What is difference between projection and datum?
A datum is simply a foundation and reference for spatial measurements. A system of coordinates is then used to describe those measurements relative to the datum, and a projection is the visual representation of those measurements on a different surface.
What is vertical datum in surveying?
A vertical datum is a surface of zero elevation to which heights of various points are referenced. Traditionally, vertical datums have used classical survey methods to measure height differences (i.e. geodetic leveling) to best fit the surface of the earth.
What is the vertical datum for WGS84?
Fig. Definitions of height (from an old NIMA web page)
| Datum | Height | Example |
|---|---|---|
| National or continental | Orthometric | NAVD 1929, NGVD 1988 |
| Geodetic | Geoid | EGM2008, EGM 96 |
| Ellipsoidal | Ellipsoidal | WGS84 |
| Tidal | Orthometric | Local chart datum from nautical chart with tide gauge |
What are examples of datum?
Examples of map datums are:
- WGS 84, 72, 66 and 60 of the World Geodetic System.
- NAD83, the North American Datum which is very similar to WGS 84.
- NAD27, the older North American Datum, of which NAD83 was basically a readjustment [1]
- OSGB36 of the Ordnance Survey of Great Britain.
- ETRS89, the European Datum, related to ITRS.
What’s the difference between a datum and a projection?
A datum is a specific, known point on or in the Earth that is used for reference. A projection uses the datum as a point of reference, it’s location on Earth. In GIS, there are two types of “coordinate systems”: Geographic Coordinate System (latitude and longitude) and Projected Coordinate System (X and Y).
Which is the correct definition of a vertical datum?
A vertical datum is a surface of zero elevation to which heights of various points are referenced. Traditionally, vertical datums have used classical survey methods to measure height differences (i.e. geodetic leveling) to best fit the surface of the earth.
What is the vertical datum for Puerto Rico?
TABLE 1: Current Vertical Datums for United States and Territories Datum Name Dates North American Vertical Datum of 1988 (N 1992-present Guam Vertical Datum of 2004 (GUVD04) 2004-present Northern Marianas Vertical Datum of 2003 2003-present Puerto Rico Vertical Datum of 2002 (PRVD 2002-present
Can a datum be referenced to a different coordinate system?
It is possible, and even common for datasets to be in the same projection, but be referenced to different datums, and therefore have different coordinate values. For example, the State Plane coordinate systems can be referenced to either NAD83 and NAD27 datums.