Why change the datum and grid?

Increased efficiency and greater accuracy will result from a modern GPS based coordinate system.

The existing datums and grids were established between 1959 and 1961 and are standard versions used around the world at that time. They are not optimized for the Cayman Islands or GPS and are separate for Grand Cayman and the Sister Islands. The increased measuring accuracy afforded by GPS can not be fully utilized by the current system.

Increased accuracy and efficiency in the new system is achieved due to:

  • The projection chosen in 1959 (the Transverse Mercator) is best suited for areas with a larger north south extent relative to their east west extent. In the case of Grand Cayman the adoption of the Universal Transverse Mercator projection system and our position within Zone 17 results in projection scale factors approaching 0.9996, equivalent to 4cm per 100m or 1ft per half mile.
  • A distortion of the order of 4cm per 100m becomes problematic for many Geospatial professionals involved with any application where measured ground distances need to be the same as calculated grid distances. In order to complete accurate survey or stake out work based on grid coordinates it is necessary to either adjust the grid coordinate to a ground coordinate or adjust the measured ground distances to a grid distance.
  • The chosen new projection is the Lambert Conformal Conic with two standard parallels (along which the scale factor is 1.00000) one bisecting Grand Cayman the other bisecting the Sister Islands.
  • Changing the projection to one more specifically suited to our three islands results in the projection scale factor becoming negligible in most areas for most purposes, ranging from -3mm to +5mm per 100m on Grand Cayman and -3mm to +4mm per 100m on the sister islands. A maximum of 1ft per 3.75 miles.

The new system improves the efficient use of GPS as a mapping tool in the Cayman Islands due to:

  • A single new geodetic datum (CIGD11) replaces the two existing datums (GCGD59 and SIGD61).
  • CIGD11 latitude, longitude, ellipsoid heights are projected directly to CING11 easting northings using the Lambert Conformal Conic projection without the use of a local ellipsoid or the associated datum transformation.
  • The two existing datums both use locally fitted ellipsoids (the Clarke 1866 ellipsoid). The new geodetic datum is geocentric; using the GRS80 ellipsoid (designed to best fit the globe as a whole) with its origin at the earth’s gravitational centre.
  • The new datum is coincident with the ITRF05 realization of the WGS84 datum at epoch 2011.0 i.e. midnight on New Years Eve 2010.
  • GPS becomes a powerful elevation tool as CIGD11 ellipsoid heights can be converted directly to ground elevations by use of the appropriate island geoid model.