We have an iOS app that switches between different sets of custom map tiles depending on users' needs. Most are standard third-party sources (OpenStreetMap, NOAA marine charts), but we have a new set using image files we've created.
On the iPad these tiles are flickering on and off as iOS keeps asking for them again and again. (With other tile sets each unique tile is only requested once.) After the first request (which we get from our server) these files are in our local cache, and we deliver their data from there, but iOS keeps replacing them with its own tiles and then requesting/redisplaying ours.
The same tiles display just fine on the iPhone, so it doesn't appear to be an issue with the tile image files themselves. Comparing them to a tile set that does work as expected (NOAA's tileservice.charts.noaa.gov) there are differences, but nothing that would seem to explain this behavior.
Both sets are 8-bit RGB, 256 x 256 PNG images, non-interlaced. The NOAA tiles are 72 pixels/inch while ours are 96, and ours have an alpha channel.
Any suggestions as to what might be causing this, and how we might fix it?
Thanks,