Coordinate Systems, Projections, and Datums

Overview

An fundamental understanding of coordinate systems, projections, and datums is vital when preparing data for use in WMS or GIS.  Several coordinate systems have been developed to help describe a point in space based on different projections, datums, and units.  Those who learned geometry and trigonometry are familiar with several coordinate systems used to define points in space.  These include the cartesian coordinate system (x,y,z) and polar coordinates (r,q,z), among others.  For engineering purposes, state plane, UTM (Universal Transverse Mercater), and geographic (latitude/longitude) are some of the more common coordinate systems used in the United States.  Click here to learn more about coordinate systems from the University of Colorado at Boulder.

Most coordinate systems take the earth's surface (which is fairly elliptical) and "flatten" it out into a 2-dimensional map.  These attempts to portray the earth's surface onto a flat surface are called projecting the map.  There are different methods of projecting onto a flat surface.  Examples of projections include cylindrical and conic.  The UTM coordinate system is an example of a cylindrical projection.  Click here to learn more about map projections at from the University of Colorado at Boulder.

Each coordinate system must have a datum.  A datum helps describe the size and shape of the earth, and more importantly the origin and orientation of the coordinate system.  A location in one coordinate system and datum may be several hundred meters away from the same point described by the same coordinate system, but with a different datum.  Datums are used for horizontal coordinate systems (e.g., UTM) as well as for vertical position (e.g., elevation or height).  NAD27 (North American Datum of 1927) and NAD83 (North American Datum of 1983) are two commonly used datums in the United States.  You should include the datum(s) and units when stating what coordinate system you are using.  For example, UTM NAD27 meters.  Click here to learn more about datums from the University of Colorado at Boulder.

Different types of data coming from different sources may be in different coordinate systems.  When using multiple data layers together, they all must be in the same coordinate system.  For example, land use and soil type data is often in geographic coordinates (latitude, longitude).  Elevation data such as a DEM might be in UTM NAD27 in meters.  To overlay the soil type and land use data on top of the DEM, all data must be converted to a common projection.  It does not matter which projection is used, but if the majority of your data is in a UTM NAD27 coordinate system, it may be easiest to convert all the rest to this same coordinate system.  Sometimes it is not obvious what coordinate system data is in.  In this case, you must find the metadata.  The metadata (data about the data), which should tell you what projection the data is in, is typically available on the same website or in text documents attached to the data itself.  If you cannot find information about the data's projection, import the data into WMS or a GIS to see if it is in geographic coordinates (latitude/longitude).  Stand alone programs such as Corp Con will convert a single point (or a batch file of multiple points) from one coordinate system into another.  GIS systems such as ArcView can easily convert shapefiles from one coordinate system to another through the use of the Projector extension.  WMS v6.1 is also capable of converting many types of spatial data into different coordinate systems.