The earth's surface is approximated in size and shape by the geometric surface that is termed an ellipsoid. The NAD 27 is an example of horizontal geodetic datums the NGVD 29 and NAVD 88 are the most two common nationwide vertical datum systems. The first definition represents an idealization of a geodetic datum, and the second definition expresses the realization of a geodetic datum.īefore the advent of manmade satellites, geodetic positions in surveying were determined separately, either horizontally in two-dimensions as latitudes and longitudes, or vertically in the third dimension as heights or depths. The second definition, which is prevalent in mapping and charting, is realized, for example, by specification of ellipsoid, origin, and orientation in combination with a self-consistent set of observed reference coordinates. The first definition is realized, for example, by specificating an ellipsoid and associated origin and orientation information. A geodetic datum is as defined above, together with the coordinate system and the set of all points and lines whose coordinates, lengths, and directions have been determined by measurement and calculation.A geodetic datum is a set of constants specifying the coordinate system used for geodetic control.This broad characterization, in turn, leads to two related definitions of a geodetic datum. Geodetic DatumĪs a general definition, a datum is any quantity or set of quantities that may serve as a reference or basis for calculating other quantities. If you selected such an option, then your data will probably be in the selected datum. As above, augmented GPS data collection systems may provide the option of output in different datums. If you are using National CORS data for carrier phase post-processing, and if you are processing with ITRF2000 coordinates, then your output coordinates will be in ITRF2000. Coast Guard Maritime Differential GPS or National Differential GPS (NDGPS) base stations, then those broadcast pseudorange correctors are established using NAD 83 (86) coordinates, and your output coordinates will be in NAD 83 (86). If however, you are collecting data georeferenced by augmented GPS, then you are probably in NAD 83 (86). However, data collection systems may provide the option of output in different datums. When collecting data georeferenced with autonomous GPS, then you are in the WGS 84 (G1150) datum. Even if the features are horizontally referenced to NAD 83, this does not mean the vertical datum is a 3-D vertical datum. Since then, the horizontal features have been based on WGS 84 or in other geodetic reference systems which are compatible, such as NAD 83 or the ITRF combined with the GRS80 ellipsoid. On a global basis, the International Hydrographic Organization (IHO) designated the use of the World Geodetic System as the universal datum. Also note that charts (like maps) have a horizontal datum. Note: to support harbor and river navigation, bridge clearances are typically referenced to mean high water (MHW) not to a low water. In the United States, Mean Lower Low Water (MLLW) is the typical low water reference surface. Nautical charts will have depths referred to different tidal surfaces, which may vary from chart to chart. Except for the largest map scales, the horizontal components of WGS 84 and NAD 83 may be considered equivalent. Topographic maps (from USGS, for example) will have elevations referenced to either NAVD 88 or the older NGVD 29. In fact, both horizontal and vertical datum are usually provided. When dealing with paper maps and charts, the datums are typically listed in the legend of the map. If you generate new data, you should probably provide the datum identification along with those new data. A supplier of geospatial data should provide the appropriate metadata, in this case datum identification, to allow its use. These are the key questions that need to be answered. What datum are my data in? What datum do I want?
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