Creating a roundabout
Esri Roads and Highways supports looped roads and roundabouts, which are traditionally extremely difficult to calibrate with linear referencing systems (LRS). You can easily create looped roads or roundabouts with some strategically placed calibration points. Roads and Highways leverages calibration points as a way of explicitly controlling the measure values along a route.
Because of this, calibration points trump everything else when it comes to constructing and calibrating a route, including digitized direction. When a route is created, the Roads And Highways Editing tools automatically place calibration points at the beginning and end of each contiguous section of the route. When routes self-intersect, however, Roads and Highways does not know how the calibration points should be placed.
Digitized direction, and the order in which segments appear on routes, can provide some clues but cannot provide the system with enough information to accurately and predictably calibrate the route. The use of calibration points overrides everything else and explicitly tells Roads and Highways how the route should be calibrated. You are responsible for collecting and placing calibration points in strategic locations around the roundabout.
When creating roundabouts, you must follow some very simple calibration point placement rules.
- You must have a calibration point at the entrances and exits of the roundabout.
- You must have a calibration point at the beginning and end of the roundabout. This will most likely mean that you will have two calibration points right on top of each other: one at the entrance to the roundabout and one where the roundabout circles back on itself. Note that the measure value of the calibration point that closes the loop is the same as the measure value of the calibration point that exits the roundabout. This is because the measure values of the portion of the route leaving the roundabout begin accumulating after the entire length of the roundabout has been added.
- Place at least one calibration point between the entrance and exit in the direction of accumulation. In North America this will be the counterclockwise direction. Roundabouts are calibrated in two parts: one to either side of the entrance and one to the exit of the primary route. You must control the route calibration of each part individually.
- Place at least two calibration points on the last part of the roundabout. By placing two calibration points, you force the measure values to accumulate in the correct direction. Remember that the calibration point at the exit of the roundabout has a higher measure value than any other calibration point along the roundabout, except for the last one. By placing only one calibration point with a value of 2.86 between two calibration points of 2.88, the system will not understand whether you are trying to force a loop or create a route that does not have strictly increasing measures. The second calibration point gives the system enough information to understand that you are creating a roundabout.
Keeping these rules in mind, you can now create your roundabout.
- Start ArcMap and set up your editing environment for working with Roads and Highways.
- Add calibration points so that your new roundabout will calibrate properly.
- Set your target LRS Network.
- Set your edit activity to Create Route.
- Click the Select Source Geometry button on the Roads And Highways Editing toolbar and select all the parts of the route to be created, including the roundabout geometry.
Order the segments so that the roundabout will be constructed in the proper direction (counterclockwise in North America). Refer to Using the Select Source Geometry Tool for tips on selection, order, and direction.
- Click the Apply button .
The Create Route dialog box appears.
- Populate the Create Route dialog box with the relevant information for the new route.
- Click OK.
Your new route is created with a properly calibrated roundabout.