[Xastir] Re: Interesting APRSdos map!

[Curt, WE7U]

On Thu, 4 Mar 2004, Henk de Groot wrote:

> Finally yesterday I started looking at the APRSdos map format, just
> basically trying to figure out the corner points. My math kept showing
> strange results so I looked at the Xastir code how it decodes these maps.
> It turns out the map covers a very big area, but more strangely it goes
> from 90:00:00.00E to 306:02:41.28E. Normally E/W values do never get bigger
> than 180 degrees, 181:00:00.00E would be written as 179:00:00.00W. On
> Xastir the map shows up, but half of it sticks outside the earth's grid on
> the right hand side... I can't change the 306:02:41.28E value to a west
> value since then the right border would be more left than the left border
> (hmm,how does one say this, does it still make sense?).

Yea.  Ran into a similar thing with the Martian coordinate system.
Some maps have it going with positive west, and with 0-360 degrees.


> Anyway, PocketAPRS has hard borders at 180W and 180E which cannot be
> crossed, so a map smack in the middle over his area will not work. But if
> APRSdos is able the show this map correcly, I assume Xastir should too, so
> it should have a notion that the earth is round (at least a cylinder
> approximation for this one...).
>
> I'm very interested in your reaction and if someone can make it show up
> correctly in Xastir...

Xastir is also set up with hard limits at +/- 180.  Some of the NOAA
Shapefile maps we use have polygons for islands that cross the -180
boundary, and we complain about that when those shapes are hit.  I
brought it up to them and the official response from them was that
the GIS software they were using allowed it, and so they used it.
They didn't see it as being wrong.  I do.  Something about 183
degrees in a longitude coordinate just doesn't sit right with me.


> P.S. Of course after this is fixed the next challenge is a polar map of the
> North Pole or the Antartic area... I wonder if APRSdos can do that too...

I remember doing the UPS and MGRS coordinate code for these regions.
There wasn't a lot of info on it.

I've also been asked by a user in New Zealand area to allow Xastir
to see the world as not ending at +/-180 degrees.  She has APRS
neighbors across the boundary line and must zoom out to world map
size to see all of her close neighbors.

I'd like eventually to allow the user to determine where the middle
of the map is, but right now I think it'd take quite a bit of work
to get there.  We should also convert coordinates that are
above 180 so that they get drawn correctly, but that's a tricky
operation if lines/polygons go across the boundary, as I'm sure
you've surmised.

If the map you are converting doesn't have any lines/polygons that
cross the boundary, I'd suggest converting the coordinates to within
the accepted parameters.  Anything crossing the boundary will have
to be split, even if it's an island.

--
Curt, WE7U			    archer at eskimo dot com
Arlington, WA, USA		http://www.eskimo.com/~archer
"Lotto:    A tax on people who are bad at math." -- unknown
"Windows:  Microsoft's tax on computer illiterates." -- WE7U
"The world DOES revolve around me:  I picked the coordinate system!"