# Lesson 3. GIS in R: Introduction to coordinate reference systems in R

This lesson covers the key spatial attributes that are needed to work with spatial data including: Coordinate Reference Systems (`CRS`

), extent and spatial resolution.

## Learning objectives

After completing this tutorial, you will be able to:

- Be able to describe what a Coordinate Reference System (
`CRS`

) is - Be able to list the steps associated with plotting 2 datasets stored using different coordinate reference systems

## What you need

You will need a computer with internet access to complete this lesson and the data for week 5 of the course.

## Intro to coordinate reference systems

In summary - a coordinate reference system (`CRS`

) refers to the way in which spatial data that represent the earth’s surface (which is round / 3 dimensional) are flattened so that we can “Draw” them on a 2-dimensional surface. However each using a different (sometimes) mathematical approach to performing the flattening resulting in different coordinate system grids (discussed below). These approaches to flattening the data are specifically designed to optimize the accuracy of the data in terms of length and area (more on that later too).

In this lesson we will explore what a `CRS`

is. And how it can impact your data when you are working with it in a tool like `R`

(or any other tool).

The short video below highlights how map projections can make continents look proportionally larger or smaller than they actually are.

## What is a Coordinate Reference System

To define the location of something we often use a coordinate system. This system consists of an X and a Y value located within a 2 (or more) -dimensional space.

While the above coordinate system is 2-dimensional, we live on a 3-dimensional earth that happens to be “round. To define the location of objects on the earth, which is round, we need a coordinate system that adapts to the Earth’s shape. When we make maps on paper or on a flat computer screen, we move from a 3-Dimensional space (the globe) to a 2-Dimensional space (our computer screens or a piece of paper). The components of the `CRS`

define how the “flattening” of data that exists in a 3-D globe space. The `CRS`

also defines the the coordinate system itself.

A coordinate reference system (CRS) is a coordinate-based local, regional or global system used to locate geographical entities. – Wikipedia

## The components of a CRS

The coordinate reference system is made up of several key components:

**Coordinate system:**the X, Y grid upon which our data is overlayed and how we define where a point is located in space.**Horizontal and vertical units:**The units used to define the grid along the x, y (and z) axis.**Datum:**A modeled version of the shape of the earth which defines the origin used to place the coordinate system in space. We will explain this further below.**Projection Information:**the mathematical equation used to flatten objects that are on a round surface (e.g. the earth) so we can view them on a flat surface (e.g. our computer screens or a paper map).

## Why CRS is important

It is important to understand the coordinate system that your data uses - particularly if you are working with different data stored in different coordinate systems. If you have data from the same location that are stored in different coordinate reference systems, **they will not line up in any GIS or other program** unless you have a program like `ArcGIS`

or `QGIS`

that supports **projection on the fly**. Even if you work in a tool that supports projection on the fly, you will want to all of your data in the same projection for performing analysis and processing tasks.

**Data tip:** Spatialreference.org provides an excellent online library of CRS information.

### Coordinate system & Units

We can define a spatial location, such as a plot location, using an x- and a y-value - similar to our cartesian coordinate system displayed in the figure, above.

For example, the map below, generated in `R`

with `ggplot2`

shows all of the continents in the world, in a `Geographic`

Coordinate Reference System. The units are Degrees and the coordinate system itself is **latitude** and **longitude** with the `origin`

being the location where the equator meets the central meridian on the globe (0,0).

```
# devtools::install_github("tidyverse/ggplot2")
# load libraries
library(rgdal)
library(ggplot2)
library(rgeos)
library(raster)
#install.packages('sf')
# testing the sf package out for these lessons!
library(sf)
# set your working directory
# setwd("~/Documents/earth-analytics/")
```

In the plot below, we will be using the following theme. You can copy and paste this code if you’d like to use the same theme!

```
# turn off axis elements in ggplot for better visual comparison
newTheme <- list(theme(line = element_blank(),
axis.text.x = element_blank(),
axis.text.y = element_blank(),
axis.ticks = element_blank(), # turn off ticks
axis.title.x = element_blank(), # turn off titles
axis.title.y = element_blank(),
legend.position="none")) # turn off legend
```

```
# read shapefile
worldBound <- readOGR(dsn="data/week_04/global/ne_110m_land/ne_110m_land.shp")
# convert to dataframe
worldBound_df <- fortify(worldBound)
```

```
# plot map using ggplot
worldMap <- ggplot(worldBound_df, aes(long,lat, group=group)) +
geom_polygon() +
coord_equal() +
labs(x="Longitude (Degrees)",
y="Latitude (Degrees)",
title="Global Map - Geographic Coordinate System ",
subtitle = "WGS84 Datum, Units: Degrees - Latitude / Longitude")
worldMap
```

We can add three coordinate locations to our map. Note that the UNITS are in decimal **degrees** (latitude, longitude):

**Boulder, Colorado:**40.0274, -105.2519**Oslo, Norway:**59.9500, 10.7500**Mallorca, Spain:**39.6167, 2.9833

Let’s create a second map with the locations overlayed on top of the continental boundary layer.

```
# define locations of Boulder, CO, Mallorca, Spain and Oslo, Norway
# store coordinates in a data.frame
loc_df <- data.frame(lon=c(-105.2519, 10.7500, 2.9833),
lat=c(40.0274, 59.9500, 39.6167))
# add a point to the map
mapLocations <- worldMap +
geom_point(data=loc_df,
aes(x=lon, y=lat, group=NULL), colour = "springgreen",
size=5)
mapLocations
```

## Geographic CRS - the good & the less good

Geographic coordinate systems in decimal degrees are helpful when we need to locate places on the Earth. However, latitude and longitude locations are not located using uniform measurement units. Thus, geographic ‘CRS’s are not ideal for measuring distance. This is why other projected `CRS`

have been developed.

## Projected CRS - Robinson

We can view the same data above, in another CRS - `Robinson`

. `Robinson`

is a **projected** `CRS`

. Notice that the country boundaries on the map - have a different shape compared to the map that we created above in the `CRS`

: **Geographic lat/long WGS84**.

```
# reproject data from longlat to robinson CRS
worldBound_robin <- spTransform(worldBound,
CRS("+proj=robin"))
worldBound_df_robin <- fortify(worldBound_robin)
# force R to plot x and y values without rounding digits
# options(scipen=100)
robMap <- ggplot(worldBound_df_robin, aes(long,lat, group=group)) +
geom_polygon() +
labs(title="World map (robinson)",
x = "X Coordinates (meters)",
y ="Y Coordinates (meters)") +
coord_equal()
robMap
```

Now what happens if you try to add the same Lat / Long coordinate locations that we used above, to our map, that is using the `Robinson`

`CRS`

as it’s coordinate reference system?

```
# add a point to the map
newMap <- robMap + geom_point(data=loc_df,
aes(x=lon, y=lat, group=NULL),
colour = "springgreen",
size=5)
newMap
```

Notice above that when we try to add lat/long coordinates in degrees to a map in a different `CRS`

, that the points are not in the correct location. We need to first convert the points to thenew projection - a process called **reprojection**. We can reproject our data using the `spTransform()`

function in `R`

.

Our points are stored in a data.frame which is not a spatial object. Thus, we will need to convert that `data.frame`

to a spatial `data.frame`

to use `spTransform()`

.

```
# data.frame containing locations of Boulder, CO and Oslo, Norway
loc_df
## lon lat
## 1 -105.252 40.03
## 2 10.750 59.95
## 3 2.983 39.62
# convert dataframe to spatial points data frame
loc_spdf<- SpatialPointsDataFrame(coords = loc_df, data=loc_df,
proj4string=crs(worldBound))
loc_spdf
## class : SpatialPointsDataFrame
## features : 3
## extent : -105.3, 10.75, 39.62, 59.95 (xmin, xmax, ymin, ymax)
## coord. ref. : +proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0
## variables : 2
## names : lon, lat
## min values : -105.2519, 39.6167
## max values : 10.75, 59.95
```

Once we have converted our data frame into a spatial data frame, we can then reproject our data.

```
# reproject data to Robinson
loc_spdf_rob <- spTransform(loc_spdf, CRSobj = CRS("+proj=robin"))
```

To make our data place nicely with ggplot, we need to once again convert to a dataframe. We can do that by extracting the `coordinates()`

and turning that into a `data.frame`

using `as.data.frame()`

.

```
# convert the spatial object into a data frame
loc_rob_df <- as.data.frame(coordinates(loc_spdf_rob))
# turn off scientific notation
options(scipen=10000)
# add a point to the map
newMap <- robMap + geom_point(data=loc_rob_df,
aes(x=lon, y=lat, group=NULL),
colour = "springgreen",
size=5)
newMap
```

## Compare maps

Both of the plots above look visually different and also use a different coordinate system. Let’s look at both, side by side, with the actual **graticules** or latitude and longitude lines rendered on the map.

To visually see the difference in these projections as they impact parts of the world, we will use a graticules layer which contains the mediaian and parallel lines.

```
## import graticule shapefile data
graticule <- readOGR("data/week_04/global/ne_110m_graticules_all",
layer="ne_110m_graticules_15")
# convert spatial sp object into a ggplot ready, data.frame
graticule_df <- fortify(graticule)
```

Let’s check out our graticules. Notice they are just parellels and meridians.

```
# plot graticules
ggplot() +
geom_path(data=graticule_df, aes(long, lat, group=group), linetype="dashed", color="grey70")
```

Also we will import a bounding box to make our plot look nicer!

```
bbox <- readOGR("data/week_04/global/ne_110m_graticules_all/ne_110m_wgs84_bounding_box.shp")
bbox_df <- fortify(bbox)
latLongMap <- ggplot(bbox_df, aes(long,lat, group=group)) +
geom_polygon(fill="white") +
geom_polygon(data=worldBound_df, aes(long,lat, group=group, fill=hole)) +
geom_path(data=graticule_df, aes(long, lat, group=group), linetype="dashed", color="grey70") +
coord_equal() + labs(title="World Map - Geographic (long/lat degrees)") +
newTheme +
scale_fill_manual(values=c("black", "white"), guide="none") # change colors & remove legend
# add our location points to the map
latLongMap <- latLongMap +
geom_point(data=loc_df,
aes(x=lon, y=lat, group=NULL),
colour="springgreen",
size=5)
```

Below, we reproject our graticules and the bounding box to the Robinson projection.

```
# reproject grat into robinson
graticule_robin <- spTransform(graticule, CRS("+proj=robin")) # reproject graticule
grat_df_robin <- fortify(graticule_robin)
bbox_robin <- spTransform(bbox, CRS("+proj=robin")) # reproject bounding box
bbox_robin_df <- fortify(bbox_robin)
# plot using robinson
finalRobMap <- ggplot(bbox_robin_df, aes(long, lat, group=group)) +
geom_polygon(fill="white") +
geom_polygon(data=worldBound_df_robin, aes(long, lat, group=group, fill=hole)) +
geom_path(data=grat_df_robin, aes(long, lat, group=group), linetype="dashed", color="grey70") +
labs(title="World Map Projected - Robinson (Meters)") +
coord_equal() + newTheme +
scale_fill_manual(values=c("black", "white"), guide="none") # change colors & remove legend
# add a location layer in robinson as points to the map
finalRobMap <- finalRobMap + geom_point(data=loc_rob_df,
aes(x=lon, y=lat, group=NULL),
colour="springgreen",
size=5)
```

Below we plot the two maps on top of each other to make them easier to compare. To do this, we use the `grid.arrange()`

function from the `gridExtra`

package.

```
require(gridExtra)
# display side by side
grid.arrange(latLongMap, finalRobMap)
```

## Why multiple CRS?

You may be wondering, why bother with different `CRS`

s if it makes our analysis more complicated? Well, each `CRS`

is optimized to best represent the:

- shape and/or
- scale / distance and/or
- area

of features in the data. And no one `CRS`

is great at optimizing all three elements: shape, distance AND area. Some `CRS`

s are optimized for shape, some are optimized for distance and some are optimized for area. Some `CRS`

s are also optimized for particular regions - for instance the United States, or Europe. Discussing `CRS`

as it optimizes shape, distance and area is beyond the scope of this tutorial, but it’s important to understand that the `CRS`

that you chose for your data, will impact working with the data.

We will discuss some of the differences between the projected `UTM`

`CRS`

and geographic `WGS84`

in the next lesson.

## Optional challenge

Compare the maps of the globe above. What do you notice about the shape of the various countries. Are there any signs of distortion in certain areas on either map? Which one is better?

Look at the image below which depicts maps of the United States in 4 different

`CRS`

s. What visual differences do you notice in each map? Look up each projection online, what elements (shape,area or distance) does each projection used in the graphic below optimize?

## Geographic vs. projected CRS

The above maps provide examples of the two main types of coordinate systems:

**Geographic coordinate systems:**coordinate systems that span the entire globe (e.g. latitude / longitude).**Projected coordinate systems:**coordinate systems that are localized to minimize visual distortion in a particular region (e.g. Robinson, UTM, State Plane)

We will discuss these two coordinate reference systems types in more detail in the next lesson.

## Additional resources

- Read more on coordinate systems in the QGIS documentation.
- The Relationship Between Raster Resolution, Spatial extent & Number of Pixels - in R - NEON
- For more on types of projections, visit ESRI’s ArcGIS reference on projection types..
- Read more about choosing a projection/datum.

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