The JAD 2001 Datum
The parameters for JAD2001 follow:
Lambert Conformal Conic Projection
False Easting: 750000m
False Northing 650000m
Latitude of 1st Parallel: 18 N
Latitude of 2nd Parallel: 18 N
Longitude of Central Meridian: 77 W
Latitude of origin of projection: 18 N
Spheroid Name: WGS84
It should be first noted that the spheroid is WGS84. This means that JAD 2001 is a datum in name only; it is in fact the reference spheroid against which datums are offset. There is one great advantage to this, and one significant disadvantage.
The advantage, of course, is that there is no need for a datum transformation to convert Jamaica Grid Coordinates to WGS84, and visa versa; one merely deals with the projection, the False Easting, and the False Northing. A 2D position will have the same latitude and longitude in both JAD 2001 and WGS84, thus ensuring that errors are not introduced by dodgy datum parameters when GPS derived positions are turned into Ja grid coordinates.
The disadvantage is caused by the geoid height. To put it simply, WGS84 sea-level is not the actual level of the sea in Jamaica. Because of variations in the Earth's mass concentrations and resultant equipotential surface, the planet's true sea-level, the orthometric height, is not a perfect spheroid, but instead bulges in some places and is dimpled in others. The WGS84 spheroid is just the best fit, not a perfect model of the Earth's surface. To compensate for this, we have a dataset that is not a model but is empirically derived. The name for this is the WGS84 EGM96 geoid. It will tell you the actual, local altitude for a particular WGS84 position. We can therefore see that this geoid height problem is not such a great disadvantage and is easily overcome. An online Geoid Calculator can be found at NIMA, (note: orthometric Height = GPS ellipsoidal height - geoid height).
We offer now a walk-through of how to convert WGS84 L/L to JAD2001 Grid, and JAD2001 Grid to WGS84 L/L, using the readily available freeware program, Geotrans.
Open Geotrans, and leave both the upper and lower windows at the default datum, "WGE"; this is WGS84. In the lower window, click where it says, "Universal Transverse Mercator (UTM)", and by using the drop down menu on the right, set it instead at ""Lambert Conformal Conic". You will then see a new choice of variables and these must be entered as: Central Meridian: 77 0 0.0 W, Origin Latitude: 18 0 0.0 N, 1st Std Parallel: 18 0 0.0 N, 2nd Std Parallel: 18 0 0.0 N, False Easting (m): 750000, False Northing (m): 650000. You are now ready to convert positions.
In the upper window, enter your Geodetic Lat/Lon positions and click, "Convert: Upper -> Lower". In the lower window, the result is where it says "Easting/X (m)" and "Northing/Y (m)". These are the JAD2001 Metre Grid Coords. You can reverse the procedure by entering the JMG in the lower window and converting Lower -> Upper. You will also find, by fooling around with Geotrans, that you can convert UTM to JAD2001 JMG, and visa versa, and if you want, you can even convert MAS, the Ethiopian datum, to JAD2001, (not that it will do you any good).
To convert from JAD69 to JAD2001, and back, you will need to create the JAD69 datum in Geotrans; the walk-through can be found here. Conversions from WGS84 L/L to JAD2001 L/L are not required because they are exactly the same.
Readers of the foregoing who need further assistance are welcome to contact us at firstname.lastname@example.org.
Information on maps can be found via our page, Jamaica Map Resources.
By: R.S.Stewart of the Jamaican Caves Organisation
|JAD69 and the GPS|
More info on Datums, Projections, and Grids in Jamaica
The needs for and problems associated with the adoption of a geocentric datum for Jamaica: Kirk Haughton, MSc Surveying 2001
|Maps, Georeferencing, and GIS in Jamaica|