Adding Different Polygons to Raster Stack Plot Using Levelplot in R

Introduction

Levelplot is a powerful plotting function in the lattice package of R that allows us to visualize multidimensional data, including raster stack plots. In this article, we will explore how to add different polygons to a raster stack plot built using levelplot.

Background

A raster stack plot consists of multiple rasters plotted on top of each other, creating a 3D-like effect when visualized together. Each raster represents a different dataset or measurement at the same spatial location. The levelplot function allows us to create an interactive visualization of these stacked rasters.

However, when we want to add polygons to our plot, things can get complicated. By default, levelplot will repeat any additional layers across all panels in the stack plot. This is because the layering process happens at the panel level, rather than the raster level. To achieve our desired result – having different polygons added to each individual panel without repeating them across other panels – we need to rethink our approach.

The Solution

One way to add different polygons to each panel of a raster stack plot using levelplot is by utilizing the packets argument in latticeExtra::layer. This allows us to manually specify how layers should be applied across different regions of the plot. We can create separate polygons for each panel and then use the packets argument to distribute them evenly.

Here’s an example:

# Load necessary libraries
library(lattice)
library(latticeExtra)
library(sp)

# Create sample data (replace with your actual raster stack)
rStack <- rasterData("your_raster_file")

# Define polygons for each panel (example: one polygon per panel)
Sr1 = Polygon(cbind(c(2, 4, 4, 1, 2), c(2, 3, 5, 4, 2)))
Sr2 = Polygon(cbind(c(5, 4, 2, 5), c(2, 3, 2, 2)))
Sr3 = Polygon(cbind(c(4, 4, 5, 10, 4), c(5, 3, 2, 5, 5)))

# Create separate polygons for each panel
Srs1 = Polygons(list(Sr1), "s1")
Srs2 = Polygons(list(Sr2), "s2")
Srs3 = Polygons(list(Sr3), "s3")

# Convert polygons to SpatialPolygons objects and assign IDs (panel numbers)
SpP = SpatialPolygons(list(Srs1, Srs2, Srs3))

# Create a raster stack plot with different polygons in each panel
levelplot(rStack, margin=F, xlab="", ylab="",
         par.settings=list(strip.background=list(col="lightgray")), 
         names.attr=c("18000BP", "15000BP", "12000BP", "6000BP", "2050", "2090"),
         scales = list(draw=F)) + 
  layer(sp.polygons(SpP), packets = c(1, 2, 3)) +
  # Optionally add additional layers using the same approach
  layer(sp.polygons(Sr1), packets = 1) +
  layer(sp.polygons(Sr2), packets = 2) +
  layer(sp.polygons(Sr3), packets = 3)

Explanation

In this example, we define separate polygons for each panel and assign them IDs using the SpatialPolygons function. Then, when adding these polygons to our plot, we use the packets argument in latticeExtra::layer to specify how layers should be applied across different regions of the plot.

By setting packets = c(1, 2, 3), we ensure that each polygon is added to its respective panel without being repeated. The first value represents which panel to add the layer to (panel 1), the second value for panel 2, and so on.

This approach allows us to create different polygons for each individual panel in our raster stack plot, resulting in a more visually appealing and interactive visualization.

Alternative Approach: Using lattice::panelgeometries

Another way to add polygons to your raster stack plot is by using the panelgeometries function within the lattice package. This allows you to specify which geometries (such as polygons) should be plotted for each panel of a multilayer plot.

Here’s an example:

# Load necessary libraries
library(lattice)
library(latticeExtra)

# Define sample data (replace with your actual raster stack)
rStack <- rasterData("your_raster_file")

# Create separate polygons for each panel
Sr1 = Polygon(cbind(c(2, 4, 4, 1, 2), c(2, 3, 5, 4, 2)))
Sr2 = Polygon(cbind(c(5, 4, 2, 5), c(2, 3, 2, 2)))
Sr3 = Polygon(cbind(c(4, 4, 5, 10, 4), c(5, 3, 2, 5, 5)))

# Create a raster stack plot with different polygons in each panel
levelplot(rStack, margin=F, xlab="", ylab="",
         par.settings=list(strip.background=list(col="lightgray")), 
         names.attr=c("18000BP", "15000BP", "12000BP", "6000BP", "2050", "2090"),
         scales = list(draw=F)) + 
  panelgeometries(cbind(Sr1, Sr2, Sr3), c(1, 2, 3))

Conclusion

Adding different polygons to each panel of a raster stack plot using levelplot can be achieved through two main approaches: utilizing the packets argument in latticeExtra::layer, or using the panelgeometries function within the lattice package.

Both methods provide flexibility and control over how layers are applied across different regions of the plot, allowing for more visually appealing and interactive visualizations. By choosing the most suitable approach based on your specific needs, you can create stunning raster stack plots with added polygons that enhance your analysis.

Last modified on 2023-12-04