Prefetching OSM Tiles


Lynn 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 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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