Re: [mitab] Locating features in a particular area

[Bob Young]

Title: Message

Hi Tim   The parameters for the projection are held in the header of the MAP file.   As mentioned before I do not know MITAB, but I would imagine there is a function to read the projection for any particular table. If there is not Uffe has really good documentation at his website on the content of the MAP file so you could read the parameters directly yourself.   The overhead of conversion is not a problem with regard to render times. Bear in mind you are not reprojecting the whole file, only what you render, and the time to transform a node is much smaller than the time to read the data off the disc or storage card. Ideally though the OSTN02 "look up" table wants to be loaded into memory once, so that it is constanly used from memory. Even if it isn't you'll probably find the Operating System will cache it, giving the same effect.   You can see from MapInfo that the "on the fly" transformation will not be a problem. MapInfo is continually doing this as the contents of all its MAP files are actually in integers, and need to be multiplied by a "step size" which are the widths and heights of the Bounds clause divided by 2 GB to get floating point values. Its doing even more when different layers are in different base projections or even just different Bounds clauses.     Regards     Bob   ----- Original Message ----- From: Tim Smith To: mitab@yahoogroups.com Sent: Wednesday, August 16, 2006 3:29 PM Subject: RE: [mitab] Locating features in a particular area Hi Bob,   Thanks for your reply, it was very helpful :P I sort of imagined it would work something like that. I didn't know seamless layers could be recursive! I'll only be dealing with raster seamless layers. Another problem I have is coordinate conversion so I can project features on the screen. At present I make sure all my vector data is WGS84-lat/long, but I might have some OS mastermap data that I need to convert from UK grid to WGS84 for display. I have a free coord conversion library, but I don't know the best way to work out what projection and datum is being used in the vector or raster tabs, and how I would realte this to the datums and projections in the conversion library. I guess I'll just have to have a look up table and test it well. If map data is in a different datum to the one I want to project on the screen, I wonder how much overhead there is in converting each point to another datum, especially when there might be hundreds of points on the screen.   Cheers   Tim   -----Original Message----- From: mitab@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Bob Young Sent: 16 August 2006 14:36 To: mitab@yahoogroups.com Subject: Re: [mitab] Locating features in a particular area 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 ----- From: Tim Smith To: [EMAIL PROTECTED]com 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   -----Original Message----- From: [EMAIL PROTECTED]com [mailto:mitab@yahoogroups.com] On Behalf Of Bob Young Sent: 15 August 2006 23:28 To: [EMAIL PROTECTED]com Subject: Re: [mitab] Locating features in a particular area 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 > > > > > > > ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. 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