CODEWITHSUNDEEP
binarygpshexcoordinateslatitude-longitude
vanadium
vanadium

Reputation: 41

GPS Latitude and longitude from hexadecimal value

I'm getting the following replies from a GPS to a microcontroller, rather than try parse the string and convert that to hex to send, I'd rather just use the hex value supplied to then send to my server but I'm having difficulty working out what format it's stored in.

(another example:)

I've tried answers to other questions with similar titles to no avail..

Lat in the first example looks like -7020998 dec since it's south, similar to Lon in the second example (-22942054) is also negative in the west.

I've tried dividing the numbers by the converted expected result (using degrees + minutes/60 + seconds/3600) I come up with a rough number, ie:

This looks close to 180000, but if I divide by that it doesn't look right (-7020998 / 180000 = -39.00554). What does 186413 relate to though? I feel like I'm missing something completely obvious..

Edit: I'm able to get it working using the below quick example (I know it's not pretty):

if ($GPSLatitude > 0x7FFFFFFF) // ensure correct signedness
  $GPSLatitude-=0x100000000;

$GPSf = floatval($GPSLatitude) / 186413.51334561207757602506827277;
$GPSD = floor($GPSf);
$GPSt = abs($GPSf - $GPSD) * 60;
$GPSM = floor($GPSt);
$GPSS = floor(($GPSt - $GPSM) * 6000) / 100;
echo 'GPS Latitude: ' . $GPSD . ' Deg ' . $GPSM . ' Min ' . number_format($GPSS,2,'.','') . ' Sec';

The results match perfectly, but I know there's got to be a better way than dividing by that horrible number?

Upvotes: 4

Views: 7063

Answers (1)

Gangnus
Gangnus

Reputation: 24484

According to http://en.wikipedia.org/wiki/Geotagging, the GPS coordinates could be set even in rationals:

When stored in EXIF, the coordinates are represented as a series of rational numbers in the GPS sub-IFD. Here is a hexadecimal dump of the relevant section of the EXIF metadata (with big-endian byte order):

I am afraid, you have not all hex digits and you see only the upper part of the rational number. And the invisible divisor could be that magic number.

I am not insisting it is so, I only demonstrate there could be ANYTHING hidden in the format. And you are counting in degrees, and the coord could simply be gaussian X,Y.

Anyway, what is the name of your GPS format?

Upvotes: 1

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