Dear David and Mike
Wow! I'm not sure I get that one!! Does it answer the question?
But Mike I'm not sure the DAT file is always bigger?
The DBF file actually contains a header that describes each field ( so
far this makes DBF bigger ). This description is also in the TAB file
and therefore in the Native DAT file is not necessary. When using DBF
with TAB there is some redundancy( ie duplication).
There are advantages with DAT. DAT supports short and long integers
whereas DBF only supports Decimal ( Later versions of Foxpro implemented
same as DAT). This means if you have a field needing to hold values
between -32000 to +32000 DAT needs only 2 bytes per record where as DBF
needs 6 ( to hold sign as well ). If your number ranges from -2 Gig to +
2 Gig DAT gains even more. DAT (using Long Integer ie Integer) requires
only 4 bytes whereas DBF needs 11. ( So DAT should be smaller again ).
Another restriction of DBF is field name length. I think you'll find
the results in speed (searches, updates etc ) are about the same.
I've done a lot of testing using both and now nearly always use DBF.
Although DAT should be smaller on modern machines this is incidental.
The field name length is not really an issue ( 10 chars is enough ). The
payback is that you can access the data using FoxPro or VB ( Although if
you are changing records you need to keep two sets of indexes up to date
). I think your original question related to accessing data from other
sources. Unless you go OLE automation route then I would recommend DBF.
Hope this helps.
Regards
Bob
In message <[EMAIL PROTECTED]>, David Bruce
<[EMAIL PROTECTED]> writes
>Mike,
>The linkage of database records to map objects is integral to the GIS
>functions in MapInfo [or any other GIS software]. The way in which
>MapInfo implements the linkage - enabling creation, modifying, and
>deleting of object attributes, as well as maintaining logical database
>links to graphic objects which can be created, modified, or deleted as
>well - is a proprietary expansion of the published .dbf file format.
>
>David Bruce
>Pflum, Klausmeier & Gehrum Consultants, Inc.
>[EMAIL PROTECTED]
>
>"Perry, Mike" wrote:
>>
>> If a .DAT is just a slightly modified .DBF that happens to be larger in file
>> size, why does MI use it? What is different about the native files that
>> makes them superior for MapInfo's use?
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--
Bob Young - www.bydesignwales.demon.co.uk
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