GIS Course at Sinte Gleska University:

Lakota Studies 400/600:  Special Topics:  Introduction to Geographic Information Systems and Science

Instructor:  Joseph J. Kerski, USGS, jjkerski@usgs.gov, 303-202-4315

Week 7 Notes:  Coordinate Systems

Referencing to Real-World Coordinates, Part 1:  Coordinate Systems

As you are aware by this time in the course, everything within a GIS is geo-referenced.  That is, rather than being a simple map in a computer paint program or graphics program, all GIS data is referenced to positions on the Earth's surface.   But, how do we know where things are located on the Earth's surface in the first place?  The science of geodesy is concerned with these topics, and geodesy is therefore one of the sciences associated with geographic information sciences.   Why should we care about such topics?  We want the data for a particular project to overlay, or "match up."  Not paying attention to coordinate systems and datums could mean errors and frustration with your GIS projects in the future.   

We know where things are located on the Earth by using coordinate systems and geodetic datums.  This week's notes will describe coordinate systems.  Next week's notes will discuss geodetic datums.  Both will be important in the lab exercises and throughout your future journey with GIS.  The most commonly used coordinate system today is the latitude, longitude, and height system.  The Prime Meridian and the Equator are the reference planes used to define latitude and longitude.  The geodetic latitude of a point is the angle between the equatorial plane and a line normal to the reference ellipsoid.  Here, the "ellipsoid" refers to the shape of the Earth--not a sphere, but an ellipse, because it is slightly flattened at the poles.   The geodetic longitude of a point is the angle between a reference plane and a plane passing through the point, both planes being perpendicular to the equatorial plane.  The geodetic height at a point is the distance from the reference ellipsoid to the point in a direction normal to the ellipsoid.  

We use several coordinate systems to reference positions on the Earth's surface.  These positions have been used on maps, and therefore, they find their way into digital map data and GIS.  The most common ones in North America are latitude/longitude, UTM, and the State Plane Coordinate System. 

Latitude/Longitude

Latitude/longitude begins at the Prime Meridian, at 0 degrees longitude, which runs through Greenwich, England.  The equator is 0 degrees latitude.  Here in North America, longitude is measured west of the Prime Meridian and north of the equator.  West Longitude is given in negative units in all GIS software.  North Latitude is given in positive units.  Latitude/longitude is a geographic coordinate system; it is not tied to any particular map projection.

Universal Transverse Mercator System

The UTM (Universal Transverse Mercator) coordinate system is a rectangular coordinate system tied to the Transverse Mercator projection.  It divides the Earth into 60 zone numbers of  6° wide longitudinal strips extending from 80° South latitude to 84° North latitude.  UTM coordinates define two dimensional, horizontal, positions.  Each zone has a central meridian.  For example, zone 14, for much of South Dakota, has a central meridian of 99° west longitude.  The zone extends from 96 to 102° west longitude.  Locations within a zone are measured in meters eastward from the central meridian and northward from the equator. Eastings increase eastward from the central meridian which is given a false easting of 500000 meters so that only positive eastings are measured anywhere in the zone.  Northings increase northward from the equator with the equator's value differing in each hemisphere. In the Northern Hemisphere, the Equator has a northing of 0.  In the Southern Hemisphere, the Equator is given a false northing of 10,000 km.  

State Plane Coordinate System

State plane systems were developed in order to provide local reference systems that were tied to a national datum.  In the USA, the State Plane System 1927 was developed in the 1930s and was based on the North American horizontal Datum of 1927.  The coordinates are in English units (feet).  The State Plane System 1983 is based on the North American Datum of 1983 and the coordinates are metric.  While the NAD27 State Plane System has been superceded by the NAD83 System, maps and digital data in NAD27 coordinates are still in widespread use.  For example, most USGS 7.5 Minute topographic maps show State Plane coordinates from 1927.  Each state has its own State Plane system with specific parameters and projections.  Some smaller states use a single state plane zone while larger states are divided into several zones.  State plane zone boundaries often follow county boundaries.  

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