Solving the Mystery of Colored Line Segments in RgoogleMaps: A Step-by-Step Guide

Introduction to R and RgoogleMaps

Overview of the Problem

The question presents a scenario where a user is trying to create a map using RgoogleMaps with line segments that are colored based on a third variable. The problem arises when the map displays only green, despite the presence of red and other colors in the ‘active’ column. We will explore the technical aspects of this issue and provide solutions.

Background

RgoogleMaps is an R package used for creating interactive maps. It provides various functions to create different types of maps, such as static maps, interactive maps, and more. The PlotOnStaticMap() function is used to add lines or polygons to a static map.

Understanding the Issue

Why Doesn’t the Map Display Different Colors?

The problem arises because the PlotOnStaticMap() function does not natively support colored line segments. It only supports colors for filled shapes like polygons and markers. The FUN argument of this function can be used to plot lines, but it requires specific formatting.

Solutions

Using ColorRamp

One solution is to use a color ramp from the RColorBrewer package. We will first load the required libraries:

library(RgoogleMaps)
library(RColorBrewer)

# other necessary code here...

Next, we will create a color map using RColorBrewer’s function:

# Define the color map
colMap <- colorRamp(palette = "Reds")[[1]]

Creating Colored Line Segments

Now, we can use the PlotOnStaticMap() function to add colored line segments. We will specify the col argument with our custom color map:

# Create a sample dataframe for demonstration
df <- data.frame(lat=c(37.86047, 37.860303), lng=c(122.536226, 122.536422),
                 active=c("green", "green"), segment=c(c(1,2),c(3,4)))

# Add line segments to the map
PlotOnStaticMap(mymap,lat=df$lat,lon=df$lng,col=colMap(df$active), FUN=lines, add = TRUE)

Conclusion

In this example, we explored a common problem with RgoogleMaps and provided a solution using RColorBrewer. The PlotOnStaticMap() function supports adding colored lines to the map, but requires a custom color map.

Additional Considerations

Using Line Density for Custom Colors

Another approach is to use line density for creating a heat map effect on your static map. This can be achieved by plotting the density of data points using the ggmap package.

# Install and load necessary packages
install.packages("ggmap")
library(ggmap)

# Map out the area where we want to visualize our density
mymap <- get_map(lng=-122.0, lat=37.0, zoom=10)

# Create a density plot for demonstration
plot(df, main = "Density Plot", xlab = "", ylab = "")

# Add a heat map layer using ggmap
ggmap(mymap) + geom_density2d(aes(color = ..density..), alpha = 0.5)