I'm working on a project that surveys the condition of a road or highway using a calibrated trip computer connected to a rugged-PC. An operator keys in defect codes as they travel along a pre-defined route.

I need to show an indicator on the map screen that shows the vehicles current position, taking into account the distance data from the trip computer.

I know the exact lat lon co-ordinates at the starting point of each section of road, and the road is made up of a series of points.

The question is: how can I calculate the lat lon co-ordinates of the vehicle assuming that it has continued on the route and traveled a certain distance (e.g. 1.4km). The co-ordinates would be 'locked onto' the road line, as shown in blue on the diagram below.

Thanks, Alex