CINXE.COM

<html> <head><script type="text/javascript" src="https://web-static.archive.org/_static/js/bundle-playback.js?v=7YQSqjSh" charset="utf-8"></script> <script type="text/javascript" src="https://web-static.archive.org/_static/js/wombat.js?v=txqj7nKC" charset="utf-8"></script> <script>window.RufflePlayer=window.RufflePlayer||{};window.RufflePlayer.config={"autoplay":"on","unmuteOverlay":"hidden"};</script> <script type="text/javascript" src="https://web-static.archive.org/_static/js/ruffle/ruffle.js"></script> <script type="text/javascript"> __wm.init("https://web.archive.org/web"); __wm.wombat("http://home.hiwaay.net/~taylorc/bookshelf/math-science/geodesy/positioning/index.html","20160303212325","https://web.archive.org/","web","https://web-static.archive.org/_static/", "1457040205"); </script> <link rel="stylesheet" type="text/css" href="https://web-static.archive.org/_static/css/banner-styles.css?v=p7PEIJWi" /> <link rel="stylesheet" type="text/css" href="https://web-static.archive.org/_static/css/iconochive.css?v=3PDvdIFv" />    <title>Locating a Point On the Earth</title> <link rel="stylesheet" href="/web/20160303212325cs_/http://home.hiwaay.net/~taylorc/style/basic.css" type="text/css" media="screen">  </head> <body>   <div class="navigate"><a href="/web/20160303212325/http://home.hiwaay.net/~taylorc/about/sitemap.html"><img src="/web/20160303212325im_/http://home.hiwaay.net/~taylorc/images/sitemap.gif" alt="Site Map"></a></div> <div class="logo"><img src="/web/20160303212325im_/http://home.hiwaay.net/~taylorc/images/logotop.gif" alt=""></div>  <h1 class="title">Locating a Point On the Earth</h1> <p>Giving a complete description of the location of a point on the Earth requires a lot of information--more than some might imagine. It requires three general pieces of information <em>in addition to</em> the point's coordinates.</p> <ul> <li><p>The terrestrial reference system (the geodetic datum)</p></li> <li><p>The coordinate system</p></li> <li><p>For all but <abbr title="Earth-centered, earth-fixed">ECEF</abbr> (Cartesian) coordinates, a reference for the height measurement.</p></li> </ul> <h1>Terrestrial Reference System</h1> <p>The terrestrial reference system (<abbr>TRS</abbr>) is a set of rules that define the relationship between a point's location and the coordinates that describe it. A <abbr>TRS</abbr> is also known as a <em>geodetic datum</em> and includes</p> <ul> <li><p>Rules that define the origin of the coordinate system (or systems) and in what directions the axes point</p></li> <li><p>An ellipsoid model, which approximates the shape of the Earth</p></li> </ul> <h1>Coordinate System</h1> <p>There can be any number of coordinate systems in which one may describe the location of a point on the Earth. The systems described below are the most common.</p> <h2>Earth-Centered, Earth-Fixed (<abbr title="Earth-centered, earth-fixed">ECEF</abbr>) Coordinates</h2> <p>The ECEF coordinate system is a three-dimensional Cartesian coordinate system. Its origin is at the center of Earth's mass, its x and y axes coincide with the plane of zero latitude, and the z axis coincides with the Earth's rotational axis. The x axis also passes through the point of zero longitude (the prime meridian).</p> <p>You might be tempted to believe that the ECEF coordinate system is independent of the TRS. Don't. There is almost always a translational or rotational difference (or both) between the ECEF coordinate system in different TRSes.</p> <h2>Geodetic Coordinates</h2> <p>Geodetic coordinates are the longitude, latitude, and height of a point.</p> <p>In cases where a spherical Earth model is used, the latitude is sometimes referred to as the <em>authalic</em> latitude.</p> <h2>Projected Coordinates</h2> <p>A projection coordinate system is one in which a portion of the round Earth is depicted on (usually) a flat surface. A projection coordinate system defines an origin and an orientation relative to ECEF or geodetic coordinates.</p> <p>Coordinates in a projected coordinate system describe point locations in terms of easting and northing from the coordinate system's origin.</p> <h1>Describing Height</h1> <p>In geodetic or projected coordinate systems, there is a height component. The height component is normally referenced to one of three possible baselines.</p> <ul> <li><p>The ellipsoid. A height referenced to this baseline is called the ellipsoid height.</p></li> <li><p>Mean sea level, which is described by the <em>geoid</em>. A height referenced to this baseline is called the orthometric height, or height above the geoid.</p></li> <li><p>A vertical datum, which normally declares a specific point to be at a known height above mean sea level</p></li> </ul>   <hr> <p><small>Chuck Taylor -- (<a href="/web/20160303212325/http://home.hiwaay.net/~taylorc/about/copyright.html">Copyright</a>) -- (<a href="/web/20160303212325/http://home.hiwaay.net/~taylorc/contact/">Contact</a>)<br> </small>  </body> </html>