You can accomplish it by using the QGIS plugin LRS that enables linear referencing support for QGIS. This would be the work flow:

1. Based on both a line (routes) and a point (for calibration) layer, go to the Calibration tab.

• Select the line layer with its route field (a field that identifies routes, it's very useful if your line layer has several routes).
• Select the calibration point layer with its corresponding route (point route value should match with the line route value) and measure field. As you can see, you would need a calibration point layer with at least two points, that could be located at the start and the end of the route, with values of 0 and 15000 in case that the route has 15km.
• Click on Ok to interpolate M values along the routes.

2. Go to the Measures tab.

• Select your waypoints layer.
• Fill the output field names.
• Click on Ok to get a new (memory) layer which will contain the M value (chainage) of waypoint.

3. Don't forget to save the layer you obtain from 2., since it's a memory (temporal) layer. If you want the M values in your waypoints layer, perform a table join between waypoints and the layer you obtained from 2.

In the following screenshot you can see a sample route, a couple of calibration points in blue, and a couple of points along the route (green stars) with calculated chainage (M value).

You can accomplish it by using the QGIS plugin LRS that enables linear referencing support for QGIS. This would be the work flow:

1. Based on both a line (routes) and a point (for calibration) layer, go to the Calibration tab.

• Select the line layer with its route field (a field that identifies routes, it's very useful if your line layer has several routes).
• Select the calibration point layer with its corresponding route (point route value should match with the line route value) and measure field. As you can see, you would need a calibration point layer with at least two points, that could be located at the start and the end of the route, with values of 0 and 15000 in case that the route has 15km.
• Click on Ok to interpolate M values along the routes.

2. Go to the Measures tab.

• Select your waypoints layer.
• Fill the output field names.
• Click on Ok to get a new (memory) layer which will contain the M value (chainage) of waypoint.

3. Don't forget to save the layer you obtain from 2., since it's a memory (temporal) layer. If you want the M values in your waypoints layer, perform a table join between waypoints and the layer you obtained from 2.

In the following screenshot you can see a sample route, a couple of calibration points in blue, and a couple of points along the route (green stars) with their calculated chainage (M value).