Page | 8
An ellipsoid is created when an ellipse is rotated around its polar axis. h ‘m h m c l’pop sof n llps shown nfgu 2. ‘ ’ s assigned to represent the semi-m jo x s o qu o l d us nd ‘b’ the semi-minor or polar axis. The fl n ng, ‘f’, qu ls h o of h dff nc n‘ ’ nd‘b’o ‘ ’. 1.1.1 MSL, Elevation and Height Mean Sea Level is established by measuring the rise and fall of the tides. hs s noh ‘nxc’sc nc. h d s ff c d by h jux pos on of ‘c l s l’ obj c s, mos no bly h moon bu lso h planets to varying degrees. A well-established MSL reference datum is one where tidal movement has been observed for over 18 years at what s c ll d ‘ m y po ’. h m jo y of countries with a coastline today have established these primary ports along that coastline and the predicted level of tides is reported by the US National Oceanographic and Atmospheric Administration (NOAA) and the UK Hydrographic Office tide tables.
LINKS
NOAA tide tables UKHO tide tables
In order to tie MSL to both onshore elevations and offshore depths, these observations are tied to a physical benchmark usually in a nearby
Figure 2: Mathematical properties of an ellipse.
building wall or some other place unlikely to be inundated by the sea. This benchmark is quoted as a certain height above mean sea level. Sometimes MSL is used also as chart datum for the reduction of depth measurements to a common reference. Sometimes chart datum is established as the lowest level of low water, in order to provide mariners with the least possible depth at a point (i.e. the worst case). Onshore selected benchmarks represent an origin or starting point that can be used to provide the starting point for levelling across the whole country and continent.
h f ncsfo ohAm c h‘
l umof1929’-l n m d o h ‘ on lG od cV c l
um’( GV 29)- nd c nly djus d‘ o hAm c nV c l
um of 1988’ ( AV 88).
Figure 1 shows the difference between heights (h) above the reference ellipsoid and the height above the geoid (H) also known s ‘o hom c' h gh . A more general picture of this relationship with the definitions is in figure 3. The caution is that the GPS sys mpo d s‘h gh’ bo h llpsod,no M elevation. These heights have to be adjusted to make sure they match elevations from other datasets. 1.1.2 Coordinate Systems There are three fundamental types of coordinate systems that are used to define locations on the Earth: ‘G oc n c’ coo d n s m su ng X, Y, Z f om h centre of n ll pso d, ‘G og ph c l’ - latitude and longitude and height (figure 4) nd ‘ oj c on’ - easting and northing and elevation. Various subsets of h s c n lsob us d s‘2 ’conss ngofonly latitude and longitude or easting and northing.
Figure 3: Shows the relationship between surfaces.
Made with FlippingBook flipbook maker