=== Grande échelle - règle de placement ===
To create a realistic experience for flights beyond a tile, there need to be matching rules for tiles. Since each tile represents a particular development in a larger weather environment, only certain types of tiles match. For instance, one cannot place a ' high-pressure region' tile next to a ' low pressure region' tile without crossing a front. With a moderately large library of different tile types and a set of placement rules, out of which ( in offline mode) a credible set of neighbouring tiles is chosen, a more or less realistic long-distance flight experience can result.
If each tile type contains default values for weather parameters ( to be overwritten by METAR info if available), the actual value of any parameter such as atmospheric pressure of visibility can be obtained by interpolation between tile centers, and the parameters will vary even in the absence of METAR info simpy due to the tile placement rules mocking up the presence of real large-scale weather systems.
Tile placement rules can be implemented in a Nasal loop which monitors the local position and initializes a new tile as soon as it is needed.
with the METAR system ==
Connecting the above concepts with real METAR info is not straightforward. The METAR info should conceptually influence three different things: