Polyline elevation relative to a reference path

Pythagoras can automatically calculate the vertical design of one or more polylines based on a reference path, either by a fixed slope (grade) or by a fixed height offset (elevation difference).

This is especially useful in road or infrastructure design when you want to generate edges, ditches, or curb lines that must follow the road axis at a specific slope or elevation difference.

Before you start you will need

• Reference polyline (Path):
  This is the line with valid height (Z) information. It will serve as your vertical reference.

• Target polyline(s):
  These are the basic polylines you want to adjust. They can be parallel to the path, but don’t have to be.

• Optional: Point(s) on the target polyline

WORKFLOW

1. Create a Path on your reference line

The path acts as the vertical reference.

• Go to Create Path.

• Click the first point of the reference polyline.

• Double-click a segment to generate a path along the entire polyline.

2. Select the reference and target polylines

• Select the Path (with Z).

• Shift+select one or more target polylines.

• Go to Calculation > Design > Polyline relative to Path
  or right-click → Edit > Design Polyline relative to Path.

The dialog Polyline relative to Path opens.

3. Choose how elevations are calculated

A. Fixed Gradient (Slope / Grade)

Keeps the target polyline at a constant slope from the path.

• Enter the slope (e.g. –2% to create a ditch falling away from the road).

B. Fixed Height Difference (Elevation Offset)

Keeps the target polyline at a fixed vertical offset from the path.

Two ways to define it:

Point Nr (requires a point on the target polyline)

• Select the point number from the dropdown.

• The tool compares the Z of the point and the Z of the path at that XY location.

• It then applies this difference to the full target polyline.
  

Delta Z (no point required)

• Enter a chosen vertical difference (e.g. –0.20 m for a curb edge 20 cm lower than the road axis).

4. Confirm and check

• Click OK.

• Review results in 2D and 3D.

Pythagoras calculates the elevation of every vertex on the target polyline relative to the path, using the slope or height offset rule you selected.

This saves you from manual height editing and keeps your design polylines consistent with the reference.

Video:

The following video shows how this works: