[Freegis-list] Need help converting GPS waypoint data

[Mikael Rekola]

Allan Metts <ametts2 at mindspring.com> wrote:

> 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.

This data shows a slight resemblance to NMEA-format. At least the
coordinates look similar, so the the correct format would be degrees +
decimal minutes.

The most obvious explanation for the mirroring and shifting would be
that you calculated your longitude wrong. As all the longitudes you have
are negative while the latitudes are not, it's possible that you
applied the sign incorrectly. All your waypoints should be on the
western side of the 84°W longitude line, but if you forgot to apply the
sign to the minutes part of the coordinate, your data would be mirrored
and shifted to the eastern side.

> $G,042604,162316,+3557.478,-08400.106,007.17

So you propably calculated the first longitude this way:

longitude=-84+0.106/60

while you should have done it like this:

longitude=-(84+0.106/60)

If you got that wrong, your data would be mirrored, and you first
waypoint would be shifted 0.212 minutes or about 300m to east.

--
Mikael Rekola