Mapping Matters: The layman's perspective on technical theory and practical applications of mapping and GIS

Abstract

Dr. Abdullah: Your concern about the concept of using two different datums to represent the horizontal and vertical position and height is valid, and using a single datum for both horizontal and vertical coordinates sounds like a good idea. A single datum representing horizontal coordinates and height is possible and may be used for certain applications. Unfortunately it is not practical for many of us in the mapping industry. The World Geodetic Systems of 1984 (WGS84) and the International Terrestrial Reference System (ITRS) are examples of such three-dimensional systems. WGS84 and ITRS provide users with unique position and height values based on the three-dimensional Cartesian system. The Cartesian system, when associated with a geo-centric system (in which the center of the system’s ellipsoid coincides with or near the mass center of Earth), is known as Earth Centered, Earth Fixed (ECEF). Therefore, position and height of a point near or on the surface of the Earth as defined by ECEF systems are referenced to the mass center of the Earth (or near it). GPS provides global positions (X,Y,Z) in WGS84-based ECEF systems. To many users, expressing coordinates in ECEF is not practical as positions are referenced to the mass center of the Earth. The following values are the three-dimensional coordinates for the CORS station AMC2 located in Colorado Springs, Colorado, USA based on ITRF2000 ECEF: X = -1248596.072 m | Y = -4819428.218 m | Z = 3976506.023 m The above coordinates are also published in geographic representation as follows: latitude = 38o 48’ 11.249150” N longitude = 104o 31’ 28.53276” W ellipsoid height = 1911.393m Examining the above coordinates one can easily realize that the height value of 3,976,506.023m is too large to deal with or to interpret. In addition, the geo-centric derived height, or Z, is not practical from the operational sense as it represents the perpendicular distance between the reference point and the plane of the equator and not the distance from the reference point to the local geoid as users are accustomed to. There are other reasons that have prevented users from using the three-dimensional ECEF for day-to-day operations, but there is no room in this column to discuss those. While using heights based on ellipsoidal, such GPS-derived heights based on WGS84, may serve the field commanders of the armed forces, it does not serve the needs of the larger community of users who are conducting accurate engineering operations for dams, sewers and pipelines, and tunnels. The previous operations require topographic details that reflect and explain the actual direction of water flow as influenced by gravity. Also, aircraft navigation databases need to provide pilots with accurate ground elevation to maintain constant altitude above the actual surface of the Earth. These are just examples on the reasons behind The orthometric height, which is derived from the geoid model that is modeled through gravity measurements, has a physical meaning and is considered to be the natural definition of heights as gravity is what causes water to flow down hill.