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 8 Notes:  Datums

Referencing to Real-World Coordinates, Part 2:  Datums

As you know, everything within a GIS is geo-referenced.  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?  As you read last week, we have been dealing with topics in the science of geodesy.  Because we want our data for a particular project to overlay, or "match up," we need to pay attention to coordinate systems (last week) and datums (this week).  

Datums

We know where things are located on the Earth by using geodetic datums.  These datums define the reference systems that describe the size and shape of the Earth, and the orientation of the coordinate systems used to map the Earth.  These can be "flat-earth" models used in plane surveying on a local scale to complex models for applications that span continents, and can describe the size, shape, orientation, gravity, and angular velocity of the planet.  Just like there is no "one best map projection," or "one best coordinate system," there is no "one datum is best" model.   Different nations and organizations use different datums for GIS and navigation systems, depending on their needs. 

True geodetic datums were employed only after the late 1700s, when measurements showed that the Earth was an ellipse; close to a sphere, but the circumference is longer around the equator than from pole to pole.  The slight flattening of the earth at the poles results in about a twenty kilometer difference at the poles between an average spherical radius and the measured polar radius of the Earth.  If the GIS user is not aware of the issues surrounding datums, the positions on the map can be off by hundreds of meters. 

Reference ellipsoids are usually defined by the semi-major axis (equatorial radius) and flattening (the relationship between equatorial and polar radii).  Some geodetic datums are based on ellipsoids that touch the surface of the earth at a defined point.  For our purposes, the North American Datum 1927 (NAD27) is commonly used. It is tangent to the mean sea level surface at  Meades Ranch in Kansas.  NAD27 is not a global datum, but only for North America.  Some USGS data is referenced to NAD 27.  Other datums are "topocentric" datums with a reference ellipsoid that has its center at the center of mass of the earth.  The World Geodetic System 1984 (WGS-84) is an example of a global datum.  These global datums can be better fits to the gravity surface for the entire earth but can be less accurate in specific areas.  WGS 84 is the basis for that used in Global Positioning Systems (GPS) units. 

Two Types of Datums

Two major types of datums exist -- horizontal and vertical.  Horizontal datums define the relationship between the physical Earth and horizontal coordinates such as latitude and longitude.  The North American Datum of 1927 is an example.  Vertical datums define the height of surfaces, such as the National Geodetic Vertical Datum of 1929 (based on sea-level measurements and leveling networks) and the North American Vertical Datum of 1988, based on gravity measurements.  Some USGS and other commonly used data is in the 1929 datum, and others are in the 1988 datum.  The WGS 84 datum describes both vertical and horizontal coordinates.  It considers the rotation rate of the Earth and various physical constants such as the angular velocity of the earth and the Earth's gravitational constant.   We will be using several datums here in this course.  

Metadata

Metadata files are reference files about your spatial data, including who created the data, when it was created, what the data contains, and other information.  These files should indicate the horizontal and vertical datums for the spatial data.

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