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Prefetching OSM Tiles

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  • Lynn W. Deffenbaugh
    What s with this new feature of Prefetching OSM Map Tiles? Why would I want to clutter up my hard drive with such stuff? Why not just let them download
    Message 1 of 1 , Feb 9, 2010
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      What's with this new feature of "Prefetching" OSM Map Tiles? Why would
      I want to clutter up my hard drive with such stuff? Why not just let
      them download when they're needed?

      Well, with the new KISS and AGW interfaces, APRSISCE/32 has moved beyond
      the assumption of Internet connectivity. In an RF-only APRS
      environment, it would still be nice to see the OSM map tiles; hence the
      prefetch option.

      To use it, simply zoom in or out and pan around until your map view
      shows the area whose maps you want to prefetch. Then simply select
      View/Map/Prefetch. APRSISCE/32 will calculate how many tiles will be
      necessary to have the current zoom and two additional zoom levels at the
      current size of your window. When you accept this confirmation, ALL of
      those tiles are queued for fetching and you get to watch an expanding
      circle moving slower than you've probably seen it move to date. Don't
      be alarmed if the number of tiles queued doesn't match the original
      estimate. The second number takes into account tiles that were already
      resident in your tile cache.

      After prefetching the tiles for a fairly large area, I zoom down a bit
      and continue prefetching more details and panning around fetching more
      and more details. It is, however, important to wait for each prefetch
      to complete before dragging the map around. APRSISCE/32 is smart enough
      to think that you've moved the map, so you must not need the stuff that
      was queued, so it will flush the queue and start over at the new
      location. Probably not what you want.

      When you DO go mobile without an Internet connection, just toggle off
      Enables/OSM Fetch Enabled and APRSISCE/32 will not attempt to load any
      additional tiles. If you zoom in further than you did while
      prefetching, APRSISCE/32 will simplly stretch the closest available tile
      resulting in a grainier looking map than you would otherwise have, but
      at least you have a map!

      Again, let me know if any of you are using this feature and how I can
      make it better (other than making the prefetch-queued tiles not flush
      when you pan around to queue more). BTW, double-clicking an empty space
      on the map will tell you how many tiles covering how much space you
      currently have on your machine as well as how long it has been since
      they've been referenced (since creation currently on Windows Mobile).
      The XML configuration file has a parameter called OSM.RetainDays. If
      this is set to zero, tiles will never be purged. Probably a good idea
      for a normally-disconnected machine on which a prefetch has been done.

      Lynn (D) - KJ4ERJ - Author of APRSISCE for Windows Mobile and Win32
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