[Freegis-list] Need help converting GPS waypoint data

[Allan Metts]

I'm trying to overlay some GPS waypoints onto a city street map.  The GPS 
drive test data has somehow formed a mirror image (flipped in the east-west 
direction).  And it's shifted several miles due east.

All I have are cryptic GPS files like the excerpt you see below, and not 
much else.  Using C++ code, I'm reading in the $G lines, and interpreting 
the 4th and 5th fields in various ways.  I've tried Decimal Degrees, 
Degrees + Decimal Minutes (with the degrees being the most significant 
digits), and Degrees, Minutes, and Decimal Seconds. Different Map 
projections don't help much.

I convert to Decimal Degrees by dividing Minutes by 60 and Seconds by 3600.

I'm obviously landing in the right part of the world, but obviously 
something is quite wrong.  Any idea what could cause the mirror-image flip 
and eastward shift?

Thanks in advance,
Allan


$H,UNIT ID,none,1.110702tc,2,,3.03
$A,042604,162254,1,1,0,3,001,000,006,000,030,00000
$+i,042604,162256
$S,042604,162301,1
$s,042604,162301,1
$G,042604,162316,+3557.478,-08400.106,007.17
$G,042604,162317,+3557.477,-08400.107,009.62
$G,042604,162318,+3557.477,-08400.109,009.96
$G,042604,162319,+3557.476,-08400.111,009.48
$G,042604,162320,+3557.476,-08400.112,008.33
$G,042604,162321,+3557.476,-08400.113,006.29
$G,042604,162322,+3557.476,-08400.114,005.69
$G,042604,162323,+3557.476,-08400.116,008.50