Thanks to the wonderful people on Stackoverflow, I’m making some progress with figuring out how some geographic information is saved in a DB2 database that I’m working with.
it turns out that, rather than using the spatial functionality of DB2 (which would make sense, but considering the source of this database, I wouldn’t ask for too much), coordinate pairs are being stored as Morton codes in a 16-byte Character column.
So far, I’ve sorted out decoding the coordinate values, and can shuffle points predictably on my test map; now I’m just trying to figure out what grid that these coordinates are on. I’m fairly certain (not that I know anything about these things) that it’s somehow related to the MGA grid, because 0 east is at almost exactly 114°, which is where MGA zone 50 starts (the ‘almost’ comes from my point-and-click measurement technique).
But I don’t know where I’m going next; I’ll leave it till the morning.
My test code for this has been in PHP, and I needed something to convert big numbers between bases; hadn’t realised that the BC math library didn’t include base_convert(), so I found the one below.
/**
* Convert an arbitrary-length number between arbitrary bases.
* Copied, and very slightly translated, from
* http://www.technischedaten.de/pmwiki2/pmwiki.php?n=Php.BaseConvert
*
* @param $value
* @param $from_base
* @param $to_base
* @return string
*/
function bc_base_convert($value, $from_base, $to_base)
{
$valid_digits = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
if (min($from_base, $to_base) < 2)
{
trigger_error('Bad Format min: 2', E_USER_ERROR);
}
if (max($from_base, $to_base) > strlen($valid_digits))
{
trigger_error('Bad Format max: ' . strlen($valid_digits), E_USER_ERROR);
}
$dezi = '0';
$level = 0;
$result = '';
$value = trim(strval($value), "\r\n\t +");
$sign = ('-' === $value{0}) ? '-' : '';
$value = ltrim($value, "-0");
$len = strlen($value);
for ($i = 0; $i < $len; $i++)
{
$wert = strpos($valid_digits, $value{$len - 1 - $i});
if (FALSE === $wert)
{
trigger_error('Bad Char in input 1', E_USER_ERROR);
}
if ($wert >= $from_base)
{
trigger_error('Bad character in input 2', E_USER_ERROR);
}
$dezi = bcadd($dezi, bcmul(bcpow($from_base, $i), $wert));
}
if ($to_base == 10)
{
return $sign . $dezi; // Shortcut for base 10
}
while (1 !== bccomp(bcpow($to_base, $level++), $dezi));
for ($i = $level - 2; $i >= 0; $i--)
{
$factor = bcpow($to_base, $i);
$zahl = bcdiv($dezi, $factor, 0);
$dezi = bcmod($dezi, $factor);
$result .= $valid_digits{$zahl};
}
$result = empty($result) ? '0' : $result;
return $sign . $result;
}