Examples of map data are:
The difference in co-ordinates between data is commonly referred to as datum shift. The datum shift between two particular datums can vary from one place to another within one country or region, and can be anything from zero to hundreds of metres (or several kilometres for some remote islands). The North Pole, South Pole and Equator may be assumed to be in different positions on different datums, so True North may be very slightly different. Different datums use different estimates for the precise shape and size of the Earth (reference ellipsoids).
The difference between WGS84 and OSGB36 is up to 140 metres (450 feet), which for some navigational purposes is an insignificant error. For most applications, such as surveying and dive site location for SCUBA divers, 140 metres is an unacceptably large error.
The main reason that there are a number of datums is that before the advent of GPS positioning, national map making organizations did not have a common surveying reference point and only produced maps for their locality.
In surveying and geodesy, a datum is a reference point or surface against which position measurements are made, and an associated model of the shape of the earth for computing positions. Horizontal datums are used for describing a point on the earth's surface, in latitude and longitude or another coordinate system. Vertical datums are used to measure elevations or underwater depths.
In common usage, elevations are often cited in height above sea level; this is a widely used tidal datum. Because ocean tides cause water levels to change constantly, the sea level is generally taken to be some average of the tide heights. Mean lower low water — the average of the lowest points the tide reached on each day during a measuring period of several years — is the datum used for measuring water depths on some nautical charts, for example; this is called the chart datum. Whilst the use of sea-level as a datum is useful for geologically recent topographic features, sea level has not stayed constant throughout geological time, so is less useful when measuring very long-term processes.
A geodetic vertical datum takes some specific zero point, and computes elevations based on the geodetic model being used, without further reference to sea levels. Usually, the starting reference point is a tide gauge, so at that point the geodetic and tidal datums might match, but due to sea level variations, the two scales may not match elsewhere. One example of a geoid datum is NAVD88, used in North America, which is referenced to a point in Quebec, Canada.
In geodetic coordinates the Earth's surface is approximated by an ellipsoid and locations near the surface are described in terms of latitude (), longitude () and height (). The ellipsoid is completely parameterised by the semi-major axis and the flattening .
It is important to note that geodetic latitude () is different than geocentric latitude (). Geodetic latitude is determined by the angle between the normal of the spheroid and the plane of the equator, whereas geocentric latitude is determined around the centre (see figure). Unless otherwise specified latitude is geodetic latitude.
|Reciprocal of flattening||1/f|
From a and f it is possible to derive the semi-minor axis b, first eccentricity e and second eccentricity e′ of the ellipsoid
|semi-minor axis||b = a(1-f)|
|First eccentricity squared||e2 = 1-b2/a2 = 2f-f2|
|Second eccentricity||e′2 = a2/b2 - 1 = f(2-f)/(1-f)2|
A more comprehensive list of geodetic systems can be found here
AGD66 and AGD84 both use the parameters defined by Australian National Spheroid (see below)
|semi-major axis||a||6378160.000 m|
|Reciprocal of Flattening||1/f||298.25|
Both GDA94 and GDA2000 use the parameters defined by GRS80 (see below)
|semi-major axis||a||6378137 m|
|Reciprocal of flattening||1/f||298.257222101|
see GDA Technical Manual document for more details.
The global positioning system (GPS) uses the world geodetic system 1984 (WGS84) to determine the location of a point near the surface of the Earth.
|semi-major axis||a||6378137.0 m|
|Reciprocal of flattening||1/f||298.257223563|
|Semi-minor axis||b||6356752.3142 m|
|First Eccentricity Squared||e2||6.69437999014x10-3|
|Second Eccentricity Squared||e′2||6.73949674228x10-3|
see The official World Geodetic System 1984 document for more details.
The Earth-centred Earth-fixed (ECEF) or conventional terrestrial coordinate system rotates with the Earth and has its origin at the centre of the Earth. The axis passes through the equator at the prime meridian. The axis passes through the north pole. The axis can be determined by the right-hand rule to be passing through the equator at 90o longitude.
In many targeting and tracking applications the local East, North, Up (ENU) Cartesian coordinate system is far more intuitive and practical than ECEF or Geodetic coordinates. The local ENU coordinates are formed from a plane tangent to the Earth's surface fixed to a specific location and hence it is sometimes known as a "Local Tangent" or "local geodetic" plane. By convention the east axis is labeled , the north and the up .
In an aeroplane most objects of interest are below you, it is therefore sensible to define down as a positive number, the NED coordinates allow you to do this as an alternative the ENU local tangent plane. By convention the north axis is labeled , the east and the down . To avoid confusion between and , etc in this web page we will restrict the local coordinate frame to ENU.
To convert from geodetic coordinates to local ENU up coordinates is a two stage process
Geodetic coordinates (latitude , longitude , height ) can be converted into ECEF coordinates using the following formulae:
Where and are the semi-major axis and the square of the first numerical eccentricity of the ellipsoid respectively
To transform from ECEF coordinates to the local coordinates we need a local reference point, typically this might be the location of the radar. If a radar is located at and an aircraft at then the vector pointing from the radar to the aircraft in the ENU frame is
Note: is the geodetic latitude. A prior version of this page showed use of the geocentric latitude (). The geocentric latitude is not the appropriate up direction for the local tangent plane. If the original geodetic latitude is available it should be used, otherwise, the relationship between geodetic and geocentric latitude has an altitude dependency, and is captured by:
Note that is also called the Normal, and is the length of the line segment, co-linear with the altitude vector and normal to the ellipsoid, which runs from the geodetic ellipsoid at the specified latitude/longitude to the intersection with the line connecting the north and south poles.
Obtaining geodetic latitude from geocentric coordinates from this relationship requires an iterative solution approach, otherwise the geodetic coordinates may be computed via the approach in the section below labeled "From ECEF to geodetic coordinates." Reference
for another example of computing ENU coordinates.
The geocentric and geodetic longitude have the same value. This is true for the Earth and other similar shaped planets because their latitude lines (parallels) can be considered in much more degree perfect circles when compared to their longitude lines (meridians).
Note: Unambiguous determination of and requires knowledge of which quadrant the coordinates lie in.
This is just the inversion of the ECEF to ENU transformation so
The conversion of ECEF coordinates to geodetic coordinates (such WGS84) is a much harder problem. A number of techniques are available but the most accurate according to Zhu (Ref 6), is the following 15 step procedure summarised by Kaplan. It is assumed that geodetic parameters are known
Note: arctan2[X,Y] is the four quadrant inverse tangent function
This code was written in MATLAB. This would be useful for converting GPS measurements into the local ENU coordinate system.
This code was written in MATLAB. This would be useful for converting from the local ENU coordinate system into the coordinate system that GPS uses.
Note: atan2(Y,X) uses quadrant information to return a value of lambda between and .