Coordinate Reference Systems

Jim Graham & Alex Daniels

Colorado State University

Warner College of Natural Resources

Coordinate Reference System

• References coordinates to the earth

• Also known as CRS

• Always has 4 components:– Coordinate System– Units– Projection– Datum

Coordinate Reference Systems

• Coordinate System– Cartesian or Rectangular– Spherical

• Units

• Projection– Geographic or Un-projected– UTM– State Plane

• Datum

Spherical Coordinates• Longitude:

Degrees East or West from the prime meridian

• Latitude: Degrees North or South from the Equator

Geographic “Projection”

-180° 180°90°

-90°

0°

0°

Units

• Spherical:– Degrees

• Rectangular:– Metric: Meters, Kilometers– English: Miles, Feet– Nautical: Nautical Miles

• Elevation:– Metric: Meters– English: Feet

Projected Coordinate Systems

• Use projection to display 3 dimensional locations on to a surface in 2D

• Uses Cartesian coordinates (rectangular)

Cartesian Coordinates(Rectangular)

• X,Y

• Easting, Northing

• Units: Miles, Meters, Feet, Nautical Miles

YNorth

XEast

Cogito Ergo Sum

Projections

• Different orientations, different projections

http://welcome.warnercnr.colostate.edu/class_info/nr502/lg1/lg1_master.html

Mercator Distortion

Projections

• There are hundreds of projections

• 98% of the time you will be using:– Geographic (Un-Projected)– Universal Trans-Mercator (UTM)– State Plane

The Earth is an oblate Spheroid

Datums

• There are hundreds of Datums

• If you only work in the US, 98% of the time you will be using:– NAD 27– NAD 83– WGS 84– HARN

• Recommendation: “Project” everything to WGS 84

Summary

• Coordinate Reference Systems (CRS) reference coordinates to the earth

• They are composed of:– A spherical or Cartesian Coordinate System– Units– Projection– Datum

CRS in GIS

• Your data must have a CRS!– We can guess at units, coordinate system,

and even the projection– Must find the datum!

• Sources:– “prj” file– Internal to file– From metadata– Over the phone, email, whatever!

Defining the CRS in ArcCatalog

http://jan.ucc.nau.edu/~rcb7/namNm15.jpg

Shapefile with defined CRS

• Units: Degree

• Projection:– Geographic– GCS=Geograp

hic coordinate system

• Datum:– NAD 83

Shapefile without a CRS

• Click on “Select”

• Or…

Finding the Right CRS

http://jan.ucc.nau.edu/~rcb7/namNm15.jpg

WGS 84

North American Datums

NAD 83

Defining the CRS in ArcGIS

• ArcToolBox:• Data Management Tools

• Projections and Transforms• Feature

• Project• Raster

• Project• Define Projection (they mean CRS)

http://jan.ucc.nau.edu/~rcb7/namNm15.jpg

http://jan.ucc.nau.edu/~rcb7/namNm15.jpg

Unknown CRS

Incorrect CRS’s are a nightmare!

Changing the CRS

• ArcToolBox:• Data Management Tools

• Projections and Transforms• Feature

• Project• Raster

• Project• Define Projection (they mean CRS)

http://jan.ucc.nau.edu/~rcb7/namNm15.jpg