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[aprssig] Map Symbol Placement

'Scott Miller' scott at opentrac.org
Thu Apr 19 15:38:14 UTC 2007

I think programs like UI-View just assume it's linear - i.e., if the
longitude of the point to be plotted is exactly between the left and right
reference points (x = x1 + (x1-x2)/2) and the screen is 1000 pixels wide,
then it's plotted at 1000/2 = 500.

Of course, maps aren't going to be linear if they're flat, and the Earth
isn't perfectly spherical.  To do it right you need to know what projection
the map uses.  Xastir is the only APRS program that I'm pretty sure knows
about map projections, and I think it uses an external library to handle

I suppose it all depends on the scale you're dealing with.  If you're
showing a small area, the error will probably be smaller than the size of
the symbol and it won't really matter.

Maybe someone with more geodesy knowledge than me can point you in the
direction of some appropriate resources.


> -----Original Message-----
> From: aprssig-bounces at lists.tapr.org 
> [mailto:aprssig-bounces at lists.tapr.org] On Behalf Of M J
> Sent: Thursday, April 19, 2007 8:07 AM
> To: aprssig at lists.tapr.org
> Subject: [aprssig] Map Symbol Placement
> I notice that within some APRS programs, it is necessary to 
> identify two lat/lon points on a map, and link them to the 
> corresponding map's pixels.  Has anyone more publicly 
> documented the equations used to plot a symbol on a map if 
> you know the lat/lon that the symbol is supposed to appear on 
> (once you know the previous info)?  I'm interested in 
> learning more about that.  Thanks!  MJ
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