The Essential Elements of a 3-D Geographic Coordinate John W. Dix 13 September 2005 ESRI Homeland Security GIS Summit - Denver, CO John.W.Dix@nga.mil NATIONAL GEOSPATIAL-INTELLIGENCE AGENCY
Overview is a framework for reporting the essential elements of a three dimensional (3-D) geographic coordinate 2
Background Global Positioning System (GPS) provides a worldwide network for determining precise geodetic positions GPS geographic coordinates based on the WGS-84 ellipsoid are global, standardized, precise, and reliable GIS provide geographic coordinates in a variety of formats which may not be referenced to a datum and lack information about source and accuracy of a coordinate 3
Comparison of GPS and GIS GPS GIS Geocentric Absolute Positioning Coherent Network Precise WGS-84 Datum Worldwide Image Space Object Characterization Various Implementations Inconsistent Accuracy Various Datums Worldwide 4
The 5 Elements of L: Latitude and Longitude E: Elevation A: Accuracy D: Datum S: Source 5
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Height Above Ellipsoid & Mean Sea Level Surface of Earth Height Above Ellipsoid WGS-84 Ellipsoid Mean Sea Level Elevation Above Mean Sea Level 7
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The 5 Elements of L: Latitude and Longitude E: Elevation Height Above Ellipsoid (GPS) Mean Sea Level (MSL) A: Accuracy Horizontal Vertical D: Datum WGS-84 Ellipsoid North American Datum 1983 Other S: Source GPS Receiver Paper map or chart series Geographic Information System Other 9
Value Added Provides producers and users a common framework for transferring and assessing geographic coordinates from different sources Allows users to correlate geographic coordinates from various sources, such as GIS, paper maps and charts, and GPS receivers 10
Process System Design & Development Standardized Production Process Standards Based > Systems Engineering > Application Interfaces Procedures & Training Processed Geographic Coordinates 11
Summary defines the minimum set of data elements required to accurately characterize a geodetic coordinate does not satisfy every need, but is a simplified format for describing the most important elements of a geographic coordinate concept can be broadly applied in military, scientific, and technical applications 12
Recommendations Know the heritage of the coordinate(s) you are using Build quality into the process instead of inspecting for quality after production (W. E. Deming) Define community best practices for determining, communicating, integrating, and displaying geographic coordinates Develop Graphical User Interfaces, applications, mensuration software, and Application Program Interfaces (API) that implement best practices Use display format for geographic coordinates that is readily understood Degrees/Minutes/Seconds (D/M/S) versus Decimal degrees (D.dddddd) Adopt best practices in software and system engineering that display geographic coordinate at a level of precision consistent with the accuracy of the source from which the coordinate is derived 13
Conclusion Geospatial information community would benefit from a standard methodology for correlating geographic coordinates from GIS, imagery, and paper maps and charts with geographic coordinates from GPS receivers. 14
Technical Details Worth Remembering Image resolution is not an indicator of geopositional accuracy One meter pixels do not mean one meter geopositional accuracy Adding decimal precision to an inaccurate coordinate does not make it more precise and dilutes its integrity Measuring a coordinate to nearest 0.001 seconds of arc (1.2 inches), is rarely possible, leaves the impression that a coordinate is precise, and complicates correlating coordinates from multiple sources 15
Technical Details Worth Remembering At the equator: One minute of arc equals one nautical mile (~6080 feet) One second of arc equals approximately 101 feet: 0.01 arc seconds = 1 foot 0.001 arc seconds = 0.1 feet (1.2 inches) One degree is approximately 364,800 feet: 10-5 degrees ~ 3.6 feet 10-6 degrees ~ 4 inches 10-7 degrees ~ 1/2 inch 16
NATIONAL GEOSPATIAL-INTELLIGENCE AGENCY 17