Map reading and field study

FOCUS QUESTION

How does one draw and interpret cross sections and sketch sections?

Topographic profiles show us the side view of a landscape or relief of the terrain along which a line is drawn between two locations on the map. They are cross sections, showing us how the land rises and falls between two points. A cross section is a diagram which shows the shape of a landform represented on a map.

HOW TO DRAW TOPOGRAPHY PROFILES

Before starting, study the area of the cross section to the general shape or lie of the land; that is, always have an idea of the shape before you begin plotting heights or drawing the section. 1. Label the elevation of every contour line on the map. 2. Line up a paper between the two points. 3. Mark the two end points on the paper.

4. Make a mark for every contour line you cross and label with the elevation of the line. 5. Line up your paper with the bottom of the grid. 6. Move straight up from every mark on your paper to the matching elevation on the grid and make a point.

7. Connect the points with a smooth, flowing line.

Vertical exaggeration simply means that your vertical scale is larger than your horizontal scale (in the example, you could use: one inch = 1,000ft for your vertical scale, while keeping the horizontal scale the same). Vertical exaggeration is often used if you want to discern subtle topographic features or if the profile covers a large horizontal distance (miles) relative to the relief (feet).

HOW TO CALCULATE VERTICAL EXAGGERATION

When you have done a cross section, the height and gradient of features on the cross section may be distorted. They can appear much steeper, narrower and higher than they actually are.

The vertical exaggeration shows you by how much the vertical scale has been distorted. Five times is a relatively good representation. However, an exaggeration bigger than five times becomes too distorted.

The vertical scale chosen will, therefore, have a huge impact on the distortion of the cross section.

INTERVISIBILITY

Intervisibility is the ability to see one point from another point on the landscape.

The motor car at B is not intervisible from the man on the hill, but the motor car at A is intervisible. So, on a cross section, if you draw a straight line with a ruler and see the topography intersect that line, that means there will be intervisibility.

This is the concept of whether one place on a map can be seen from another.

It is decided upon by studying the heights between the two places.

Any ground which cannot be seen behind a higher height is known as dead ground.

If a convex slope is between the two places, the second cannot be seen. A rough cross-section sketch shows this more easily. Intervisibility can also be affected by presence of buildings or vegetation.

TO REINFORCE

To draw a topographic profile along a desired line on a topography map, put the straight edge of a piece of paper between the end points of the line. Mark with a tick. Mark on the edge of the paper wherever the paper crosses a contour line.

Label each tick mark with the elevation of the corresponding contour line.

Place the edge of the paper along the x axis of a graph paper.

Note the minimum and maximum elevations along the line you have recorded.

Label the graph’s y axis with elevation values, ensuring that they encompass the minimum and maximum values recorded previously. Therefore, the x axis corresponds to the horizontal distance of the line on map.

The y axis represents the elevation of points along the line. On the graph paper, plot the corresponding elevation above each tick mark.

By connecting the dots, the elevation profile along the line of interest is drawn. Next lesson, we will look at calculating gradients using ratios.