Reversed – r.iresmi.net

Author

Michaël

Published

2025-11-05

Modified

2025-11-05

A photo of beach seen through a glass ball — Calvi - Noël 2017 – CC-BY-NC-NA by Valerie Hukalo

Day 5 of 30DayMapChallenge: « Earth » (previously).

What if the bathymetry of the Mediterranean Sea is reversed? We would get a Western Island and an Eastern Island, a Sardinia-Corsica Bay, the Balearic Lagoon… Let’s see!

library(terra)
library(sf)

terraOptions(progress=0)

A global bathymetry dataset (7.5 Go zipped NetCDF) is available at GEBCO. We crop it around the Mediterranean Sea.

med <- c(xmin = -7, ymin = 30, xmax = 39,  ymax = 45) |> 
  st_bbox() |> 
  st_as_sfc()

gebco <- rast("~/data/gebco/GEBCO_2025_sub_ice.nc") |> 
  crop(med)

We set all positive values to 0 and all negative values are inversed. The relief is more pronounced with a light lower resolution, then we compute the hillshading.

mountain_med <- aggregate((gebco <= 0) * -1 * gebco, 10, mean)
slope <- terrain(mountain_med, "slope", unit = "radians")
aspect <- terrain(mountain_med, "aspect", unit = "radians")
hill <- shade(slope, aspect, angle = 45, direction = 315, normalize = TRUE)

Map

plot(hill, col = grey(0:255/255), 
     main = "Mediterranean Sea upside down",
     legend = FALSE, mar = c(2,2,2,4))
plot(mountain_med, col = topo.colors(30, alpha = 0.3),
     plg = list(title = "Elevation (m)"), add = TRUE)
mtext(paste("data: GEBCO\nhttps://r.iresmi.net", Sys.Date()), 
            side = 1, line = 3, cex = 0.5, adj = 1)

Map of inverted Mediterranean elevation — Figure 1: Mediterranean Sea upside down

---
title: "Reversed"
description: "Day 5 of 30DayMapChallenge"
author: "Michaël"
date: "2025-11-05T08:00:00"
date-modified: last-modified
categories:
  - R
  - 30DayMapChallenge
  - spatial
  - raster
  - datavisualization
draft: false
freeze: true
editor_options:
  chunk_output_type: console
chunk_output_type: console
execute:
  eval: true
  message: false
---

[![Calvi - Noël 2017 -- CC-BY-NC-NA by Valerie Hukalo](images/24506957907_2d890bcd05_c.jpg){fig-alt="A photo of beach seen through a glass ball"}](https://www.flickr.com/photos/hukalo/24506957907)

Day 5 of [30DayMapChallenge](https://30daymapchallenge.com/): « Earth » ([previously](/#category=30DayMapChallenge)).

What if the bathymetry of the Mediterranean Sea is reversed?
We would get a Western Island and an Eastern Island, a Sardinia-Corsica Bay, the Balearic Lagoon...
Let's see!

```{r}
#| label: setup

library(terra)
library(sf)

terraOptions(progress=0)
```

A global bathymetry dataset ([7.5 Go zipped NetCDF](https://dap.ceda.ac.uk/bodc/gebco/global/gebco_2025/ice_surface_elevation/netcdf/gebco_2025.zip?download=1)) is available at [GEBCO](https://www.gebco.net/data-products/gridded-bathymetry-data#global).
We crop it around the Mediterranean Sea.

```{r}
#| label: data

med <- c(xmin = -7, ymin = 30, xmax = 39,  ymax = 45) |> 
  st_bbox() |> 
  st_as_sfc()

gebco <- rast("~/data/gebco/GEBCO_2025_sub_ice.nc") |> 
  crop(med) 
```

We set all positive values to 0 and all negative values are inversed.
The relief is more pronounced with a light lower resolution, then we compute the hillshading.

```{r}
#| label: processing

mountain_med <- aggregate((gebco <= 0) * -1 * gebco, 10, mean)
slope <- terrain(mountain_med, "slope", unit = "radians")
aspect <- terrain(mountain_med, "aspect", unit = "radians")
hill <- shade(slope, aspect, angle = 45, direction = 315, normalize = TRUE)
```

## Map

```{r}
#| label: fig-map
#| fig-cap: Mediterranean Sea upside down
#| fig-alt: Map of inverted Mediterranean elevation
#| out-width: 100%
#| fig-height: 4

plot(hill, col = grey(0:255/255), 
     main = "Mediterranean Sea upside down",
     legend = FALSE, mar = c(2,2,2,4))
plot(mountain_med, col = topo.colors(30, alpha = 0.3),
     plg = list(title = "Elevation (m)"), add = TRUE)
mtext(paste("data: GEBCO\nhttps://r.iresmi.net", Sys.Date()), 
            side = 1, line = 3, cex = 0.5, adj = 1) 
```