Office of GEOINT Sciences: Precise Positioning
All You Ever Wanted to Know and Couldn't Find Out About
(In Plain English)William H. Wooden
A National Geospatial-Intelligence Agency publication.
There are many methods of determining your position. If you have a gasoline station road map and see a road sign that indicates it is five miles to the next town, you can estimate your approximate position. If you have a more accurate product, such as a paper map or chart, or better yet a digital image product from the National-Geospatial Intelligence Agency (NGA), you can get a better position. Finally, if you have an electronic device such as a Navstar Global Positioning System (GPS) satellite receiver, you can get an extremely precise position.
Precise positioning is needed to determine your exact current location (What is my starting point?) and/or your destination (Where is my target?). These questions have different answers depending on your frame of reference and what you intend to do with the information. If you are in a hangar and want to move your airplane outside, you only need to know where you are with respect to the door. An eyeball estimate would be sufficient. It would not be necessary to know your location or the door's position to the nearest inch. The problem becomes a little more complicated if you are trying to put ordnance on target. In that case, you need to know your position very accurately relative to the target. Hence, before asking "what is my position," you need to know to what accuracy you need the information and in what frame of reference.
The focus of this paper is to give the warfighter practical information on positioning accuracies possible using NGA Geospatial Informational and Services (GI&S) products. The paper presents the underlying basic reference system of GI&S products, the difference between accuracy and precision, how errors affect positioning, the accuracies of paper and digital products, GPS positioning, future product accuracies, and factors to consider to ensure accuracy. In other words, how do you use GI&S products correctly to accomplish the mission, what are the advantages of different products, and what are the limitations of these products? offers the warfighter a "bag of tricks". The warfighter must understand the content of NGA the bag and pick the appropriate product to accomplish the mission.
A Basic Primer on World Geodetic System--
A Point of Reference
A geodetic datum is a reference system or base from which accurate positions of points on the Earth's surface can be determined. It is defined by specifying a reference surface (size and shape) and an origin (starting point with certain conditions including orientation). It identifies where you are with respect to some starting point. It is similar to knowing where you are with respect to the runway of the airfield from which you took off. In other words "where am I with respect to the airfield (reference surface) and touchdown point of the runway (origin) and how do I return home?"
Because the Earth is slightly flattened at the poles and bulges at the equator, an ellipsoid is chosen to approximate its shape. Hence, a geodetic datum is defined by its ellipsoid (reference surface) and its origin (starting point). If the system is applicable in a local area, it is called a local geodetic datum. North American Datum 1927 (NAD 27) is an example of a local geodetic system. Its reference surface is the Clarke 1866 ellipsoid; its origin is Meades Ranch in Kansas; and its area of applicability is North America. An example of a system that is applicable worldwide is the World Geodetic System 1984 (WGS 84). Its origin is at the center of the earth. NGA is responsible for GI&S products worldwide. Therefore, due to the large number and variety of such products and users, the need for commonality in relating information from one product to another, increasingly stringent accuracy requirements, and the need to support space-age activities worldwide, it is advantageous to tie GI&S products to a world geodetic reference system. WGS 84 represents NGA's best modeling of the Earth from a geometric, geodetic and gravitational standpoint. It provides the means for relating positions on various local reference systems to a single Earth-centered, Earth-fixed coordinate system. WGS 84, which is accurate to better than five centimeters, is the internal model for NGA production. All areas of the world have been mapped or charted on multiple reference systems depending on local preferences at different times, i.e., different ellipsoids and datums have been used. In order to relate this old information to new products, it must be changed or transformed to WGS 84. Consequently, NGA continues to update or improve existing datum transformation constants (which mathematically relate other systems to WGS 84) or, when required, derive completely new transformations. However, it must be noted that if you begin with poorly-known coordinates in a local reference system and change them to WGS 84, they do not magically become more accurate.
Datums & Targets
When a geodetic datum is changed, coordinates of a point will usually change. In some cases, the differences can be as large as 900 meters. Why is this important? If a soldier calling for close air support has his coordinates with respect to one datum and your coordinates are with respect to a different datum, you could fire at the targeted location and miss the requested location by hundreds of meters- the most severe consequence of your action being friendly fire! Another example would be missing a targeted command and control center and hitting a hospital instead. To ensure the success of your operation and avoid the pitfalls of coordinate misuse: (1) ensure that all coordinates are on the same datum, (2) verify all coordinates when sources are in doubt, (3) ensure that your coordinates are in the same datum as your operating system (e.g., aircraft inertial navigation system, weapon delivery system, etc.), and (4) ensure that you do not degrade the delivery accuracy of your weapon by using coordinates that are less accurate than your system permits. Some operating systems have user-selectable datums, while others are hard-wired to a specific datum. For interoperability reasons, WGS 84 is the preferred system. If your operating system is hard-wired with something other than WGS 84, special measures must be taken to compensate for the difference between the systems. Knowing your system will determine the action to take for a specific mission (e.g., transform coordinates into your system, update to WGS 84 using GPS, etc.). Whenever possible, to minimize any problems with coordinate systems and to obtain the most accurate results, use WGS 84 as the geodetic reference system throughout the entire operation.
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