Ordering a set of lines so that they follow one from the other
<pre><code>Line#       Lat.            Lon.

1a      1573313.042320  6180142.720910
..
..
1z      1569171.442602  6184932.867930

3a      1569171.764930  6184934.045650
..
..
3z      1570412.815667  6190358.086690

5a      1570605.667770  6190253.392920
..
..
5z      1570373.562963  6190464.146120

4a      1573503.842910  6189595.286870
..
..
4z      1570690.065390  6190218.190575
</code></pre>

<p>Each pair of lines above (a..z) represents the first and last coordinate pair of a number of points which together define a line.
The lines are not listed in sequence because I don't know what the correct sequence is just by looking at the coordinates (unless I look at the lines in a map).</p>

<p>Hence my question: how can I find programmatically (in Python) what the correct sequence is, so I can join the lines into one long line, keeping in mind the following problem:
- a 'z' point (the last point in a line) may well be the first point if the line is described as proceeding in the opposite direction to other lines. e.g. one line may go from left to right, another from right to left (or top to bottom and viceversa).</p>

<p>Thank you  in advance...</p>
