Understanding spplot with Layers: A Deep Dive into Map Alignment

Introduction

As a data visualization enthusiast, you’ve likely encountered maps and spatial data while working on various projects. When combining different layers of data, such as polygons or grids, onto a map, it’s common to encounter alignment issues. In this article, we’ll delve into the world of spplot with layers in R, specifically addressing why spplot with layers are not aligned.

What is spplot and Layers?

The latticeExtra package provides a convenient way to create maps using spatial data. The spplot() function allows you to plot points on a map by overlaying the spatial data onto a base map.

{< highlight r >
library(latticeExtra)
</ highlight >}

# Create a sample dataset
data("mtcars")
df <- mtcars[, c(1, 3:9)]

spplot(df, "mpg", border = "green", col = NA,
       main = "Sample Data Map",
       xlab = "Mileage",
       ylab = "Acceleration",
       pch = 19,
       bg = "lightblue")

layers() is a new function introduced in R 4.0.0, which allows you to add additional layers on top of your existing plots.

{< highlight r >
library(latticeExtra)

# Create sample data
data("mtcars")
df <- mtcars[, c(1, 3:9)]

spplot(df, "mpg", border = "green", col = NA,
       main = "Sample Data Map",
       xlab = "Mileage",
       ylab = "Acceleration",
       pch = 19,
       bg = "lightblue") +
  layers(frame.data = df, aes(x = mileage), type = "point")
</ highlight >}

Projecting Spatial Data

When working with spatial data, it’s essential to consider the projection of your objects. The choice of projection depends on the source and purpose of the data.

In this example, we’re using ggmap (version 3.12.0) to overlay an Italy boundary map onto our sample dataset. However, we encountered alignment issues when combining these two layers.

Why Automatic Centering Doesn’t Work

Automatic centering in spplot with layers relies on the base map’s projection. In our case, ggmap uses the OpenStreetMap (OSM) tileset with its own coordinate system. This means that our Italy boundary layer is not aligned with the OSM coordinates.

The problem arises when we try to align our spatial data to a projected coordinate system like UTM or Web Mercator. Since these projections are different from ggmap’s native system, the spplot layers don’t properly center on top of each other.

Solution: Update Your Projection

To resolve this issue, you need to update your projection when working with spplot() and layers.

{< highlight r >
library(raster)
library(ggmap)
library(RColorBrewer)
library(latticeExtra)

# Set working directory
setwd("c:\\temp")

# Load GADM data
gadm <- get_data('GADM', country = 'Italy', level = 2)

# Extract bounding box points
bbPoints <- bbox(gadm)
gmap <- get_map(c(bbPoints), maptype = 'watercolor', source = 'stamen', crop = FALSE)

# Calculate bounding box dimensions and center coordinates
bbMap <- attr(gmap, 'bb')
height <- with(bbMap, ur.lat - ll.lat)
width <- with(bbMap, ur.lon - ll.lon)
latCenter <- with(bbMap, ll.lat + ur.lat)/2
lonCenter <- with(bbMap, ll.lon + ur.lon)/2

# Convert data to projected coordinates (UTM zone 32N)
df_projected <- spTransform(df, crs = "EPSG:32632")

# Create base map and overlay Italy boundary layer
gmap_base <- spplot(gadm, "PID", border = "green", col = NA,
                    main = "Base Map",
                    xlab = "", ylab = "")

spplot(df_projected, "mpg", border = "black", col = NA,
       main = "Projected Data Map",
       xlab = "Mileage",
       ylab = "Acceleration",
       pch = 19,
       bg = "lightblue") +
  layers(frame.data = df_projected, aes(x = mileage), type = "point")

Conclusion

When working with spatial data and map overlays in R, it’s crucial to consider the projection of your objects. Automatic centering may not work as expected due to differences between base maps’ projections.

By updating your projection when using spplot() and layers, you can resolve alignment issues and create visually appealing maps that effectively display your data.

Additional Resources

For further information on spatial data in R:

For more details on ggmap and spatial visualization:

Last modified on 2024-05-11