[Freegis-list] Need help converting GPS waypoint data

[Shane Brennan]

The joys of NMEA! I have some C++ code that parses the NMEA data from a 
Lowrance GPS/Sonar unit that's logged to hyperterminal. You can 
configure what NMEA information is output from the unit and in our case 
we used Latitude, Longitude and Depth so that rough bathymetric maps can 
be generated for our city's many small lakes very rapidly. We add in a 
water line with a known elevation as a nice breakline and then create 
TIN's using ESRI's 3D Analyst. Is there an open source alternative that 
writes to TIN or something similar?

Shane Brennan

> 
> Subject:
> Re: [Freegis-list] Need help converting GPS waypoint data
> From:
> Mikael Rekola <mrekola at acev.fi>
> Date:
> Wed, 02 Jun 2004 10:06:31 +0300
> To:
> Allan Metts <ametts2 at mindspring.com>, Freegis-list at intevation.de
> 
> 
> 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
> 
> 
> 
>------------------------------------
> 
> Subject:
> [Freegis-list] Need help converting GPS waypoint data
> From:
> Allan Metts <ametts2 at mindspring.com>
> Date:
> Tue, 01 Jun 2004 18:53:41 -0400
> To:
> freegis-list at intevation.de
> 
> To:
> freegis-list at intevation.de
> 
> 
> 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
> 
> 
> 
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