Solved

Instead of plotting one long line for the track I split it up into segments no more than 15 minutes journey time long. Each journey segment is assigned a shape name which includes the listbox index of the first vertex on the line. When the user clicks on the track I use the name of the shape selected to work back to the listbox ensure the originating reading is selected and visible.

So the user doesn't get to the nearest point on the track but within a reasonable region of the journey (given that reading at most every 30 seconds). It is slower to plot the track for a long journey but 15 minute segments is a good compromise.


Thanks