Hi Tim
The seamless table has a normal table at its
root. So the MAP file for the seamless table contains region objects that
represent the MBR extents of each base table. Its a similar concept to the
spatial index within the MAP file and allows vector layers to exceed the 2 GB
limit as well as allowing rendering of multiple rasters.
I have not used MITAB. However if you can render
regions from a MapInfo table, and you can render a single raster table then
you should be able to render the seamless as well.
You will need to read from the base table all the
MBRs that intersect your area of interest. This is the same as reading all the
regions that intersect your area of interest in a normal vector table. Having
done this use the record number to read into the DAT file to get the Table
name. This is similar to the technique needed to be able to thematically
render where again you need to read the DAT record to be able to decide the
render style based on attribution. I assume MITAB allows you to retrieve from
the DAT file, but if it doesn't they are fixed length records, and the
structure is defined both at the top of the DAT file and within the TAB
file.
Now you have the table name you can read the TAB
file for each raster so that you can render the raster in the appropriate
position - the same as you would for a single image.
To render a seamless vector table you need to
make your code recursive so that you can step down through possibly more than
one level but I would have expected your seamless raster to only be one level
deep ie the base table points directly to the raster
tables.
If you need more guidance feel free to get in
touch.
Hope this answer doesnt take 12 hours to reach
you. Having assumed the last message was not making it - after no activity for
12 hours - I posted a second and as you probably saw they both arrived
virtually together!
Regards
Bob
----- Original Message -----
Sent: Wednesday, August 16, 2006 1:14
PM
Subject: RE: [mitab] Locating features
in a particular area
Hi
Bob,
Thanks for the info.
I
worked how to use the spatial index in MITAB. I just need to add raster
map rendering now. One problem I have is reading a seamless raster tab
- I don't think seamless layers are supported in MITAB, and I don't know the
format to create my own reader.
Cheers
Tim
Hi Tim
A very late reply to your query on mitab on 5 June this
year regarding
finding features in an area:
To speed up access
of your maps of Europe reduce the number of tables by
combining them
to best use the performance of RTREE spatial index.
The index is
already built for you - in the MAP file. It is implemented
as
documented in several text books. Basically you only load a
percentage of
the file.
The deeper the tree the more impressive
the performance is. As already
answered through MapInfo L we have
implemented a 64 bit version and the
performance is still just as fast.
I parameterised the code so that I could
test different block sizes etc
and tested against National Coverage of
MasterMap.
Typically
each parent has ten to twenty children ( depending on object type
and
MBR resolution), so for just one extra disc read the MAP file can be
ten
to twenty times bigger, for two reads one hundred to four hundred
times
bigger and so on. The trick is to keep the tree balanced as you
add objects
to it.
Limit with 64 bit pointers in ID ( instead of
32 bit ) is 16 Terrabytes.
MapInfo do not think there is a need for
this now that Oracle Spatial is so
popular, but did take a look at the
work we did. We have had virtually zero
interest in what we did on 64
bit format from customers so perhaps they were
right not to commit
programming time to it, and now many public sector
organisations are
implementing storage in Oracle.
I find it incredibly frustrating
that so much money is spent on Oracle to
store data when it can be
stored for free using MapInfos elegant native
solution - and can
actually be written and retrieved significantly faster.
Missed your
original postings in June. Hope this belated answer, and the
MapInfo L
one is relevant for you.
Cheers
Bob
----- Original
Message -----
From: "scumhampton" <[EMAIL PROTECTED]com>
To:
<[EMAIL PROTECTED]com>
Sent:
Monday, June 05, 2006 9:59 AM
Subject: [mitab] Locating features in a
particular area
> Hi,
>
> I have to say I'm really
impressed with mitab.
>
> I'm creating my own map renderer -
much like MapX. Primarily I will
> use Navteq Street
data.
>
> The problem I have is with large data sets.
Currently my application
> loads an entire layer into ram at the
start (e.g. minor roads). This
> is not a problem for Luxembourg,
but when I load the whole of Europe
> into ram, as you can imagine,
things start to become an issue.
>
> I know loading all the
data at once is not the right thing to do - I
> only need to load
the area that I am currenly displaying. But how?
> How do I find the
ID of the features that should be displayed? I
> guess I need some
sort of geospatial look-up table. How does MapInfo
> do it - it must
load from the file dynamically as you pan. Is this
> possible with
mitab - or would I need to create my own special look-
> up
table?
>
> Any help would be appreciated.
>
> Kind
regards
>
>
Tim
>
>
>
>
>
>
>
>
>
>
>
Yahoo! Groups
Links
>
>
>
>
>
>
>
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