[aprssig] Map Symbol Placement

Curt, WE7U archer at eskimo.com
Thu Apr 19 10:38:24 CDT 2007

On Thu, 19 Apr 2007, Curt, WE7U wrote:

> On Thu, 19 Apr 2007, M J wrote:
> > 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
> I've worked on the code for this before.  First the program
> translates the pixels & locations you gave it to the pixels at
> diagonal map corners.  Once you have the new numbers you have the
> beginning/ending latitude and longitude for the area the map covers,
> assuming a rectangular map.
> After that it's simple math to subtract min-lat from max-lat and
> min-long from max-long, then each up by the number of pixels along
> that dimension of the map.

Should have been: "then divide each up by the number of pixels..."

> Of course the fact that both lat and long can use negative numbers
> complicates things slightly.  For the Xastir project we translate
> everything to our own coordinate system that uses only positive
> numbers which makes this part easier.

Curt, WE7U.   APRS Client Comparisons: http://www.eskimo.com/~archer
"Lotto:    A tax on people who are bad at math." -- unknown
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