4-crs Grace Evans | March 30, 2026 4-crs class: title-slide background-size: cover <h1>Coordinate Reference Systems (CRS)</h1> <h3></h3> <div></div> <div></div> --- # Can't we just skip this section? --- # Please!? --- # You might avoid the pitfalls at first...  --- # But sooner or later...  --- # Problems will arise * Layers won't match up * Distances won't be correct * Circles turn to ellipses for no apparent reason --- # I guarantee that this section will pay dividends in your future work --- # Motivating example Map these together r landmarks <- read_sf("landmarks.geojson") boroughs <- read_sf("boroughs.shp") --- # Landmarks plot just fine r st_geometry(landmarks) %>% plot(pch = 16, col = "green4", cex = 2) <img src="4-crs_files/figure-html/unnamed-chunk-3-1.svg" /> --- # Boroughs also plot fine r st_geometry(boroughs) %>% plot() <img src="4-crs_files/figure-html/unnamed-chunk-5-1.svg" width="80%" /> --- # Plot both layers together *I promise this code is correct...* r plot(st_geometry(boroughs)) plot(st_geometry(landmarks), add = TRUE) --- # Dude where are my points! <img src="4-crs_files/figure-html/unnamed-chunk-7-1.svg" width="85%" /> --- # CRS mismatch is the most likely culprit --- # Knowing about coordinate reference systems is important .footnote[ [A nice reference from QGIS]() ] --- class: center, middle # Big picture --- # The earth is not a perfect sphere -- it's a spheroid/ellipsoid  --- # The earth's shape is defined by a mathematical formula  --- # And that formula has changed through time --- # A "geographic" coordinate reference system The spheroid definition is part of a geographic coordinate system -- a way to define locations on the earth --- # With a geographic CRS coordinates are in latitude, longitude --- class:center, middle # 3-D to flat map --- # Converting to a 2-D map is not so straightforward --  --- # Another mathematical formula!  --- # This conversion from 3-D to 2-D is called a projection Or a "projected coordinate reference system" --- # But no projection is perfect -- <br> <br>  --- # There are always distortions of shape, distance and relative angles --- # So you pick a projection based on what you're doing * One that minimizes distortion of the type you care about * For example, local distance measurement vs shower curtain global map? --- # Famous example, the mercator vs peters projection  --- # For mapping and geoprocessing you should use a projected CRS -- * Otherwise R/GIS will "flatten" a geographic CRS resulting in tons of distortion -- * Many R functions require a projected CRS (computations are much easier with a projected CRS) --- class: center, middle # CRS can be described by a pre-defined<br>string or a code --- # Both unprojected and projected CRS have a set of parameters --- # A CRS can be specified using a "proj4string" <br> <br> r "+proj=longlat +ellps=WGS84 +no_defs" --- # An unprojected (geographic) CRS has the word `longlat` -- r "+proj=longlat +ellps=WGS84 +no_defs" --- # And most CRS have a short-hand "EPSG" code -- r 4326 --- # The proj4string for a projected CRS is much longer This is the EPSG for State Plane Long Island (feet) -- "+proj=lcc +lat_1=41.03333333333333 +lat_2=40.66666666666666 +lat_0=40.16666666666666 +lon_0=-74 +x_0=300000.0000000001 +y_0=0 +ellps=GRS80 +to_meter=0.3048006096012192 +no_defs" --- # But this one also has an EPSG code r 2908 --- # EPSG vs proj4string -- * You can use either to assign/define a CRS in R for vectors -- * For rasters you'll need the proj4string --- # A note for the future... * There are some big changes upcoming in how CRS are defined * There will be a move away from proj4strings * The implications of this for {sf} and {raster} are uncertain at the moment --- exclude:true # Occasionally handy function to get proj4string from EPSG r # sp is loaded with raster sp::CRS("+init=epsg:4326") ## CRS arguments: ## +init=epsg:4326 +proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 ## +towgs84=0,0,0 --- # spatialreference.org is useful  --- class:center, middle # What CRS should I use!? --- # Thankfully, in most cases you do not have to decide for yourself -- * In most cases, the data you receive will already have a CRS you can use -- * If it doesn't come with a CRS you can ask the source of the file what the CRS is --- # When do you have to choose a CRS yourself? * If the data you receive is unprojected (geographic) and you want to use a projected CRS. --- # Nice guidance in Geocomputation with R A good discussion by Robin Lovelace, Jakub Nowosad and Jannes Muenchow [here](). --- # If you can, find out what government agencies are using for the area of interest * This is usually step 1 for me --- # I often google "best map projection for [area of interest]" * This is often step 2 for me --- # I use state plane for relatively small US areas  --- # I use UTM zones for global areas  .footnote[ ] --- # For both state plane and UTM, again, I often google: * "State plane zone for San Francisco" * "UTM zone for Ghana" --- exclude: true # For global I like the Robinson  .footnote[ [Source]() ] --- class: center, middle # The CRS in R --- # Hopefully your spatial data come with a CRS * Usually this is in the metadata of files you receive --- # {sf} and {raster} will read the CRS if it exists in the metadata * `st_read()` will read the CRS * `raster()` and `brick()` will read the CRS --- # Once your data is in R you can look at the CRS -- * `st_crs()` for vectors -- * `crs()` for rasters --- # An example with vectors r boroughs <- read_sf("boroughs.gpkg") landmarks <- read_sf("landmarks.gpkg") schools <- read_sf("schools.gpkg") --- # An example with vectors -- r st_crs(boroughs) ## Coordinate Reference System: ## No EPSG code ## proj4string: "+proj=lcc +lat_1=41.03333333333333 +lat_2=40.66666666666666 +lat_0=40.16666666666666 +lon_0=-74 +x_0=300000 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=us-ft +no_defs" -- r st_crs(landmarks) ## Coordinate Reference System: ## EPSG: 4326 ## proj4string: "+proj=longlat +datum=WGS84 +no_defs" --- # Trick question: Does this layer **have** a CRS? r st_crs(schools) ## Coordinate Reference System: NA --- # Answer: Yes but it's not defined in R -- * All spatial data *has* a CRS but it's not always in the metadata -- * Go back to the original source and find out what it is -- * Or, gulp, make an educated guess --- # For points with lat/long coordinates you can generally assume WGS84 -- Latitude values range between -90 to 90 and longitude ranges between -180 to 180 ## X Y ## 1 -74.01177 40.64915 ## 2 -73.98576 40.60125 ## 3 -73.97096 40.57757 ## 4 -73.99175 40.59758 ## 5 -74.02895 40.63418 ## 6 -74.00362 40.63315 -- With much larger values the CRS is not geographic ## X Y ## 1 980985.1 175780.8 ## 2 988205.1 158329.6 ## 3 992317.3 149703.0 ## 4 986541.2 156991.8 ## 5 976215.3 170325.0 ## 6 983246.6 169950.6 --- # If WGS84 doesn't work you probably need to contact the creator of the data --- # In the example of a spatial file without a specified CRS * Once you determine what the CRS should be * You assign/define the CRS --- # Assign/Define the CRS for vectors with `st_crs()` -- r # Using a proj4string st_crs(schools) <- "+proj=lcc +lat_1=41.03333333333333+ lat_2=40.66666666666666 +lat_0=40.16666666666666 + lon_0=-74 +x_0=300000 +y_0=0 +ellps=GRS80 +towgs84= 0,0,0,0,0,0,0 +units=us-ft +no_defs" -- r # Using an EPSG code st_crs(schools) <- 2908 --- # You can even use an existing layer to assign/define Take note, this is useful! -- r st_crs(schools) <- st_crs(boroughs) --- # For rasters use `crs()` in the same way -- * Except `crs()` does not accept an EPSG code as a number --- # For example... r canopy <- raster("canopy.tif") -- Get the CRS r crs(canopy) ## CRS arguments: NA --- # I went to the website and looked at the documentation for the CRS --- # Assign/Define the CRS for rasters with `crs()` r # Requires proj4string crs(canopy) <- "+proj=lcc +lat_1=41.03333333333333 +lat_2=40.66666666666666 +lat_0=40.16666666666666 +lon_0=-74 +x_0=300000 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=us-ft +no_defs" --- # If you want to use an EPSG code with `crs()` -- r crs(canopy) <- "+init=epsg:2908" --- # Subtle distinction between `st_crs()` and `crs()` output --- # With `st_crs()` you can extract EPSG and proj4string r vect_crs <- st_crs(schools) -- r vect_crs$epsg ## [1] 2908 -- r vect_crs$proj4string ## [1] "+proj=lcc +lat_1=41.03333333333333 +lat_2=40.66666666666666 +lat_0=40.16666666666666 +lon_0=-74 +x_0=300000.0000000001 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=us-ft +no_defs" --- # With `crs()` you can only get the proj4string r rast_crs <- crs(canopy) -- r rast_crs@projargs ## [1] "+proj=lcc +lat_1=41.03333333333333 +lat_2=40.66666666666666 +lat_0=40.16666666666666 +lon_0=-74 +x_0=300000 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=us-ft +no_defs" --- class: center, middle # Changing your layer's CRS --- # Often this is referred to as "projecting" --- # An example might be converting WGS84 to a projected CRS --- ## Change the CRS in R * Use `st_transform()` for vectors * Use `projectRaster()` for rasters --- # Back to our motivating example --- # Remember these two have different CRS r st_crs(boroughs) # projected ## Coordinate Reference System: ## No EPSG code ## proj4string: "+proj=lcc +lat_1=41.03333333333333 +lat_2=40.66666666666666 +lat_0=40.16666666666666 +lon_0=-74 +x_0=300000 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=us-ft +no_defs" r st_crs(landmarks) # unprojected ## Coordinate Reference System: ## EPSG: 4326 ## proj4string: "+proj=longlat +datum=WGS84 +no_defs" --- # We will choose to use the projected CRS --- # Transform landmarks to match boroughs r landmarks <- st_transform(landmarks, crs = st_crs(boroughs)) --- # Do they map now? -- r plot(st_geometry(boroughs)) plot(st_geometry(landmarks), add = TRUE, pch = 16, col = "red") <img src="4-crs_files/figure-html/unnamed-chunk-37-1.svg" width="75%" /> --- # One more great thing about {tmap} -- r landmarks <- st_transform(landmarks, crs = 4326) st_crs(landmarks) ## Coordinate Reference System: ## EPSG: 4326 ## proj4string: "+proj=longlat +datum=WGS84 +no_defs" r st_crs(boroughs) ## Coordinate Reference System: ## No EPSG code ## proj4string: "+proj=lcc +lat_1=41.03333333333333 +lat_2=40.66666666666666 +lat_0=40.16666666666666 +lon_0=-74 +x_0=300000 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=us-ft +no_defs" --- # Projections on the fly! r tm_shape(boroughs) + tm_polygons() + tm_shape(landmarks) + tm_dots(size = 0.3) <img src="4-crs_files/figure-html/domap-1.svg" /> --- # But don't let this allow you to get sloppy Most `sf` functions require the same CRS so this is good practice. --- class: center, middle, doExercise # open_exercise(4)
