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[aprssig] Re: TIGER maps what-s the problem?

Tom Russo russo at bogodyn.org
Tue Jun 13 15:16:27 UTC 2006


On Tue, Jun 13, 2006 at 08:12:09AM -0600, we recorded a bogon-computron collision of the <w0ep at frii.com> flavor, containing:
> On Tue, 2006-06-13 at 07:43, KC2MMI (Jared) wrote:
> 
> > If the guy writing the software can't confirm the map data, why is he using the
> > most misleading possible icon, a pinpoint or speck, to (mis)represent that data?
> > 
> 
> Big circle, little circle, pinpoint or speck,
> it makes no difference in the least.  The guy writing the software
> has zero, none, zip, nada, information about accuracy
> or lack thereof, so he shouldn't make a presentation that he
> knows.  Making a big fuzzy circle would be more bogus than
> making a pinpoint.

That's not entirely correct.  If position ambiguity is enabled, there is a
fairly clear indication of the precision of the posit --- drawing a blob 
as big as the ambiguity is a reasonable thing as long as the map is zoomed
in to the point where the blob is bigger than the icon chosen.  That's all
that Bob asked for in the mails that started this friendly little discussion.  
(FWIW, Xastir will shade a rectangle corresponding to the range of lat/lon 
that is allowed by the ambiguity of the posit).

And for regular uncompressed posits, there's something like a 60 foot 
resolution just because of the limits of the transmitted data.  If zoomed in 
to the point that the icon/symbol/pinprick is substantially smaller than 60 
feet on the map, it would be reasonable to display a 60 foot circle or
rectangle instead, even for a posit with no deliberate ambiguity.  A similar
argument can be made for compressed posits with their extra digit or so of
precision, and !DAO! posits. (FWIW, Xastir is as guilty as the next program
on this bit, but it wouldn't take much to fix just by having an extra piece of 
data stored with the station record ("how many digits of precision did we
decode from the packet's position fields?") and drawing a circle around the 
station marker if that circle would be big enough to be visible)  This, again,
is all that Bob said.

All of this is completely independent of the precision of the underlying
map data (which Bob never mentioned), and dependent only on the number of 
digits of position that come in and how zoomed in the map is, which the 
programmer of the APRS application certainly *DOES* have access to.

-- 
Tom Russo    KM5VY     SAR502  DM64ux         http://www.swcp.com/~russo/
Tijeras, NM  QRPL#1592 K2#398  SOC#236  SKCC#2002
"And, isn't sanity really just a one-trick pony anyway? I mean all you get is
 one trick, rational thinking, but when you're good and crazy, oooh, oooh,
 oooh, the sky is the limit!"  --- The Tick




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