Wentworth’s Method of Slope Analysis

This practical demonstrates how to calculate the average slope of an area using contour crossings.

Step 1: Draw the Grid

Draw equally spaced horizontal and vertical lines across the contour map.

1. Use a ruler to draw horizontal lines.
2. Draw vertical lines at equal intervals.
3. Label horizontal lines H1, H2, H3, etc.
4. Label vertical lines V1, V2, V3, etc.

Step 2: Count Contour Crossings

Count the number of times each grid line intersects the contour lines.

Count repeated intersections with the same closed contour separately.

Step 3: Measure Ground Distance

Measure each grid line using a ruler.

For example, a line measuring 5 cm on the correctly sized map represents 5 km.

Step 4: Straight-Line Observation Table

Enter the measurements for all horizontal and vertical grid lines.

Step 5: Calculate Average Crossings

Step 6: Calculate Average Slope

Since the contour interval is assumed to be 100 metres and the ground distance is in kilometres:

Step 7: Verification Using Diagonal Lines

Draw diagonal lines across the same contour map.

1. Measure the length of every diagonal line.
2. Convert each length into ground distance.
3. Count contour crossings.
4. Enter the observations below.

Step 8: Compare Results

A small difference indicates that the calculated slope is less sensitive to sampling direction.

Worked Numerical Example

The following values are hypothetical and are not measurements from the contour map.