3.8 Manual design method: Stepping out a roadline on a topo

For larger or more complex projects, it will be worthwhile to ‘walk the block’ prior to developing map-based plans to get a good idea of the terrain (see section 3.9). However, stepping out roadline alternatives on your contour map can help guide the feasibility of road options, and help guide what areas of the block you should focus on during the site visit. As part of good environmental practice, you should always plan two alternative routes for each new segment of road. Once they have been checked in the field, you will then be able to select the best alternative and make a clear case that a lower impact road option has been chosen.

Stepping out a road is a simple and effective technique for plotting a roadline of a given average grade using contour maps. The diagram below shows a roadline marked on a contour map, with control points at A, B, C and D.

The procedure for stepping out a roadline is:

1. Mark known control points on the map

Typical roadline stepped out with dividersSource: Walbridge, 1997Positive control points are points which the road must pass through. They include:

• Start point or area (usually an existing road)
• Finish point (landing)
• Preferred river crossing sites
• Other points that the road must pass through (for example, other landings)

Negative control points are areas that the road must avoid. They include:

• Rocky bluffs or unstable soil areas (active slips)
• Wetlands
• Cultural sites requiring protection
• Water intake structures.

2. Determine an appropriate grade

Percent and degree relationshipRelationship between percent, 1 in x, and degreesThe road grade is the slope of the road and should be presented in percent, but it is still sometimes given as a ratio. Road grade in percent is the ratio of the rise over run; for a given road segment this is the difference in elevation over the length of the road segment.

For example, a 150 m road that gains 15 m in elevation is a road at 15 / 150 x 100% = 10% grade. A road at 10% can also be represented as a ‘1 in 10’ ratio.

Road grade should not be presented in degrees (degrees are used for angles, not slope). If you come across road standard information using degrees or ratios, the adjacent table is useful to convert between the different methods of measuring grade.

Ideally, a segment of road should be planned to have the lowest possible average grade, which should not exceed the maximum grade given by the road standard. For example, a secondary road where trucks are expected to operate at speed might have a maximum grade limit of 10%, whereas a small spur road might have a maximum grade of 14% (for trucks travelling loaded, or a maximum of 16% unloaded).

There are specific locations where the grade needs to be lower, or even become flat (ie 0%). These include rivers crossing approaches, crossings saddles, on curves and around tight ridges. Approaching landings and road intersections, the maximum grade should be < 6% for at least 30 m to ensure that trucks can stop and move off safely.

To obtain the lowest possible grade from a map, the elevations of the control points are measured using the contours on the topographical map. The distance between the control points is measured with a ruler on the map, and the map scale is then used to determine the actual distance. Alternatively, a scale rule can be used, and the distance read directly from it.

Then, the average grade between adjacent control points can be calculated using the following formula:

Where:

G is the average grade between control points A and B (%)

EA, EB are the elevations at points A and B

DAB is the distance between A and B

Worked example

Measuring distance on a topographical mapSource: Walbridge, 1997The elevation of control points A and B are read from the topographical map as: EA = 240 m, EB = 270 m. The distance between A and B is measured and scaled as: DAB = 495 m. The average grade can then be calculated:

That is, if we lay the road out on the map or in the field, at 6.1%, we will have a road on a constant grade between the two points. However, if the calculated grade between two points exceeds the maximum allowable grade, then we must use the maximum grade and extend the road length using curves or switchbacks.

3. Step out the road between the control points using dividers

Using dividers to step out a roadlineSource: Walbridge, 1997It is recommended to start stepping out a planned road on a map from a control point such as a landing or river crossing, and work outwards from these points to establish the location of the remainder of the road. The reason for this is that there is less option to move these than the intervening road sections.

Set the dividers to a distance which will achieve the required grade between adjacent contours (Note: You can also use a ruler). This distance is given by the following formula:

Where:

DDivider is the spacing that the divider is to be set to (min)

DCont is the map contour interval (m)

G is the average grade determined in step 2 (%)

X is the map scale (1:X).

The dividers are then used to step out a trial road between the two control points. Start at the most critical control point, then step from one contour to the next until the end control point is reached. Sketch the roadline along the points that were marked by the dividers on the contours.

Plotting a roadline on the topographical map is only an initial step to ensure that the proposed road is feasible. This roadline must be checked on site, re-marked on the topographical map, and then correctly set out in the field by running a grade line.

The above steps are identical for working on digital maps such as in GIS, Google Maps and Google Earth, whereby most mapping software has a ruler function that can be used directly in the application.