# [GRASS5] Need help converting GPS waypoint data

Allan Metts ametts2 at mindspring.com
Tue Jun 1 18:58:52 EDT 2004

```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?

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

```