Road Creation

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Road Creation

In this description:

•Firstly the concepts and parameters used to control the road shape are presented.

•Then the method of road creation is related to the usual 3d-DigPlus terrain excavation.

The Create Roads dialog box allows users to control excavation of roads on the terrain surface. The description here concentrates on the method used to generate road excavations and the meaning of controlling parameters.

Road Shape

Road creation can be thought of as follows:

Apply a template (i.e. a fixed 2D shape) to points along the smoothed road centre line. This template defines the road cross section. Road template parameters determine the template shape. The template elevation at each point along the road is determined by the road centre line elevation. The template centre is positioned on the road centre line.

The road template extends in two directions from the road centre line to the road edges. Direction is perpendicular (in plan) to the road centre line at each point. The surface formed by the road template in this way is merged into the terrain using the cut and fill batter angles defined for each road segment by the road batter parameters.

The final road shape is thus determined by:

•Road direction (road segments and smoothed road centre).

•Road elevation (road segment end heights).

•Road template parameters (road width and road crossfall defining the road edges).

•Road batter parameters (cut and fill batter angles in each road segment).

The smoothed road centre is sometimes referred to as the road feature.

Road Segments

Road segments are drawn by the user as a series of straight lines (in plan view) on the terrain surface. Angles between these segments are optionally smoothed in plan view to generate the smoothed road centre.

Segment End Heights

The segments are actually 3D straight lines. The end height of each segment depends on the height of that segment's start point and the segment slope. Since the first segment starts on the terrain surface, the height of each segment end point can then be calculated. Three options are offered for determining the segment end height:

•From from user-defined segment slope ("Constant slope").

In this option, the user inputs a segment slope. This is used to calculate the segment end height using the segment start height and segment (East, North) end position i.e. plan view end position.

•From user-defined end height directly ("Constant height").

In this option, the segment end height is determined directly by an input parameter value. The segment slope can then be calculated from the two end heights, and so is not a free parameter.

•From terrain height at segment end position, plus offset ("Terrain z with Offset").

Here the segment end height is set to the terrain height at that point, with an added offset. This offset may be either positive or negative to give end heights above or below the terrain surface respectively.

As the last segment always ends on the terrain surface its end height is calculated according to the third option with zero offset.

Smoothed Road Centre

Road position is determined by the road centre line on the terrain surface. The centre line is generated from straight-line road segments drawn by the user. This generation involves "smoothing" the corner between each pair of segments. A large part of the road centre line usually coincides exactly with the user-defined road segments. They differ only in the smoothed corners.

Smoothing Corners ("Segment End Junction")

A road corner is formed at the junction of two user-defined road segments. Road corners are smoothed in 2D by replacing the corner by a smooth curve tangential to each segment. Three options are offered for smoothing the corners. These options are independently applied to the junction of each segment with its next neighbouring segment. So more than one of these options may be used in a given road.

The options are:

•Linear end junction (no smoothing).

No smoothing is performed in this option. The user-defined road segments form the road centre line directly.

•Circular curve end junction.

This option replaces the road corner with a section of a circle. The position of the circle section for a given corner angle is determined by the circle radius parameter. The curve never takes up more than half the length of a segment. So if the nominated radius is too large (i.e. a segment is too short) the radius is adjusted to occupy half of the short segment.

•Bezier curve end junction.

This option replaces the road corner with a Bezier curve. The position of the Bezier curve for a given corner angle is determined by the maximum corner size parameter. The curve never takes up more than half the length of a segment. So if the nominated maximum corner size is too large (i.e. a segment is too short) the corner size is adjusted to occupy half of the short segment.

The shape of the curve (how far it extends into the corner) is determined by the Bezier sharpness parameter. Larger values of this parameter lead to curves extending far into the corner (Bezier sharpness greater than about 1.0). Smaller values lead to curves which approach joining across the corner by a straight line (Bezier sharpness less than about 0.2).

Heights of Smoothed Road Centre

The centre of the smoothing curve between two segments is set to the segment end height. This point is the boundary between smoothed segments. The heights of other points along the smoothed road centre are calculated in proportion to plan-view distance along each smoothed segment.

Road Template Parameters

The road shape is created by applying a template (i.e. a fixed 2D shape) to points along the smoothed road centre. This template defines the road cross-section. Road template parameters determine the template shape. There are two such parameters:

Road Width

This is the plan view distance between the two road edges perpendicular to the smoothed road centre. This parameter determines the width of the road template.

Road Crossfall

This parameter determines the slope of the road surface perpendicular to the road centre line. It is measured in degrees below horizontal. Positive values thus correspond to the road edges sloping away from the road centre. In general the crossfall angle is quite small.

Road Edges

The road edges may be thought of as the 3D curves generated by moving the 2D road template along the smoothed road centre.

The elevation of the road edges depends on the heights of the smoothed road centre and also on the road template parameters i.e. road width and road crossfall.

The 2D shape of the road edges depends on the 2D shape of the smoothed road centre, and on the road width.

Road Batter Parameters

The surface formed by the smoothed road centre and road edges (i.e. by the road template as described above) is merged into the terrain using cut and fill batter angles. These are defined for each road segment by the road batter parameters. Each road segment has two road batter parameters: a cut batter angle and a fill batter angle.

Cut occurs when the road edges are below the terrain surface, fill when they are above. Cut and fill batter angles are always the angles of the resulting material measured from the horizontal. So a smaller angle represents more moved material in both cut and fill situations.

The parts of the road edges which end up above the terrain surface use the fill batter angle. Such portions of the road edges are extended at the fill batter angle until they meet the terrain surface. The space below the extending surface is filled with material.

Conversely the parts of the road edges below the terrain surface use the cut batter angle. Such road edges are extended at the cut batter angle until they meet the terrain surface. Cut material is removed from the space above the extending surface.

The Same Excavate/Fill Method

"Under the hood", roads in 3d-DigPlus are created by terrain excavation using outer-from-inner polygon generation. To generate a road:

1.Firstly an appropriate inner polygon is generated together with the correct batter angles.

2.These are then used to generate an outer polygon using outer-from-inner polygon generation..

3.Excavation using polygons is then done in the usual way.

Users do not see this process explicitly. However it is useful to keep it in mind when using Dig2000 for excavating roads.

Stages of Road Generation

How to generate the "appropriate" inner polygon for a given road design? Briefly this is done in the following steps:

1.The user draws straight-line road segments on the terrain surface to define the road centre line. Each segment represents a 3D line. The segment end height is determined in one of the three ways described below.

2.In plan view the angles between the straight-line segments are smoothed to generate the smoothed road centre. This generates smooth road curves. Three smoothing options are offered, as described below.

3.Road edges are calculated using the road template parameters road width and road crossfall. Road edges are also defined as a series of connected 3D lines. These lines are smoothed as a result of the smothing of the centre line.

4.The two road edges are connected to form the inner polygon. Edge elevation relative to the terrain surface is used to assign cut and/or fill batter angles to the polygon. Road batter parameters for each road segment determine these batter angles.

5.The inner polygon is bound to a surface formed by linear interpolation between the road centre line and road edges. This is the final road surface.

6.Road formation then proceeds by the excavation using polygons method, as above.