Horizontal Curvature¶

First, let's briefly review the elements in a horizontal curvature. When constructing the road's horizontal curvature (centerline), we use the elements:

Straight line (L)
Circular arc (R)
Clothoid (A)

The figure below shows an example of combining different elements in the horizontal curvature.

Figure: R = Circle, A = Clothoid, L = Straight line

Common Combinations¶

The most commonly used combinations:

L-R-L
L-A-R-A-L
R-A-A-R (reverse clothoid)
R-A-R (egg clothoid)

In municipal engineering, it is common to use only straight lines and circular arcs. Furthermore, it is important to construct the centerline as a continuous line with tangency between elements.

Creating SFI Model¶

Before we can start constructing the horizontal curvature, we must create an SFI model.

Procedure

Activate the models tab
Select SFI models and choose Create... from the right-click menu
Click ... and select template. Then click OK
Enter the model name and click OK which opens the properties dialog for SFI properties
Click OK in the properties dialog
Line editing via the properties dialog is now started
Construct the horizontal curvature (available functions are described below)

Note that commands for line annotation and road body for the surface are automatically inserted in the drawing.

Tip

If we only want to create a line, for example to generate a cross-section, we can take a shortcut. Select Horizontal curvature (line definition) from the Planning, design and execution toolset.

If we have selected a road class in the SFI model, the program will use the requirements during editing.

In Gemini Terrain, we can edit the horizontal curvature both via the properties dialog and via grip handles (grips). Grips are especially effective for editing existing lines. Note that we can press cancel when editing via the properties dialog and thereby undo what we have done. We do not have this option for grips editing.

Grips active

The icon to activate grips can be found on the toolbar.

Editing Options¶

When editing horizontal curvature, there are several tools available to help with precise construction:

Dynamic Helper Lines¶

Options for dynamic helper lines can be turned on/off during editing. This includes dynamic text and helper geometry.

Help options for dynamic line

Properties Dialog¶

The properties dialog can be opened by double-clicking on any object. During editing, the dialog can be turned on/off with [Ctrl] + [D]. In editing mode, the various elements in the line will be highlighted with colors.

Constructing Horizontal Curvature¶

When line editing is started, the message field shows that right mouse button opens a menu with functions for dynamic editing. The dialog box can be turned on/off with [Ctrl] + [D]. [Enter] ends and saves the line.

Line Elements¶

Straight Line (L)¶

Straight line segments connect points directly. These are the simplest elements in road design.

Circular Arc (R)¶

Circular arcs provide constant curvature with a fixed radius. The radius value determines the sharpness of the curve.

Clothoid (A)¶

Clothoids provide gradual transition between straight lines and circular arcs. The parameter A determines the length and rate of curvature change.

Construction Methods¶

There are several methods for constructing horizontal curvature elements:

Basic Construction Steps

Start with straight lines: Define the general direction of the road
Add circular arcs: Insert curves at direction changes
Insert clothoids: Add transition curves for smooth geometry
Verify tangency: Ensure all elements connect smoothly
Check design standards: Verify compliance with road design criteria

Editing Functions¶

When editing the horizontal curvature, use the right mouse button to access editing functions:

Insert point: Add new points along the line
Delete point: Remove existing points
Insert curve: Add circular arcs or clothoids
Modify radius: Change curve parameters
Move elements: Adjust line geometry

Note

All changes are made dynamically, and the properties dialog shows real-time parameter values.

Tip

Use hotkeys for efficient editing. The most common hotkeys are displayed in the context menu.

Info

You can read more about horizontal curvature design, aesthetics, and visual guidance in handbook V120 Premises for Geometric Design of Roads.