Geospatial Mapping in R

Introduction

Let me start this blog post by stating the obvious: Geospatial maps are interesting to look at and certainly make papers and presentations prettier and more impressive; however, those are not the only reasons that such maps exist. They are used to communicate various types of information including geographical locations of regions in the world.

Why R?

Several available platforms have been used for drawing spatial maps and conducting geospatial data management. An eminent example is ArcGIS, which is a popular, flexible, and user-friendly geospatial mapping tool. Although ArcGIS is powerful and has many features, I am personally interested in open source, and Linux-friendly software.

Although there are several GIS tools such as Python, GRASS, QGIS, and UbuntuGIS, in this blog post, I will explain how R as an alternative tool can be used for geospatial analysis and for drawing spatial maps. R offers several advantages. First, R is an open-source platform, whereas GIS is relatively expensive. Second, R is script based. In some situations, you might have to generate several hundred maps from post-processed results; a tool such as R could offer faster and more-flexible data processing. You can run R on Linux machines and computer clusters and link it to other models that work under the Linux operating system. Different packages in R have been developed for geospatial analysis. In this exercise, I am going to focus on “RGDAL,” a widely used R package. RGDAL is the R distribution of Geospatial Data Abstraction Library (GDAL).

I recently moved to Cornell University, and I am eager to learn more about this region, so I decided to focus on the Susquehanna River Basin (SRB) located in US mid-Atlantic. The SRB drains parts of New York, Pennsylvania, and Maryland to the Chesapeake Bay. Before I get entirely sidetracked by my interest in SRB, let’s go back to the original intention of this blog post, which is making geospatial maps in R.

Prerequisites

Download all necessary data from the following links, then unzip and save them in your preferred folder.

1- Susquehanna River Basin Boundary from here

2- Major Watersheds in the Susquehanna River Basin from here

3- Susquehanna River from here

Open a new R Script in your R-Studio, then install the following R packages, you can use the following commands to install and load the packages:

# install.packages("rgdal")
# install.packages("ggplot2")
# install.packages("RColorBrewer")

library(rgdal)
library(ggplot2)
library(RColorBrewer)

Step 1- Map of Susquehanna River Basin

The first map of this exercise is a simple map of Susquehanna River Basin.

# I) The first step is to draw the map of SRB using the following code

SRB_Boundary <- readOGR(dsn = "spatial maps/Code/Shapefiles/srb/srb.shp")
plot(SRB_Boundary, col="gray90",
     main="Figure 1", sub="Susquehanna River Basin", cex.main=2.5, cex.sub=2.5)

# II) Then we can add Susquehanna River to the map

SR=readOGR("spatial maps/Code/Shapefiles/WtrTrails/WtrTrails.shp")
plot(SR, col="skyblue3", add=T, lwd=2)

# III) Adding information from the attribute table
#Shapefiles usually contain helpful information, such as name of objects, 
#sub-basins, area/length of objects, etc. 
#We are often interested in adding some of that information to our maps. 
#Here is how we can do it in R:

text(SRB_Boundary$NAME, x=coordinates(SRB_Boundary)[1],
     y=coordinates(SRB_Boundary)[2]*1.2, cex=1.2, col="darkblue", font=2)

text(paste("Area=27,500 square miles"), x=coordinates(SRB_Boundary)[1],
     y=(coordinates(SRB_Boundary)[2]*1.15), font=3, cex=1, col="darkblue")

# Let's add coordinates to the map

llgridlines(SRB_Boundary, plotLabels = T, cex=1.5)

Step 2- Selection of objects from an attribute table

If you have already worked with Arc-GIS you probably used its selection tools. What we are doing here is equivalent to selection from the attribute table. If you are not familiar with attribute tables this short explanation from esri should be helpful.

#I) Let's add SRB map again

plot(SRB_Boundary, col="gray90", 
     main="Figure 2", sub="Sub-basins greater than 800 square kilometer", 
     cex.main=2.5, cex.sub=2.5)

#II) Then we can add all the subbasins in SRB to the map

Subbasin <- readOGR(dsn = "spatial maps/Code/Shapefiles/wshedmjr/wshedmjr.shp")
plot(Subbasin, add =T, col=alpha("darkolivegreen1", 0.9))

# III) For this exercise, we are going to select large sub-basins of SRB
# with area of greater than 800 square kilometer

LargestBasins=which(Subbasin$SQM>800) # square kilometer

# IV) Now we are going to change the color of these selected features on the map

plot(Subbasin[LargestBasins,], add =T, col=alpha("seagreen", 0.9))

Step 3- Adding a legend to the map

In this part of the exercise, we are going to add a legend to the map

# I) SRB map 

 plot(SRB_Boundary, col="gray70",lwd=4,
       main="Figure 3", sub="Precipitation contour lines", cex.main=2.5, cex.sub=2.5)

# Now let's add precipitation contours to the SRB map

 isohyet=readOGR(dsn = "spatial maps/Code/Shapefiles/precip_iso/precip_iso.shp")
 plot(isohyet, add=T, col=bpy.colors(11), lwd=4)
   
# We can use the following script to add a legend to the map
llgridlines(SRB_Boundary, plotLabels = T, cex=1.5)

legend("right",box.col = "white", legend = unique(isohyet$INCHES), 
       fill=bpy.colors(11), cex=1.75, title = "Precipitation (inches)")

In this short tutorial, we went over some basic features of RGDAL. However, R can be used for more sophisticated geospatial analysis tasks, which I might cover in my future blog posts.

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