Topics

prefetching tiles


kc9gqr <kc9gqr@...>
 

when prefetching tiles is there a way to change how many zoom levels is prefetched? i think its 5 but can you cange it to like 15 in the xml file?


Lynn Deffenbaugh
 

See my response to N7FMH to see why this is not necessarily a good idea.  It has to do with what area coverage is desired for the increasing zoom levels, all of the area originally covered or just the center as if you had manually zoomed the whole way in at a constant screen center.  The client currently does the former on the prefetch which results in a 4x tile count for each zoom closer.  The latter does a constant tile count per level, but doesn't end up fetching detail for ALL of the area originally viewed.

See the table at http://wiki.openstreetmap.org/wiki/Slippy_map_tilenames#Zoom_levels to get an idea of the total tile count for various zooms.  For your suggestion of 15, if you did this while zoomed out to the planet view, you'd end up fetching (left column is zoom tile count, right column is cumulative total).

1 1
2 4 5
3 16 21
4 64 85
5 256 341
6 1024 1365
7 4096 5461
8 16384 21845
9 65536 87381
10 262144 349525
11 1048576 1398101
12 4194304 5592405
13 16777216 22369621
14 67108864 89478485
15 268435456 357913941
16 1073741824 1431655765
17 4294967296 5726623061
18 17179869184 22906492245

Oh, and you'd have to multiply all of those numbers by the number of tiles it takes to fill the screen from which you did the Prefetch.

Lynn (D) - KJ4ERJ

kc9gqr wrote:

when prefetching tiles  is there a way to change how many zoom levels is prefetched?  i think its 5 but can you cange it to like 15 in the xml file?



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Robin Williams <phonetech@...>
 

Phew????
That's a fair few tiles.
Bet you the local telco's would love that on a mobile data connection.lol
 
Robin ZL3REW


From: aprsisce@... [mailto:aprsisce@...] On Behalf Of Lynn W. Deffenbaugh
Sent: Monday, 1 March 2010 1:20 p.m.
To: aprsisce@...
Subject: Re: [aprsisce] prefetching tiles

See my response to N7FMH to see why this is not necessarily a good idea.  It has to do with what area coverage is desired for the increasing zoom levels, all of the area originally covered or just the center as if you had manually zoomed the whole way in at a constant screen center.  The client currently does the former on the prefetch which results in a 4x tile count for each zoom closer.  The latter does a constant tile count per level, but doesn't end up fetching detail for ALL of the area originally viewed.

See the table at http://wiki.openstreetmap.org/wiki/Slippy_map_tilenames#Zoom_levels to get an idea of the total tile count for various zooms.  For your suggestion of 15, if you did this while zoomed out to the planet view, you'd end up fetching (left column is zoom tile count, right column is cumulative total).

1 1
2 4 5
3 16 21
4 64 85
5 256 341
6 1024 1365
7 4096 5461
8 16384 21845
9 65536 87381
10 262144 349525
11 1048576 1398101
12 4194304 5592405
13 16777216 22369621
14 67108864 89478485
15 268435456 357913941
16 1073741824 1431655765
17 4294967296 5726623061
18 17179869184 22906492245

Oh, and you'd have to multiply all of those numbers by the number of tiles it takes to fill the screen from which you did the Prefetch.

Lynn (D) - KJ4ERJ

kc9gqr wrote:

when prefetching tiles  is there a way to change how many zoom levels is prefetched?  i think its 5 but can you cange it to like 15 in the xml file?



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