Spatial Referencing - Coordinate Systems

Concepts
   coordinate systems
   UTM
   State Plane
   Lambert
   Geographic
   Datum
   Spheroid

Coordinate systems
A coordinate system is a defined consistent system of spatial reference.  Geographers have long used coordinate systems, for example both DaVinci and Mercator developed their own systems in the 1500s, and sailors relied heavily on early maps and coordinate systems to navigate the seas.  Coordinate systems are also known as map projections, though (as you will see in the next few pages) they can be different.  There are many types of coordinate systems, each designed to display a certain portion of the Earth accurately.


Portion of a Vespucci map clearly showing a latitude/longitude
coordinate system.

Why use coordinate systems?
A fundamental problem when making any map is that you are trying to represent a 3-D surface on a 2-D map.  There is no way to do this without some type of distortion.  Think about those maps which show the whole world.  Greenland (at 0.8 million square miles) shows up as big as Africa (almost 12 million square miles).  The coordinate system (or map projection) is designed in this case to represent the whole world, so accuracy in measurement is sacrificed in return for displaying the whole world.  Other coordinate systems are designed to represent smaller areas of the Earth and thus deliver higher accuracy for measurements.  Most coordinate systems are intended to accurately display the central section of the coordinate system on a map.  Thus the most accurate way to map any area would be to develop its own coordinate system.  However, after making 1000 maps you would see this is difficult to manage!  Thus geographers have created some standard coordinate systems which can be used for most maps, such as the ones listed below.

UTM
UTM (Universal Transverse Mercator)  is a widely-used standard coordinate system designed to map smaller areas of the Earth, such as Maine.  The UTM system divides the Earth into zones and each zone has its own coordinate system based on meters.  This system is good for areas like Maine which are small and require accurate measurements.  Large areas (for example Canada, Texas, Alaska) will likely span multiple UTM zones and thus require a different coordinate system.

State Plane
State Plane is another widely-used standard system which uses zones.  But the zones in State Plane are smaller than those in UTM and the units are typically English feet.  State Plane is typically used in surveying sites and municipal GIS.  Maine has 3 State Plane Zones, so it would not be appropriate to use this system to map the whole state.  The 3 zones are Maine East, Maine West, and Maine Central.  Maine East and Maine West are standard State Plane zones, while Maine Central was created by Maine DOT and implemented in 2000.

Lambert
Lambert is a system which allows the user to define the area in the coordinate system.  These areas do not have to be consistent like UTM zones are.  The user can also define the units in the coordinate system.  A Lambert system is used for large areas which span UTM or State Plane zones, such as Canada.  At MEDEP, Lambert is used for Canada-wide and Lower-48 state data layers.

Geographic
Geographic (latitude/longitude) is the most commonly-used coordinate system, but typically it is not used as a GIS coordinate system because it lacks a consistent unit of measurement.  Geographic is useful as a tool for global locating, but it cannot perform calculations such as area and length because a degree varies in size based on latitude (a degree of longitude at the equator is approximately 110 miles, at 45^o North it is approximately 50 miles).

Datum and spheroid
The Earth is not a perfect sphere, it is compressed at the poles and bulges at the equator.  Geodesists base coordinate systems on measurements of the Earth's curvature and surface.  The datum of a coordinate system is the set of reference data which is used to compute a coordinate system.  The spheroid is the model used to represent the Earth's shape when computing a coordinate system.  Typically, these are paired - a datum and spheroid are used together as a reference.  In Maine historically the datum that was used was North American Datum of 1927 (NAD27), paired with the spheroid Clarke1866.  As you can see, these were rather old reference data.  The newer standard of North American Datum of 1983 (NAD83), paired with the spheroid Geodetic Reference System of 1980 (GRS80) was adopted in 1998 and is now the most commonly-used system in Maine GIS.  A similar system is the World Geodetic System of 1984 (WGS84) which is very similar to NAD83 (so similar that for our purposes they are interchangeable), but it is a global reference.  WGS84 is commonly used in GPS equipment.