Highway setting-out using contour maps: For locating a contour gradient (i.e., a line that cuts successive contour lines at equal vertical intervals) for a proposed highway on hilly terrain, from which feature and in which direction is it best set out for practical alignment work?

Introduction / Context:
On contour maps, a contour gradient is a line that intersects contours at equal vertical intervals, giving a constant slope suitable for road or path location. Deciding where to begin setting out such a gradient on rugged ground (ridge, valley, or saddle) affects earthwork balance, drainage, and safety. This question checks practical route selection principles using contours.

Given Data / Assumptions:

• Contours are reliable and drawn at a uniform contour interval.
• The desired highway grade is moderate and should minimize deep cutting or high embankment.
• Hydrologic safety (avoiding concentrated runoff) is preferred.

Concept / Approach:

Ridges are naturally elevated, convex features that shed water to both sides. Aligning a constant-grade line from a ridge down the hill generally avoids cross-drainage problems, reduces the need for high retaining works, and allows cuts on the uphill side to be modest while embankments on the downhill side remain manageable. In contrast, valleys focus drainage and can require frequent cross-drainage structures. Saddles are useful for passes, but a long alignment starting from a saddle toward the valley often descends into areas requiring heavier drainage works.

Step-by-Step Solution:

Verification / Alternative check:

Check volumes: ridge routes often balance cut and fill better on hillsides. Review hydrology: ridge routes place the carriageway above runoff, reducing flooding risk.

Why Other Options Are Wrong:

Saddle down the hill can lead into converging drainage; bottom-to-ridge or bottom-to-saddle tends to demand heavy, continuous climbing with deeper cut/fill and more drainage crossings; valley routes often require culverts and higher maintenance.

Common Pitfalls:

Confusing a valley line with a ridge on contour maps; forgetting that constant contour spacing corresponds to uniform slope but not necessarily minimum earthwork unless the line follows a ridge or shoulder.

Final Answer:

Ridge down the hill