[mitab] Locating features for particular area

Wed, 16 Aug 2006 04:28:12 -0700

I apologise if this hits list twice, but a posting done 12 hours ago has not appeared yet so I am trying to send it again. The first went to [email protected] as a reply. This one is to [EMAIL PROTECTED]
 
 
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  the 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 by working down through the tree. Items that are close on the ground in the real world are also organised to be close on the same branch in the MAP 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]>
To: <[email protected]>
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
>
>
>
>
>
>
>
>
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