Re: GPSMAP 60Cx or 76Cx?


Harald Hanche-Olsen <hanche@xxxxxxxxxxxx> writes:
Assuming one trackpoint per second, that is about 120 bytes per
trackpoint. That's a lot of bytes; enough to make me wonder if they
are using an XML based format. 8-)

Sigh, they are using xml. I was really hoping it would be something
along the lines of:

float-lat
float-lon
float-altitude
int32 time

That would have been 16 bytes per trackpoint. Instead we have:

<?xml version="1.0" encoding="UTF-8" standalone="no" ?>
<gpx xmlns="http://www.topografix.com/GPX/1/1"; creator="MapSource 6.5" version="1.1" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"; xsi:schemaLocation="http://www.topografix.com/GPX/1/1 http://www.topografix.com/GPX/1/1/gpx.xsd";>
<trk>
<name>ACTIVE LOG145605</name>
<trkseg>
<trkpt lat="37.123456" lon="-121.123456">
<ele>26.106</ele>
<time>2006-06-05T21:56:05Z</time>
</trkpt>
...
</trkseg>
</trk>
</gpx>

They also only log 6 decimal places of lat/lon which seems awfully
disproportionate seeing how no other aspect of this format strives for
any kind of efficiency. If they are still using what they call
semi-circles internally, the internal representation is roughly
equivalent to 7-8 decimal places. Arbitrarily truncating it from 8
bytes to 6 in order to save 4 bytes total seems silly. After the fact
averaging could have benefited from the extra information.

;; semi-circle is 360 degrees divided into 2^32 equal parts.
(/ 360.0 (expt 2.0 32)) 8.381903171539307e-08

-wolfgang
--
Wolfgang S. Rupprecht http://www.wsrcc.com/wolfgang/
.

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