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The .ov2 speed camera downloads on this website use type 2 (POI) and type 1 (Skipper) records.
So how are the "skipper" records used? They appear to describe a rectangle that TomTom can use for efficiency so that it does not have to read every POI record in the rectangle to know if there's a match.
There are 2764 skipper records out of 13384 (20%) in the current download. These are interspersed "randomly" throughout the file, and in some cases, there are more Skipper records than actual POI records! This would surely make the use of a skipper record LESS efficient than not using one at all...?
So what are the "rules" for when a skipper record should be used, as my impression is that their current use is sub-optimal...?
PS: I notice too that tools such are PoiConverter will strip off the Skipper records, presumably at the cost of efficiency, speed or resources.
Joined: 02/11/2002 22:41:59 Posts: 11878 Location: Massachusetts, USA
Posted: Fri Oct 20, 2006 10:04 pm Post subject:
Skipper records are like indexes in a relational database. You sacrifice space (to store the index) and gain speed. When not using skipper records your search time will go up proportional to the number of POIs.
When using skipper records the search time will be a constant, no matter how many thousand POIs you have. your impression about this being sub-optimal is wrong.
makeov2.exe will create skipper records by default.
Skipper records have no usefulness for programs like POIConverter, and are stripped out correctly. They only make sense in the application itself (TomTom) _________________ Lutz
Joined: May 20, 2006 Posts: 389 Location: Sydney, Australia
Posted: Fri Oct 20, 2006 10:45 pm Post subject:
lbendlin wrote:
Skipper records are like indexes in a relational database. You sacrifice space (to store the index) and gain speed. When not using skipper records your search time will go up proportional to the number of POIs.
When using skipper records the search time will be a constant, no matter how many thousand POIs you have. your impression about this being sub-optimal is wrong.
makeov2.exe will create skipper records by default.
Skipper records have no usefulness for programs like POIConverter, and are stripped out correctly. They only make sense in the application itself (TomTom)
I understand the indexing principle. My point is simply that the data has perhaps been carved up too much, so that there are too few or no POI's in the smallest indexing rectangles. i.e. Why index a single record - you might as well read the POI record itself (1 I/O) rather than the index then the record (2 I/O's)
e.g. , the last record in the file is preceeded by an index record as follows:
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