[Xastir] Open Street Map support (read this)

[Jerry Dunmire]

Thanks for the feedback. Help is always appreciated Tom and Curt. I'll
try to keep up :-)

It will take me a bit to try out the great example you provided, but
I'll get started.

The OSM code is really just a (simple) modification of the tigermaps
code. The slow speed seems to be all related to pulling images from
the server. It doesn't appear to me like the images in the cache get
re-used if the center point changes. So even minor location changes
can cause a new map to be pulled. A far better solution would be to
cache OSM tiles (256x256), ala web based maps, but it I opted for the
quick fix first...

The zoom levels are 'binned' because I could not get the scaling to
work otherwise. OSM has 19 fixed levels (less for some rendering such
as the one I choose to use here). I had to reverse engineer the scale
factor because I could not find it documented at the OSM site. The
levels work out to 360/2^(L+8) degrees/ pixel, and tiles (the large
image I'm using, http://wiki.openstreetmap.org/wiki/StaticMap are
built from the tiles) are 256x256. So I pre-calculated a table of OSM
levels to xastir scale factors.

The next complication is that the OSM scale factor seems to be related
to latitude, so I had to map the scale_x value. To a user this is very
confusing at this point because the scale set in xastir is for the Y
axis.

Hope that helps with the understanding.

...jerry

On Wed, Jun 9, 2010 at 1:48 PM, Curt, WE7U <curt.we7u at gmail.com> wrote:
> On Wed, 9 Jun 2010, Tom Russo wrote:
>
>> I also note that the code appears to be changing requested zoom levels to
>> match
>> OSM zoom levels.  This may or may not be desirable (I was puzzled when I
>> tried
>> to set a custom zoom level to a particular value, and the code repeatedly
>> set it to something else).
>
> Tom:  I can send you the original patch if you'd like to see what
> was done.
>
> Perhaps you can help Jerry with the tweaks (if he needs/wants help).
> Perhaps I can too.
>
> --
> Curt, WE7U.                         <http://www.eskimo.com/~archer>
>   APRS:  Where it's at!                    <http://www.xastir.org>
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