Handling Spatial Projection & CRS
Last updated on 2024-08-19 | Edit this page
Overview
Questions
- What do I do when vector data don’t line up?
Objectives
- Plot vector objects with different CRSs in the same plot.
In an earlier episode we learned how to handle a situation where you have two different files with raster data in different projections. Now we will apply those same principles to working with vector data. We will create a base map of our study site using United States state and country boundary information accessed from the United States Census Bureau. We will learn how to map vector data that are in different CRSs and thus don’t line up on a map.
We will continue to work with the some of the ESRI
shapefiles that we loaded in the previous episodes.
R
library(sf)
library(ggplot2)
library(dplyr)
R
point_HARV <- st_read("data/NEON-DS-Site-Layout-Files/HARV/HARVtower_UTM18N.shp")
# If you are getting an error, check your file path:
# You might need change your file path to:
# "data/2009586/NEON-DS-Site-Layout-Files/HARV/HARVtower_UTM18N.shp"
Working With Spatial Data From Different Sources
We often need to gather spatial datasets from different sources and/or data that cover different spatial extents. These data are often in different Coordinate Reference Systems (CRSs).
Some reasons for data being in different CRSs include:
- The data are stored in a particular CRS convention used by the data provider (for example, a government agency).
- The data are stored in a particular CRS that is customized to a region. For instance, many states in the US prefer to use a State Plane projection customized for that state.
Notice the differences in shape associated with each different projection. These differences are a direct result of the calculations used to “flatten” the data onto a 2-dimensional map. Often data are stored purposefully in a particular projection that optimizes the relative shape and size of surrounding geographic boundaries (states, counties, countries, etc).
In this episode we will learn how to identify and manage spatial data in different projections. We will learn how to reproject the data so that they are in the same projection to support plotting / mapping. Note that these skills are also required for any geoprocessing / spatial analysis. Data need to be in the same CRS to ensure accurate results.
We will continue to use the sf package in this
episode.
Import US Boundaries - Census Data
There are many good sources of boundary base layers that we can use
to create a basemap. Some R packages even have these base layers built
in to support quick and efficient mapping. In this episode, we will use
boundary layers for the contiguous United States, provided by the United
States Census Bureau. It is useful to have vector layers in ESRI’s
shapefile format to work with because we can add additional
attributes to them if need be - for project specific mapping.
Read US Boundary File
We will use the st_read() function to import the
/US-Boundary-Layers/US-State-Boundaries-Census-2014 layer
into R. This layer contains the boundaries of all contiguous states in
the U.S. Please note that these data have been modified and reprojected
from the original data downloaded from the Census website to support the
learning goals of this episode. We only care about the x and y
coordinates, not the z or m dimensions, so we will use
st_zm() to drop the coordinates that we do not need.
R
state_boundary_US <- st_read("data/NEON-DS-Site-Layout-Files/US-Boundary-Layers/US-State-Boundaries-Census-2014.shp") %>%
st_zm()
OUTPUT
Reading layer `US-State-Boundaries-Census-2014' from data source
`/Users/echelleburns/Documents/2024-07-01-ucsb-intro-geospatial/site/built/data/NEON-DS-Site-Layout-Files/US-Boundary-Layers/US-State-Boundaries-Census-2014.shp'
using driver `ESRI Shapefile'
Simple feature collection with 58 features and 10 fields
Geometry type: MULTIPOLYGON
Dimension: XY, XYZ
Bounding box: xmin: -124.7258 ymin: 24.49813 xmax: -66.9499 ymax: 49.38436
z_range: zmin: 0 zmax: 0
Geodetic CRS: WGS 84R
# If you are getting an error, check your file path:
# You might need change your file path to:
# "data/2009586/NEON-DS-Site-Layout-Files/US-Boundary-Layers/US-State-Boundaries-Census-2014.shp"
Next, let’s plot the U.S. states data:
R
ggplot() +
geom_sf(data = state_boundary_US) +
labs(title = "Map of Contiguous US State Boundaries") +
coord_sf()
U.S. Boundary Layer
We can add a boundary layer of the United States to our map - to make
it look nicer. We will import
NEON-DS-Site-Layout-Files/US-Boundary-Layers/US-Boundary-Dissolved-States.
R
country_boundary_US <- st_read("data/NEON-DS-Site-Layout-Files/US-Boundary-Layers/US-Boundary-Dissolved-States.shp") %>%
st_zm()
OUTPUT
Reading layer `US-Boundary-Dissolved-States' from data source
`/Users/echelleburns/Documents/2024-07-01-ucsb-intro-geospatial/site/built/data/NEON-DS-Site-Layout-Files/US-Boundary-Layers/US-Boundary-Dissolved-States.shp'
using driver `ESRI Shapefile'
Simple feature collection with 1 feature and 9 fields
Geometry type: MULTIPOLYGON
Dimension: XYZ
Bounding box: xmin: -124.7258 ymin: 24.49813 xmax: -66.9499 ymax: 49.38436
z_range: zmin: 0 zmax: 0
Geodetic CRS: WGS 84R
# If you are getting an error, check your file path:
# You might need change your file path to:
# "data/2009586/NEON-DS-Site-Layout-Files/US-Boundary-Layers/US-Boundary-Dissolved-States.shp"
If we specify a thicker line width using linewidth = 0.5
for the border layer, it will make our map pop! We will also manually
set the colors of the state boundaries and country boundaries.
R
ggplot() +
geom_sf(data = state_boundary_US, color = "gray60") +
geom_sf(data = country_boundary_US, color = "black", alpha = 0.25, linewidth = 0.5) +
labs(title = "Map of Contiguous US State Boundaries") +
coord_sf()
Next, let’s add the location of a flux tower where our study area is. As we are adding these layers, take note of the CRS of each object. First let’s look at the CRS of our tower location object:
R
st_crs(point_HARV)$proj4string
OUTPUT
[1] "+proj=utm +zone=18 +datum=WGS84 +units=m +no_defs"Our project string for point_HARV specifies the UTM
projection as follows:
+proj=utm +zone=18 +datum=WGS84 +units=m +no_defs
- proj=utm: the projection is UTM, UTM has several zones.
- zone=18: the zone is 18
- datum=WGS84: the datum WGS84 (the datum refers to the 0,0 reference for the coordinate system used in the projection)
- units=m: the units for the coordinates are in METERS.
Note that the zone is unique to the UTM projection. Not
all CRSs will have a zone.
Let’s check the CRS of our state and country boundary objects:
R
st_crs(state_boundary_US)$proj4string
OUTPUT
[1] "+proj=longlat +datum=WGS84 +no_defs"R
st_crs(country_boundary_US)$proj4string
OUTPUT
[1] "+proj=longlat +datum=WGS84 +no_defs"Our project string for state_boundary_US and
country_boundary_US specifies the lat/long projection as
follows:
+proj=longlat +datum=WGS84 +no_defs
- proj=longlat: the data are in a geographic (latitude and longitude) coordinate system
- datum=WGS84: the datum WGS84 (the datum refers to the 0,0 reference for the coordinate system used in the projection)
- no_defs: ensures that no defaults are used, but this is now obsolete
Note that there are no specified units above. This is because this geographic coordinate reference system is in latitude and longitude which is most often recorded in decimal degrees.
CRS Units - View Object Extent
Next, let’s view the extent or spatial coverage for the
point_HARV spatial object compared to the
state_boundary_US object.
First we’ll look at the extent for our study site:
R
st_bbox(point_HARV)
OUTPUT
xmin ymin xmax ymax
732183.2 4713265.0 732183.2 4713265.0 And then the extent for the state boundary data.
R
st_bbox(state_boundary_US)
OUTPUT
xmin ymin xmax ymax
-124.72584 24.49813 -66.94989 49.38436 Note the difference in the units for each object. The extent for
state_boundary_US is in latitude and longitude which yields
smaller numbers representing decimal degree units. Our tower location
point is in UTM, is represented in meters.
Reproject Vector Data or No?
We saw in an earlier episode
that when working with raster data in different CRSs, we needed to
convert all objects to the same CRS. We can do the same thing with our
vector data - however, we don’t need to! When using the
ggplot2 package, ggplot automatically converts
all objects to the same CRS before plotting. This means we can plot our
three data sets together without doing any conversion:
R
ggplot() +
geom_sf(data = state_boundary_US, color = "gray60") +
geom_sf(data = country_boundary_US, linewidth = 0.5, alpha = 0.25, color = "black") +
geom_sf(data = point_HARV, shape = 19, color = "purple") +
labs(title = "Map of Contiguous US State Boundaries") +
coord_sf()
Challenge 1: Plot Multiple Layers of Spatial Data
Create a map of the North Eastern United States as follows:
- Import and plot
Boundary-US-State-NEast.shp. Adjust line width as necessary. - Layer the Fisher Tower (in the NEON Harvard Forest site) point
location
point_HARVonto the plot. - Add a title.
- Add a legend that shows both the state boundary and the Tower location point.
R
NE.States.Boundary.US <- st_read("data/NEON-DS-Site-Layout-Files/US-Boundary-Layers/Boundary-US-State-NEast.shp") %>%
st_zm()
OUTPUT
Reading layer `Boundary-US-State-NEast' from data source
`/Users/echelleburns/Documents/2024-07-01-ucsb-intro-geospatial/site/built/data/NEON-DS-Site-Layout-Files/US-Boundary-Layers/Boundary-US-State-NEast.shp'
using driver `ESRI Shapefile'
Simple feature collection with 12 features and 9 fields
Geometry type: MULTIPOLYGON
Dimension: XY, XYZ
Bounding box: xmin: -80.51989 ymin: 37.91685 xmax: -66.9499 ymax: 47.45716
z_range: zmin: 0 zmax: 0
Geodetic CRS: WGS 84R
# If you are getting an error, check your file path:
# You might need change your file path to:
# "data/2009586/NEON-DS-Site-Layout-Files/US-Boundary-Layers/Boundary-US-State-NEast.shp"
ggplot() +
geom_sf(data = NE.States.Boundary.US, mapping = aes(color ="State Boundary")) +
scale_color_manual(values = c("State Boundary" = "gray18")) +
geom_sf(data = point_HARV, mapping = aes(shape = "Fisher Tower"),
color = "purple") +
scale_shape_manual(values = c("Fisher Tower" = 19)) +
labs(color = NULL,
shape = NULL,
title = "Fisher Tower location") +
coord_sf()
