Coordinate reference systems

Acknowledgments

This lecture was based on materials from Dr. George Vellidis, Geocomputation with R e-book and Intro to spatial data programmin with R e-book

Learning outcomes

  • What is a coordinate reference system (CRS)?
  • Geographic vs. Projected CRS
  • Why should you care?
  • What are some of the most common CRSs used?
  • EPSG codes

What is a coordinate reference system (CRS)?

  • Geospatial data like vector and raster layers require a CRS to be correctly placed in space.

  • A CRS defines how the spatial elements of the data relate to the surface of the Earth (or other bodies)

  • CRSs are either geographic or projected

  • CRSs rely on a datum

Datum

  • Contains information on what ellipsoid to use and the precise relationship between the coordinates and location on the Earth’s surface

  • Two types of datum: geocentric and local

Geographic CRS

  • Normally uses a geocentric datum

  • Uses angular measurement units (e.g., degrees of longitude and latitude) to locate something in space

  • Longitude: location in the East-West direction

  • Latitude: location in the North-South direction

  • Popular example: WGS84

Projected CRS

  • Normally uses a local datum

  • Convert the three-dimensional surface of the Earth into Easting and Northing (x and y)

  • Uses linear measurement units, as for ex., meters

  • Popular example: Universal Transverse Mercator (UTM)

Projected CRS: UTM

  • Divides the Earth into 60 zones, each 6° of longitude in width.

  • Each zone uses a transverse Mercator projection that maps a region of large north-south extent with low distortion.

  • Each partition has a numerical code followed by the N-S hemisphere

Projected CRS: UTM

Projected CRS: UTM in the USA

  • Georgia is within zones 16 and 17 North

Geographic vs. Projected CRS

- Can you spot the difference(s)? Which is which?

Geographic vs. Projected CRS

- What about now?

How to refer to a CRS?

CRSs can be described in many ways, including:

  • Simple and ambiguous statements like “it’s in lon/lat coordinates”

  • Formalized yet now outdated ‘proj4 strings’ (also known as ‘proj-string’) such as +proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs

  • With an identifying ‘authority:code’ text string such as EPSG:4326 (preferable)

  • With a WKT description (most thorough, though long)

Why is EPSG preferable?

  • Machine readable
  • Short and easy to remember and reference to
  • Easily findable online on EPSG website

Why should I care about CRS?

When working with multiple geospatial layers, it is important to have them on the same CRS so layering operations can work properly.

Why should I care about geographic vs. projected CRS?

  • Knowing which CRS your data is in, and whether it is in geographic (lon/lat) or projected (typically meters), is important and has consequences for how R handles spatial and geometry operations

  • R has become better to project on-the-fly geographic CRSs for distance-based operations like buffering, clipping, etc.

  • However, still not fool proof:

Why should I care about geographic vs. projected CRS?

Bottom line is…

  • Need to have geospatial layers within a project containing the same CRS

  • If CRS is absent, need to set the original CRS of that object

  • If CRS is present but different, need to transform it to a common one

  • If conducting distance-based operations, preferable to have layers in a projected CRS

Recapping

  • CRS sets the spatial location of an object
  • Geographic vs. projected CRS
  • Need to have layers in a project under same CRS
  • Prefer projected CRS when performing distance-based operations

Some common CRS and their EPSG codes for this class