Download Guidance notes on Native MapInfo Format A user guide

APPENDIX F Other possible candidates for “bounds”
The default bounds clause of 8.24 mm resolution on the x axis and 9.997 on
Y does not have any particularly good characteristics. Whilst 9.997 might
seem close to 10 millimetres, it is not 10 millimetres. Bearing in mind
measurements start from the origin, by the time you get to West Wales the
multiples of 9.997 could be falling anywhere.
At stated at the start of this article, no user should expect absolute accuracy
of 500 millimetres never mind one millimetre.
Provided all users are aware of a chosen “bounds” and the results of taking a
particular bounds clause there are four clear choices, for users working in
metres such as all Ordnance Survey customers.
There are some advantages in using larger step sizes. More objects will fit
within 6500 steps, so more objects are stored in less space, and are retrieved
and rendered to screen quicker.
Also, not to be recommended, but possible none the less is to map objects
such as North Sea Oil rigs that might otherwise not fit in our 2000 km bounds
clause. The British National Grid has been designed to be accurate for Great
Britain. However with Transverse Mercator errors will rapidily increase as you
move East or West of the coastline ( see Appendix D ).
The four candidates are
1 metre resolution ( specify a 2,000,000 km square for bounds )
0.100 metre resolution (specify a 200,000 km square)
0.010 metre resolution (specify a 20,000 km square)
0.001 metre ( specify a 2,000 km square – ie new MapInfo Version 8.0 option)
The “squares” defined do not have to start from (0,0). For example a 20,000
km square might be defined with bounds of (-5000000,-5000000)
(15000000,15000000).
The 1 metre step is a good option for data that is already rounded to 1 metre.
For example Meridian 2 from Ordnance Survey can be stored with no loss of
resolution and draw faster than using MapInfo default or any other value listed
here as objects whose MBR is less than 65 km ( most if not all ) are stored as
short integers.
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