``` r
plot(processed_bilayer_global)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-7-1.png" alt="plot of chunk unnamed-chunk-7"  />
<p class="caption">plot of chunk unnamed-chunk-7</p>
</div>

The same occurs if the order of downloaded variables is inverted:


``` r
processed_bilayer_global_1 <- par_set() %>%
  melc(vars = c("ice")) %>%
  chelsa(vars = c("pr"), months = 12, year = 2015)
```


``` r
plot(processed_bilayer_global_1)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-9-1.png" alt="plot of chunk unnamed-chunk-9"  />
<p class="caption">plot of chunk unnamed-chunk-9</p>
</div>


``` r
# define the extents of the first SpatRaster object and of the second to check if they are the same

ext1 <- round(ext(processed_bilayer_global), 1)
ext2 <-round(ext(processed_bilayer_global_1), 1)

# print the resulting extents 
print(ext1)
```

```
## SpatExtent : -180, 180, -60, 84 (xmin, xmax, ymin, ymax)
```

``` r
print(ext2)
```

```
## SpatExtent : -180, 180, -60, 84 (xmin, xmax, ymin, ymax)
```

## 3.2. Country/continent

### 3.2.1. Country

A country can be specified by name (case‑insensitive). The output is cropped/masked to that country.


``` r
processed_bilayer_country <- par_set(country = "Italy") %>%
  melc(vars = c("ice")) %>%
  chelsa(vars = c("pr"), months = 12, years = 2015)
```


``` r
plot(processed_bilayer_country)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-12-1.png" alt="plot of chunk unnamed-chunk-12"  />
<p class="caption">plot of chunk unnamed-chunk-12</p>
</div>

### 3.2.2. Continent


``` r
processed_singlelayer_africa <- par_set(continent = "Africa", buffer = 10) %>%
  melc(vars = c("forest"))
```


``` r
plot(processed_singlelayer_africa)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-14-1.png" alt="plot of chunk unnamed-chunk-14"  />
<p class="caption">plot of chunk unnamed-chunk-14</p>
</div>

All countries and continents are retrieved with internal calls to the _rnaturalearth R_ package, which allows the download of shapefiles from the Natural Earth dataset. However, the continent of Europe has a complex shape if downloaded from Natural Earth, and the invalid resulting geometry provides unrealistic outputs. Thus, if continent = "Europe" is specified, a shapefile of geographic Europe is used (including Russia only up to the Ural mountains and excluding overseas territories), based on data from the Global Administrative Areas **[GADM](https://gadm.org)**.


``` r
processed_singlelayer_europe <- par_set(continent = "Europe") %>%
  melc(vars = c("forest"))
```


``` r
plot(processed_singlelayer_europe)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-16-1.png" alt="plot of chunk unnamed-chunk-16"  />
<p class="caption">plot of chunk unnamed-chunk-16</p>
</div>

### 3.2.3. Set the scale 

When retrieving continents or countries from Natural Earth (i.e., any time the "country" or "continent" argument is used except for Europe), an additional argument ("scale") can be specified. This represents the scale at which the shapefiles are retrieved, and can assume three values: small (1:100 million), medium (the default; 1:50 m), and large (1:10 m). The boundaries will appear rough and imprecise with a small scale and finer with a large scale. The default is "medium" to download data at a resolution that is sufficiently detailed but lighter and faster than high-resolution.


``` r
# download variables over a country shape at "small" scale
processed_bilayer_country_small <- par_set(country = "Italy", scale = "small") %>%
  melc(vars = c("ice")) %>%
  chelsa(vars = c("pr"), months = 12, years = 2015)
```


``` r
plot(processed_bilayer_country_small)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-18-1.png" alt="plot of chunk unnamed-chunk-18"  />
<p class="caption">plot of chunk unnamed-chunk-18</p>
</div>


``` r
# download variables over a country shape at "large" scale
processed_bilayer_country_large <- par_set(country = "Italy", scale = "large") %>%
  melc(vars = c("ice")) %>%
  chelsa(vars = c("pr"), months = 12, years = 2015)
```


``` r
plot(processed_bilayer_country_large)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-20-1.png" alt="plot of chunk unnamed-chunk-20"  />
<p class="caption">plot of chunk unnamed-chunk-20</p>
</div>

## 3.3. Ecological boundary 

### 3.3.1. Ecoregions

It is possible to specify an ecoregion/biome/realm based on the work of Dinerstein and colleagues published on BioScience [@dinerstein2017]. Insert the full name of the ecoregion/biome/realm; to check the names visit the interactive **[website](https://ecoregions.appspot.com/)**.


``` r
processed_ecoregion <- par_set(ecoregion = "Lower Gangetic Plains moist deciduous forests") %>%
  melc(vars = c("tree"))
```


``` r
plot(processed_ecoregion)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-22-1.png" alt="plot of chunk unnamed-chunk-22"  />
<p class="caption">plot of chunk unnamed-chunk-22</p>
</div>


``` r
processed_biome <- par_set(biome = "Tundra") %>%
  melc(vars = c("ice"))
```


``` r
plot(processed_biome)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-24-1.png" alt="plot of chunk unnamed-chunk-24"  />
<p class="caption">plot of chunk unnamed-chunk-24</p>
</div>


``` r
processed_realm <- par_set(realm = "Neotropic") %>%
  melc(vars = c("tree"))
```


``` r
plot(processed_realm)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-26-1.png" alt="plot of chunk unnamed-chunk-26"  />
<p class="caption">plot of chunk unnamed-chunk-26</p>
</div>

### 3.3.2. Zoogeographic regions

It is possible to specify a zoogeographic region/realm based on the update of Wallace's zooregions by Holt and colleagues [@holt2013]. Insert the full name of the zoogeographic region/realm.

The following zooregions are available: "South American", "Australian", "Novozelandic", "African", "Madagascan", "Papua-Melanesian", "Amazonian", "Guineo-Congolian", "Indo-Malayan", "Panamanian", "Oriental", "Saharo-Arabian", "Mexican", "Chinese", "North American", "Eurasian", "Tibetan", "Japanese", "Arctico-Siberian", and "Polynesian".


``` r
processed_zooregion <- par_set(zooregion = "Madagascan") %>%
  melc(vars = c("tree"))
```


``` r
plot(processed_zooregion)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-28-1.png" alt="plot of chunk unnamed-chunk-28"  />
<p class="caption">plot of chunk unnamed-chunk-28</p>
</div>

And the following zoorealms are available: "Neotropical", "Australian", "Afrotropical", "Madagascan", "Oceanina", "Oriental", "Panamanian", "Saharo-Arabian", "Nearctic", "Sino-Japanese", and "Palearctic".


``` r
processed_zoorealm <- par_set(zoorealm = "Neotropical") %>%
  melc(vars = c("tree"))
```


``` r
plot(processed_zoorealm)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-30-1.png" alt="plot of chunk unnamed-chunk-30"  />
<p class="caption">plot of chunk unnamed-chunk-30</p>
</div>

### 3.3.3. Mountain regions

The Global Mountain Biodiversity Assessment working group has defined a classification of mountain systems [@snethlage2022]. We make available through the package the large scale selection of non-overlapping mountain systems (291). To see the available mountain regions and their names check the **[website](https://www.earthenv.org/mountains)**.


``` r
processed_mountain_region <- par_set(mountain_region = "European Alps") %>%
  melc(vars = c("tree"))
```


``` r
plot(processed_mountain_region)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-32-1.png" alt="plot of chunk unnamed-chunk-32"  />
<p class="caption">plot of chunk unnamed-chunk-32</p>
</div>

### 3.3.4. CMEC mountain regions

The Center for Macroecology, Evolution and Climate at the University of Copenhagen has developed another classification of mountain regions  [@rahbek2019]. We make available through the package the mountain regions so defined (135). To see the available mountain regions and their names check the original publication.


``` r
processed_mountain_region_cmec <- par_set(mountain_region_cmec = "Alps and central European outliers") %>%
  melc(vars = c("tree"))
```


``` r
plot(processed_mountain_region_cmec)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-34-1.png" alt="plot of chunk unnamed-chunk-34"  />
<p class="caption">plot of chunk unnamed-chunk-34</p>
</div>

### 3.3.5. Glacier regions

The Randolph Glacier Inventory team defined a set of 19 (old version) or 20 (new version) glacier regions that are broad areas that encompass different mountainous and polar areas of the globe [@pfeffer2014]. Here, we make available both the 19 regions classification and the 20 regions classification. The full name of the glacier region must be inserted and must be one of the following:

(For the 19 glacier regions)

1. Alaska                 
2. Western Canada and USA
3. Arctic Canada, North      
4. Arctic Canada, South
5. Greenland Periphery       
6. Iceland          
7. Svalbard and Jan Mayen
8. Scandinavia
9. Russian Arctic       
10. Asia, North
11. Central Europe
12. Caucasus and Middle East 
13. Asia, Central             
14. Asia, South West          
15. Asia, South East        
16. Low Latitudes              
17. Southern Andes
18. New Zealand              
19. Antarctic and Subantarctic

(For the 20 glacier regions)

1. Alaska
2. Western Canada and USA
3. Arctic Canada North
4. Arctic Canada South          
5. Greenland Periphery  
6. Iceland                    
7. Svalbard and Jan Mayen
8. Scandinavia
9. Russian Arctic
10. North Asia  
11. Central Europe              
12. Caucasus and Middle East    
13. Central Asia
14. South Asia West             
15. South Asia East
16. Low Latitudes   
17. Southern Andes
18. New Zealand
19. Subantarctic and Antarctic Islands
20. Antarctic Mainland


``` r
processed_glacier_region <- par_set(glacier_region_19 = "Central Europe") %>%
  melc(vars = c("ice"))
```


``` r
plot(processed_glacier_region)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-36-1.png" alt="plot of chunk unnamed-chunk-36"  />
<p class="caption">plot of chunk unnamed-chunk-36</p>
</div>



### 3.3.6. Freshwater ecoregions

Freshwater ecoregions have been widely used in ecological studies following their definition by [@abell2008]. As the provided shapefile does not include the full names of freshwater ecoregions but only their numeric IDs, instead of the name the user must insert the ecoregion ID. Check the interactive interface **[here](https://www.feow.org/ecoregions/interactive-map)** to find out the available freshwater ecoregions and their IDs. We consider the Florida Peninsula (ID = 156) as an example here.


``` r
processed_freshwater_ecoregion <- par_set(freshwater_ecoregion = 156) %>%
  melc(vars = c("tree"))
```


``` r
plot(processed_freshwater_ecoregion)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-38-1.png" alt="plot of chunk unnamed-chunk-38"  />
<p class="caption">plot of chunk unnamed-chunk-38</p>
</div>

### 3.3.7. Marine regions

Marine regions are available from two different data references; the first one is the Marine Ecoregions Of the World (MEOW) [@spalding2007], which provides a classification of coastal and shelf areas. The second one is the Pelagic Provinces Of the World (PPOW) [@spalding2012], providing an inventory of surface pelagic waters. The available arguments are "marine_ecoregion", "marine_realm", and "marine_province" for MEOW, and "pelagic_province", "pelagic_biome", and "pelagic_realm" for PPOW. You can explore the MEOW available names **[here](https://databasin.org/maps/new/#datasets=3b6b12e7bcca419990c9081c0af254a2)** and the PPOW available names through the relative publication.


``` r
processed_marine_ecoregion <- par_set(marine_ecoregion = "East African Coral Coast", res = 6) %>%
  biooracle(vars = c("o2"))
```


``` r
plot(processed_marine_ecoregion)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-40-1.png" alt="plot of chunk unnamed-chunk-40"  />
<p class="caption">plot of chunk unnamed-chunk-40</p>
</div>

## 3.4. Custom shapefile

### 3.4.1. Points

If a POINT shapefile is used and no buffer is applied, the output will be a data.frame containing columns: ID (integer going from 1 to the length of the dataset), X (longitude), Y (latitude), and the extracted values over the specified points.


``` r
points <- sf::st_sample(Alps, size = 100, type = "random")

processed_singlelayer_points <- par_set(shape = points, crs=3035) %>%
  melc(vars = c("ice"))

processed_bilayer_points <- par_set(shape = points, crs=3035) %>%
  melc(vars = c("ice")) %>%
  chelsa(vars = c("pr"), months = 12, year = 2015)
```


``` r
# visualize the points shapefile
plot(Alps$geometry)
plot(points, add =T)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-42-1.png" alt="plot of chunk unnamed-chunk-42"  />
<p class="caption">plot of chunk unnamed-chunk-42</p>
</div>


``` r
# visualize the resulting data.frame with the extracted values when only one source was used
head(processed_singlelayer_points)
```

```
##   ID       X       Y ice
## 1  1 4087407 2393913   0
## 2  2 4673719 2517564   0
## 3  3 4702000 2673471   0
## 4  4 4074334 2309450   0
## 5  5 4255502 2642772   0
## 6  6 3962724 2486021   0
```


``` r
# visualize the resulting data.frame with the extracted values when two or more sources were used
head(processed_bilayer_points)
```

```
##   ID       X       Y ice pr_2015_12
## 1  1 4087407 2393913   0          2
## 2  2 4673719 2517564   0          1
## 3  3 4702000 2673471   0          2
## 4  4 4074334 2309450   0         13
## 5  5 4255502 2642772   0         21
## 6  6 3962724 2486021   0         18
```

### 3.4.2. Polygon

The user can load a shapefile of type POLYGON and use it to crop/mask the downloaded variables. Here we use the already included European Alps shapefile (`Alps()`).


``` r
processed_bilayer_shapefile <- par_set(shape = Alps) %>%
  melc(vars = c("ice")) %>%
  chelsa(vars = c("pr"), months = 12, year = 2015)
```


``` r
plot(processed_bilayer_shapefile[[2]])
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-46-1.png" alt="plot of chunk unnamed-chunk-46"  />
<p class="caption">plot of chunk unnamed-chunk-46</p>
</div>

## 3.5. Data frame of points

Points can be fed into the package in the form of a data.frame with only two columns that represent coordinates. Column names must be **X** and **Y**, reporting the longitude and latitude, respectively. The CRS is assigned based on user input (default: 4326).


``` r
points3035 <- st_transform(points, 3035)
points3035df <- as.data.frame(st_coordinates(points3035))

processed_singlelayer_pointsdf <- par_set(pointsdf = points3035df, crs = 3035) %>%
  melc(vars = c("ice"))

processed_bilayer_pointsdf <- par_set(pointsdf = points3035df, crs = 3035) %>%
  melc(vars = c("ice")) %>%
  chelsa(vars = c("pr"), months = 12, year = 2015)
```


``` r
# visualize the resulting data.frame when using only one source
head(processed_singlelayer_pointsdf)
```

```
##   ID       X       Y ice
## 1  1 4087407 2393913   0
## 2  2 4673719 2517564   0
## 3  3 4702000 2673471   0
## 4  4 4074334 2309450   0
## 5  5 4255502 2642772   0
## 6  6 3962724 2486021   0
```


``` r
# visualize the resulting data.frame when using multiple sources
head(processed_bilayer_pointsdf)
```

```
##   ID       X       Y ice pr_2015_12
## 1  1 4087407 2393913   0          2
## 2  2 4673719 2517564   0          1
## 3  3 4702000 2673471   0          2
## 4  4 4074334 2309450   0         13
## 5  5 4255502 2642772   0         21
## 6  6 3962724 2486021   0         18
```

## 3.6. Buffer
### 3.6.1. Apply a buffer to a shape

A positive buffer expands the shape by the specified number of kilometers; a negative buffer shrinks it. Units are always km for the user. If the CRS is left to the default (crs="EPSG:4326"), the function used by the package (st_buffer) automatically transforms the kilometers in degrees so that the buffer is circular on the 3D surface (and the buffer might thus appear non-circular if plotted in 2D). However, it is recommended to always specify a projected CRS when using buffers to avoid potential distortions.


``` r
processed_singlelayer_buffer <- par_set(country = "Italy", crs = 3035, buffer = 10) %>%
  melc(vars = c("ice"))
```


``` r
plot(processed_singlelayer_buffer)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-51-1.png" alt="plot of chunk unnamed-chunk-51"  />
<p class="caption">plot of chunk unnamed-chunk-51</p>
</div>

Negative buffer example:


``` r
processed_singlelayer_negative_buffer <- par_set(country = "Italy", crs = 3035, buffer = -10) %>%
  melc(vars = c("ice"))
```


``` r
plot(processed_singlelayer_negative_buffer)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-53-1.png" alt="plot of chunk unnamed-chunk-53"  />
<p class="caption">plot of chunk unnamed-chunk-53</p>
</div>

A buffer at global scale (no shape or data.frame or country defined) triggers an error (not shown here):


``` r
processed_singlelayer_global_buffer <- par_set(buffer = 10) %>%
  melc(vars = c("ice"))
```

### 3.6.2. Apply a buffer when using points

A buffer (in km) expands the points to a polygon and all the variables that are downloaded are subsequently cropped to that shape. This option can be very useful 
for instance in Species Distribution Modelling, whereby records of species presence
are often known but not absences. The so-called pseudo-absences (i.e. simulated absences of species) and background points (points used to define the background environmental conditions and compare them to presences, as done in the Maximum Entropy algorithm) are typically picked in a defined radius around the presences,
to define areas that could theoretically be reached by the species via dispersal. Additionally, picking pseudo-absence/background points in a buffer around presences
allows to better simulate the spatial sampling effort patterns and replicate them. 


``` r
processed_bilayer_points_buffer <- par_set(pointsdf = points3035df, crs = 3035, buffer = 50) %>%
  melc(vars = c("ice")) %>%
  chelsa(vars = c("pr"), months = 12, year = 2015)
```


``` r
plot(processed_bilayer_points_buffer)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-56-1.png" alt="plot of chunk unnamed-chunk-56"  />
<p class="caption">plot of chunk unnamed-chunk-56</p>
</div>

## 3.7. Further options

### 3.7.1. Land mask 

When downloading  data, it is possible to apply a land mask to remove all sea/ocean cells from the output. This can be done by setting the argument "land" to TRUE (default to FALSE, i.e., no land mask is applied). Here we use the glacier region centered on the European Alps to display a region with both land and sea, to show that sea is masked as NA when "land" is set to TRUE. As the land outlines are retrieved from the Natural Earth database, it is possible to use the argument "scale" as for countries/continents, to refine the precision of coastline definition (check paragraph 3.2.3. "Set the scale" above).

Note that the land mask only makes a difference for sources that provide values over the sea in the first place. Several sources (e.g. the land cover ones) are land-only by construction and already return NA over the sea, so applying the mask to them changes nothing. Here we use the mean annual air temperature (bio1) of the CHELSA 1981-2010 climatology, which is also interpolated over the sea.

Without land mask:


``` r
processed_nolandmask <- par_set(glacier_region_19 = "Central Europe") %>%
  chelsa(vars = c("bio1"), years = "1981-2010")
```


``` r
plot(processed_nolandmask)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-58-1.png" alt="plot of chunk unnamed-chunk-58"  />
<p class="caption">plot of chunk unnamed-chunk-58</p>
</div>

With land mask:


``` r
processed_landmask <- par_set(glacier_region_19 = "Central Europe", land = TRUE) %>%
  chelsa(vars = c("bio1"), years = "1981-2010")
```


``` r
plot(processed_landmask)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-60-1.png" alt="plot of chunk unnamed-chunk-60"  />
<p class="caption">plot of chunk unnamed-chunk-60</p>
</div>

### 3.7.2. Species range

For many applications, it is important to quantify the surface where a species is found as precisely as possible, for instance when estimating the Area Of Occupancy (AOO) for IUCN assessments. Thus, it is possible to use the alpha_hull = TRUE argument to define a polygon that surrounds at least 99% of occurrence records of the species, defining the maximum range extent [@daru2025]. This option works only if occurrence points are specified as X and Y coordinates in the pointsdf argument. We use the Apollo dataset as an example, a dataset of occurrences of the _Parnassius apollo_ butterfly; the dataset is already included in the package.


``` r
processed_range <- par_set(pointsdf = Apollo, alpha_hull = TRUE) %>% 
  melc(vars=c("ice"))
```


``` r
plot(processed_range)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-62-1.png" alt="plot of chunk unnamed-chunk-62"  />
<p class="caption">plot of chunk unnamed-chunk-62</p>
</div>

It can also be used in conjunction with a buffer. In this case, an alpha hulll will be created and then expanded by the specified buffer.


``` r
processed_range_buffer <- par_set(pointsdf = Apollo, alpha_hull = TRUE, buffer = 10) %>% 
  melc(vars=c("ice"))
```


``` r
plot(processed_range_buffer)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-64-1.png" alt="plot of chunk unnamed-chunk-64"  />
<p class="caption">plot of chunk unnamed-chunk-64</p>
</div>

# 4. Project to another reference system

The single layer is downloaded and maintains its original extent. But additionally it is projected to the desired Coordinate Reference System (CRS). The default CRS in _envar_ is the Geographic Reference System (GRS) WGS 84 (EPSG: 4326). This is the most commonly used GRS for global analyses. GRS assume that the Earth is an ellipsoid (or spheroid) and provide coordinates based on a 3D surface with degrees and latitude-longitude values. However, these systems are not ideal to compute distances as the meaning of degrees changes based on the location on earth (e.g., 1 degree of longitude corresponds to ~ 111 km at the equator but approaches zero towards the poles). Additionally, users might want to improve spatial analysis accuracy, visualize data in a specific projection, or align results with other data that use a different CRS. Thus, a Projected Reference System (PRS) can be used, where coordinates are based on the location on a 2D system. The conversion from a GRS to a PRS always implies a certain amount of distortion, and thus different PRS are optimized for different areas of the globe to locally reduce distortion when moving from a 3D to a 2D representation. Thus, users can use the argument "crs" in the `par_set()` function to specify the desired CRS. Here we show an example of download of a single variable from a single source, in this case the 1 km land cover based on ESA [@loparrino2025], and its projection to the local equal-area projection (EPSG:3035). Inside the crs argument, a number or character representing the code of the PRS must be provided. If nothing is specified, 4326 is assumed. The crs can also be specified with the prefix 'ESRI' or 'EPSG' according to the system, e.g. "EPSG:4326" or "ESRI:54009".


``` r
processed_singlelayer_projected <- par_set(country = "Italy", crs=3035) %>% 
 melc(vars=c("ice"))
```


``` r
print(processed_singlelayer_projected)
```

```
## class       : SpatRaster 
## dimensions  : 1249, 1194, 1  (nrow, ncol, nlyr)
## resolution  : 982.9062, 982.9062  (x, y)
## extent      : 4004865, 5178455, 1484656, 2712306  (xmin, xmax, ymin, ymax)
## coord. ref. : ETRS89-extended / LAEA Europe (EPSG:3035) 
## source(s)   : memory
## name        :      ice 
## min value   :  0.00000 
## max value   : 99.62711
```

``` r
plot(processed_singlelayer_projected)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-66-1.png" alt="plot of chunk unnamed-chunk-66"  />
<p class="caption">plot of chunk unnamed-chunk-66</p>
</div>

# 5. Aggregate to a different resolution

A multiple of the native 30 arcsecond grid (~1 km at the equator) can be set by using an aggregation factor, defined by the argument **res**. A value of `res = 1` keeps the native resolution; any value > 1 aggregates the raster by that factor. For instance, `res = 10` aggregates the native grid to a resolution of ~10 km at the equator. Aggregation is done by computing the mean value of all cells that are part of the new larger cell.


``` r
processed_singlelayer_aggregated <- par_set(res = 100) %>%
  melc(vars = c("ice"))
```


``` r
print(processed_singlelayer_aggregated)
```

```
## class       : SpatRaster 
## dimensions  : 173, 432, 1  (nrow, ncol, nlyr)
## resolution  : 0.8333333, 0.8333333  (x, y)
## extent      : -180, 180, -60.16667, 84  (xmin, xmax, ymin, ymax)
## coord. ref. : lon/lat WGS 84 (EPSG:4326) 
## source(s)   : memory
## name        : ice 
## min value   :   0 
## max value   : 100
```

# 6. Correlation check

## 6.1. Add a correlation check when retrieving rasters

It is possible to call the `corr_check()` function at the bottom of the code to enable an automatic test of correlation among the downloaded variables over the defined spatial area and resolution. Doing this will produce a list containing the processed SpatRaster, Pearson pairwise correlations, VIF statistics, and a saved correlation plot.


``` r
processed_bilayer_corr_check <- par_set(shape = Alps, crs = 3035, res = 2, buffer = 10) %>% 
             chelsa(vars = c("bio1", "bio4", "bio10", "bio12", "bio19"),
                    years = "1981-2010") %>%
             topography(vars = c("elevation", "slope")) %>%
             melc(vars = c("trees", "ice")) %>%
             corr_check()
```

The output will thus be a list that contains the following elements: "data" (the SpatRaster or data.frame with your downloaded data), "correlation_matrix" containing a matrix of Pairwise Pearson's correlation coefficients between all variables, "vif" a data.frame containing the Variance Inflation Factors for each variable, a "summary" that reports if and which variables have a Pearson's correlation coefficient higher than |0.6| and/or a VIF higher than 3. Lastly, the "plot_path" specifies the local directory to which a plot of the Pearson's pairwise correlation was saved. 


``` r
# View the Pearson's pairwise correlation matrix
print(processed_bilayer_corr_check$correlation_matrix)
```

```
##                 bio1_1981-2010 bio4_1981-2010 bio10_1981-2010 bio12_1981-2010
## bio1_1981-2010       1.0000000    -0.17098433      0.99411075     -0.58856258
## bio4_1981-2010      -0.1709843     1.00000000     -0.06629979     -0.13822734
## bio10_1981-2010      0.9941107    -0.06629979      1.00000000     -0.61255277
## bio12_1981-2010     -0.5885626    -0.13822734     -0.61255277      1.00000000
## bio19_1981-2010     -0.4182732    -0.33185424     -0.45693075      0.84502782
## elevation           -0.9682340     0.06664318     -0.96993166      0.58178549
## slope               -0.7884759     0.01527641     -0.79705030      0.57544349
## trees                0.1420440    -0.28430061      0.10591260      0.02372563
## ice                 -0.4765922     0.13185174     -0.46273645      0.27137511
##                 bio19_1981-2010   elevation       slope        trees        ice
## bio1_1981-2010     -0.418273159 -0.96823403 -0.78847587  0.142043991 -0.4765922
## bio4_1981-2010     -0.331854239  0.06664318  0.01527641 -0.284300607  0.1318517
## bio10_1981-2010    -0.456930752 -0.96993166 -0.79705030  0.105912597 -0.4627365
## bio12_1981-2010     0.845027822  0.58178549  0.57544349  0.023725631  0.2713751
## bio19_1981-2010     1.000000000  0.44083530  0.39996749 -0.003624083  0.2299007
## elevation           0.440835298  1.00000000  0.83277881 -0.120815882  0.4857175
## slope               0.399967493  0.83277881  1.00000000  0.223848319  0.2433729
## trees              -0.003624083 -0.12081588  0.22384832  1.000000000 -0.3266344
## ice                 0.229900683  0.48571755  0.24337291 -0.326634442  1.0000000
```


``` r
# View the Variance Inflation Factor values
print(processed_bilayer_corr_check$vif)
```

```
##         Variables         VIF
## 1  bio1_1981-2010 2933.270334
## 3 bio10_1981-2010 2549.226924
## 6       elevation   37.362654
## 2  bio4_1981-2010   34.871455
## 7           slope    5.654897
## 4 bio12_1981-2010    5.320165
## 5 bio19_1981-2010    4.673379
## 8           trees    1.805555
## 9             ice    1.533835
```

To better understand the structure of correlation, we can also analyze the correlation plot that was locally stored. 

<div class="figure" style="text-align: center">
<img src="images/Corr_plot_example.png" alt="plot of chunk unnamed-chunk-72" width="100%" />
<p class="caption">plot of chunk unnamed-chunk-72</p>
</div>

## 6.2. Add a correlation check when using points

This will create a similar result as for the correlation test applied to a SpatRaster object, but instead of a set of rasters, the first element of the resulting list will be a data.frame with the extracted values.


``` r
processed_bilayer_points_corr_check <- par_set(shape = points) %>%
  melc(vars = c("ice")) %>%
  chelsa(vars = c("pr"), months = 12, year = 2015) %>%
  corr_check()
```


``` r
head(processed_bilayer_points_corr_check[[1]])
```

```
##   ID         X        Y ice pr_2015_12
## 1  1  7.061866 44.61497   0          2
## 2  2 14.523698 45.67426   0          1
## 3  3 15.015153 47.05874   0          2
## 4  4  6.938675 43.84918   0         13
## 5  5  9.141245 46.89501   0         21
## 6  6  5.428874 45.38758   0         18
```

# 7. Extrapolation check

It is possible to call the `extr_check()` function at the bottom of the code to enable an automatic test of extrapolation, regardless of whether you requested already a `corr_check()`. Ecological models often produce spurious predictions when predicting in environmental conditions not present in the calibration range [@elith2010]. This problem can arise under two conditions: 1) at least a variable has values outside the calibration range - strict extrapolation -, or 2) all variables fall in the calibration range, but novel combinations of predictor values are met - combinatorial extrapolation - [@zurell2012]. Inside the function it is possible to specify if strict and/or combinatorial extrapolation has to be checked (argument “type” - default to strict only), and it is necessary to specify the calibration points as data.frame with X and Y columns (argument “calib_points”) and the CRS of those coordinates (argument “calib_crs”) if different from the default (EPSG:4326). Extrapolation is checked creating an environmental overlap mask, using a method adapted from Zurell et al. (2012) [@zurell2012] and implemented natively within `envar`.

## 7.1. With rasters

The output is a list with a SpatRaster with one layer - two if both strict and extrapolation are checked - reporting as 1 cells of extrapolation and 0 cells of no extrapolation. 


``` r
processed_bilayer_extr_check <- par_set(shape = Alps, crs = 3035) %>%
  melc(vars = c("ice")) %>%
  chelsa(vars = c("pr"), months = 12, year = 2015) %>%
  extr_check(calib_points = Apollo, calib_crs = 4326, type = c("strict", "combinatorial"))
```


``` r
plot(processed_bilayer_extr_check$extrapolation$strict)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-76-1.png" alt="plot of chunk unnamed-chunk-76"  />
<p class="caption">plot of chunk unnamed-chunk-76</p>
</div>

## 7.2. With points

If the output from the download pipeline is a data.frame (see e.g. paragraphs 3.4.1 or 3.5), then, when using `extr_check()`, one column is added to the data.frame - two if both strict and extrapolation are checked -. This/these additional column(s) will report extrapolation as 1 and no extrapolation as 0. 


``` r
# for example, we use only the first 10 points of the Apollo dataset as a simulated data set to calibrate a model and then we want to apply it to a whole set of points covering a wider environmental range.
processed_points_extr_check <- par_set(pointsdf = Apollo) %>% 
  melc(vars = c("ice")) %>%
  chelsa(vars = c("pr"), months = 12, year = 2015) %>%
  extr_check(calib_points = Apollo[1:10,], calib_crs = 4326, type = c("strict", "combinatorial"))
```


``` r
head(processed_points_extr_check$extrapolation)
```

```
##   ID        X        Y  ice pr_2015_12 strict combinatorial
## 1  1 13.49513 47.10400 0.00          8      0             0
## 2  2 12.62265 47.03912 0.01         17      0             0
## 3  3  6.65878 44.16551 0.00         10      0             0
## 4  4  5.40699 44.15510 0.00          5      0             0
## 5  5  6.05580 44.58935 0.00          4      0             0
## 6  6  6.86447 44.42881 0.00          5      0             0
```

# 8. Set NAs

When the output from the download pipeline is a SpatRaster, it is possible to specify that any cell that is NA in at least one layer is NA in all layers. For instance, when downloading data from CHELSA [@karger2017] and land cover from ESA [@loparrino2025], the sea is not covered in the second source and it might thus be needless to keep information on climate over the sea. We can thus set the "set_na" argument to TRUE (default to FALSE, i.e., no NA mask is applied).

Without NA mask:


``` r
processed_bilayer_global <- par_set() %>%
  chelsa(vars = c("pr"), months = 12, year = 2015) %>%
  melc(vars = c("ice"))
```


``` r
plot(processed_bilayer_global)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-80-1.png" alt="plot of chunk unnamed-chunk-80"  />
<p class="caption">plot of chunk unnamed-chunk-80</p>
</div>

With NA mask:


``` r
processed_bilayer_global_namask <- par_set(set_na=TRUE) %>%
  chelsa(vars = c("pr"), months = 12, year = 2015) %>%
  melc(vars = c("ice"))
```


``` r
plot(processed_bilayer_global_namask)
```

<div class="figure" style="text-align: center">
<img src="figs/package_overview-unnamed-chunk-82-1.png" alt="plot of chunk unnamed-chunk-82"  />
<p class="caption">plot of chunk unnamed-chunk-82</p>
</div>


# 9. Save results

## 9.1. Save results as raster

If the "path" argument is used (default to NULL, i.e., no output stored locally), the retrieved and processed variables will be stored locally. Remember to specify the full path, including the filename and extension.


``` r
processed_stored_raster <- par_set(shape = Alps, crs = 3035, path = "../Out/storedraster.tif") %>%
  melc(vars = c("ice")) %>%
  chelsa(vars = c("pr"), months = 12, year = 2015)
```

## 9.2. Save results as table

The same can be done if the output is a data.frame. In this case, the file written has to be a .csv file; specify the path accordingly. 


``` r
processed_stored_table <- par_set(pointsdf = Apollo, path = "../Out/storedtable.csv") %>%
  melc(vars = c("ice")) %>%
  chelsa(vars = c("pr"), months = 12, year = 2015)
```

Results cannot be stored locally if the correlation or extrapolation check is/are added, as the output will be a list and not a SpatRaster/data.frame.

# 10. Conclusion

In this tutorial we have gone through all the potential ways in which the user can tailor the download and processing of data. For simplicity we have relied only on a few sources, but many more are available and can be checked at the **[reference](https://animalbiodiversitylab.github.io/envar/reference/)** or at the detailed explanation of all **[sources and variables](https://animalbiodiversitylab.github.io/envar/articles/variables)**.

# 11. Appendix: A runnable mini-example

The download chunks in this article need network access, so they are **shown but
not executed** when the article is built through GitHub pages at every deploy. Every one of them returns the same
kind of object: a `SpatRaster` of stacked environmental layers over a study area. So that the processing steps stay reproducible and automatically tested,
here you can find an example without any download but with a test raster — a small **real** WorldClim [@fick2017] extract for Switzerland (`bio1`, `bio12`, `elevation`, `slope` at ~9 km). The chunk below loads it **in place of a download** and demonstrates the study-area operations covered in this overview: aggregating the grid, reprojecting to another CRS, and checking collinearity.

```{r mini-example, message=FALSE, fig.height=3, fig.width=6.5}
library(envar)

# This stack stands in for the output of a download pipeline such as
#   par_set(country = "Switzerland", crs = 3035, res = 2) %>%
#     worldclim(vars = c("bio1", "bio12")) %>% topography(vars = "elevation")
switzerland <- terra::rast(
  system.file("extdata", "switzerland.tif", package = "envar")
)

switzerland

# Aggregate to a coarser grid, as par_set(res = ...) would do internally:
terra::aggregate(switzerland, fact = 2, fun = "mean")

# Reproject to a projected CRS in metres, as par_set(crs = 3035) would do internally:
terra::project(switzerland, "EPSG:3035")

# Check collinearity across the study area:
corr_check(switzerland)$summary
```

# 12. References

