Output coordinates of sample points
Use the Path Tool
If you change the Path Tool to "polygonal" mode (in its Options dialog) and hold down the SHIFT key while placing your points then you can later export the path to an SVG file that uses a simple XML-based format from which it should be easy to parse the data points.
The following extract shows how the coordinates of six path points would appear in the exported file -- the "M" indicates a "move to" operation (i.e., no connecting line is drawn) and can be ignored.
d="M 99.00,106.00M 258.00,112.00M 307.00,290.00M
66.00,318.00M 504.00,136.00M 576.00,305.00"
<li>Open image in GIMP</li>
<li>Select path tool</li>
<li>press shift key & select paths using path tool</li>
<li>goto menu select windows</li>
<li>select Dockable Dialogs</li>
<li>select layer of your path</li>
<li>right click to export path</li>
<li>set type Expor active path</li>