Hi,
To be concise, Ain el Abd 1970 is not a projection, it is a datum based on
the International ellipsoid. An ellipsoid is a three-dimensional
mathmatical model for the shape of the earth - there are numerous such
models, the International ellipsoid of 1910 is one of them. In simple
terms, a datum is defined by fixing the position and orientation of a
reference ellipsoid with respect to a some control point on the earths
surface. So in 1970 someone in Bahrain did that and created the Ain el Abd
datum of 1970.
The fundamental problem is that there is no "normal" Latitude and
Longitude". When Lat/Lon coordinates are quoted, the particular datum and
associated reference ellipsoid on which they are based need to be defined.
The same physical point on the earths surface can have any number of
differing Lat/Lon co-ordinates, depending upon the datum being used. So if
you know the Lat/Lon co-ordinates of a point in terms of one datum, but you
want to express them in terms of a different datum, a datum transformation
process is required.
Projections only come into the discussion when you want to create a 2
dimensional map of your 3 dimensional world. The various projections,
Mercator, Lambert etc. are methods of projecting the model of the earth and
associated features and co-ordinate system onto a flat plane or piece of
paper.
MapInfo confuses the issue somewhat in that it lumps everything together
under the 'Projections' button/function, but if you take a look at chapter
21 and Appendix H of the Users guide it gives some more detail. The
unqualified Lon/Lat system used as MapInfo's default does not explicitly
reference any datum, so in geodetic terms it is rather a loose cannon. If
you are working with data or maps that are being used for anything other
than illustration, you need to be sure that you know the datum of the
co-ordinates or map objects you are handling, otherwise calculations based
on the data can be significantly in error.
You can prove this to yourself, create two tables with columns for lat and
lon in each. Set the "Projection" for one table as Longitude/Latitude
(ED50), and the other as something else, say Longitude/Latitude (WGS84).
Enter the same co-ordinates in each table (say Lon 1.00, Lat 57.00). Open a
map window, and use create point to create a point symbol in the map window
from each of the tables. If you do this right, and then zoom right in, you
will find two distinct points 122 metres apart.
Geodesy - the study of the form of the earth is a science in itself. If you
want to find out a bit more, take a look at:
http://www.utexas.edu/depts/grg/gcraft/notes/datum/datum_f.html
-----Original Message-----
From: Bradley Browne [SMTP:[EMAIL PROTECTED]]
Sent: 27 March 2000 08:53
To: MapInfo mailing list
Subject: MI Comparison of Map Projections
I have a file that has a projection of "Longitude / Latitude (Ain el Abd
1970)\p4204". What is the difference between this and a normal long/lat
? If anyone has a URL for that can fully explain it, then that
information would be heaps better than telling me ... though that would
be appreciated too ;-) Is there a site that has comparisons of
projections and descriptions of the differing systems ?
Cheers,
Brad
--
Bradley Browne
Support Manager
Desktop Mapping Systems Pty Ltd
1st Floor 646 Newcastle St
Leederville, WA 6007
Ph: +61 8 9328 2715 Fax: +61 8 9328 2716
email: [EMAIL PROTECTED]
http://www.mapsys.com.au/
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