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°
0°
180°
90°
0°
-90°
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)
Cogito Ergo
Sum
Cartesian Coordinates
(Rectangular)
• X,Y
• Easting, Northing
• Units: Miles, Meters, Feet, Nautical Miles
Y
North
X
East
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
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
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)
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)