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]> 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 > > > > > > > > > > > Yahoo! Groups Links > > > > > > > Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/mitab/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
