[Xastir] [tiling] reading tiles slow

[Curt, WE7U]

On Tue, 17 Aug 2010, Jason KG4WSV wrote:

> I suspect xastir uses its own projection because a) it's easier, and
> b) it supports so many different formats, it makes as much sense to
> convert to an internal easy-to-use format as anything else.
>
> I wonder if there's a reasonably efficient way to convert the tiles to
> xastir projection (or a projection that xastir can crunch quickly).
> If the tiles are going to be cached or otherwise re-used, that
> complexity and computation cost may make sense.

One of the difficulties with re-projecting maps in tile format is
that you end up with tiles that are not rectangles anymore, so
harder to combine properly into a contiguous map later.  This
problem can be alleviated by re-projecting a larger area and then
cutting the end result into rectangles, but they'd be cut along
different lines than the original tiles you started with, so the
caching code for them would have to be a bit different.  Gets a bit
tricky.

Note that for the USGS topo (UTM projection) map code we did NOT
reproject the map, instead going for "good enough" map reprojection
with custom code focusing on speed.  The resulting 7.5' quadrangle
pixels may be off from their true position by a small number of
meters as a result, but they render very quickly in Xastir.  The
1:100k and 1:250k maps are not as well positioned, but they're also
not anywhere near as detailed.

-- 
Curt, WE7U.                         <http://www.eskimo.com/~archer>
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