Cross sections and transects: drawing the shape of the land

Picture a council planning a new cycle path in a straight line between two villages. Would it be a gentle ride? The villages are Perchley and Penbury, in the fictional land of Owlobia, three kilometres apart. From above, on the map, the route looks short and direct. Draw the shape of the land along that line, though, and a long climb rises in the middle, enough to make plenty of riders get off and push. The flat map hid the hill, and one drawing shows it. That drawing is a cross-section, and you will draw one yourself.

A cross-section is the side view of the land along a straight line between two points. Height goes up the vertical axis, and distance along the line goes along the horizontal axis. It is like cutting through a loaf and looking at the cut face. To draw one, first lay the straight edge of a piece of paper along the line, from one end to the other. Then mark every contour where it crosses the edge, and label each mark with its height. Mark any river, road or spot height on the line as well. Then move the paper down onto the graph, along its horizontal axis, and plot each mark straight up at its height. Join the points in order, with a smooth line or with straight joins from point to point. Never draw one line from the first point to the last. Finally, label the features, and put any river in a low point, because water collects in the dips. The height scale is usually stretched compared with the distance scale, which is called vertical exaggeration. It makes slopes look steeper than they really are. Before plotting anything, look at the marks on the paper. On one stretch of the line they crowd close together, and on another they sit far apart. Where will the finished drawing look steepest? The crowded stretch will look steepest. Each mark is another step up in height. Many steps in a short distance make a steep climb on the drawing. Back to the cycle path. From Perchley, at forty metres, the paper edge picks up a contour every ten metres to a hilltop of one hundred and thirty metres, halfway to Penbury. How big is the climb from Perchley to the top? The climb is about ninety metres. One hundred and thirty take away forty is ninety. On the drawing, that climb rises out of the middle of the path, which the view from above could never show.

Finishing a cross-section that is already half drawn uses the same method. One examiner's report calls completing a cross-section "a challenging skill and one which many seemed unsure about". It says many students did not attempt it, or only did one part. It is hard the first time. What helps is taking it one crossing at a time, in order, with the same steps every time. A line from X to Y is ten centimetres long on the map. The section is already drawn from X, over a hilltop of one hundred and sixty-eight metres, down to the one hundred and sixty metre contour at four point eight centimetres. After that, the paper edge shows the one hundred and forty metre contour at five point six centimetres, one hundred and twenty at six point eight, and one hundred at eight centimetres. A river crosses at nine centimetres, just below one hundred metres. Y sits on the one hundred metre contour at ten centimetres. The job is to finish the drawing from where it stops all the way to Y. Pause it there and work it out for yourself. Plot one hundred and forty at five point six, one hundred and twenty at six point eight, and one hundred at eight. Then dip just below one hundred at nine centimetres for the river, label it, and rise back to one hundred at Y. Join every point in order from the hilltop down. The slope is steepest just below the top, where the contours are nought point eight centimetres apart, and gentler lower down, where they are one point two apart. Now three students finish that same section in three different ways. Student A rules one straight line from the hilltop down to Y. Student B plots every crossing and joins them in order. Student C plots every crossing and leaves the points unjoined. Which student has finished the cross-section? Student B, with every point plotted and joined in order. Now the reason: why is it B and not C? Points without a line are not a profile yet. The line is what shows the shape of the land between the points. One examiner's report, on a cross-section of a reservoir basin, records it: "A few candidates accurately plotted the 300m point but then did not 'join the dots' to show the whole profile. These candidates were awarded 1 mark." The habit is plot, then join, every time, so the profile is whole. Student A's ruled line misses the river dip and every change in slope. Another report records students who "made up points or just joined the first and last dot with a straight line". Instead, plot only where a contour crosses, and let the line bend wherever the marks say it should. Most students on that reservoir question did get there. The report on it says "Most candidates correctly completed the cross section." Plot, then join, and you are with them.

A cross-section can also be read the other way, from the drawing back to the map. Work along the section from its start. Picture a stretch that runs long and flat along the top, with steep sides dropping away. Which landform would that match on the map? A plateau, with few contours on top and crowded ones round its edges. The same reading works for the rest. The steepest parts of a section match the closest contours on the map. Match each label by two things together: its distance along the line and its height. A section runs west to east across a one to twenty-five thousand map, about three point eight kilometres, from grid reference six one two three four oh to six four eight three five two. From the west, it rises steeply from sixty metres to a summit of two hundred and forty-five metres at nought point eight kilometres. Then it falls gently to one hundred and ten metres at two point five kilometres. It drops into a narrow dip of seventy metres at three point one kilometres, then runs flat at about seventy-five metres to the end. Along the line, the map shows a triangulation pillar, a river and a railway, and each one needs a label. Where on the section does the river go? In the narrow dip at three point one kilometres. That dip is the lowest point on the line, at seventy metres, and water collects in the lowest ground. The triangulation pillar goes on the summit, two hundred and forty-five metres high at nought point eight kilometres, because the pillar marks the highest point. And the railway: where does it belong, and what gives it away? On the flat ground near the end of the line. That is about seventy-five metres high, roughly three point three to three point eight kilometres along. Railways follow level land, because trains cannot climb steep slopes, and that last stretch is the only flat ground on the line.

A transect is a line along which change is measured or recorded. One might run inland from the sea across sand dunes, recording the plants. Another might cross an eroded footpath, recording how deep and wide it is worn. A third might cross a river, recording its depth. When a question says describe, you say what changes along the line, with distances and figures. You give no reasons unless the question asks for them. A transect runs inland across sand dunes. At nought metres, the strand line where the tide leaves its debris, there is bare sand. At twenty metres, sea couch grass gives ten per cent plant cover. At sixty metres, marram grass covers forty-five per cent. At one hundred and fifty metres, grasses and mosses cover eighty per cent. At two hundred and fifty metres, heather and scrub cover all of it. One examiner's report says many students struggled to take data from beach profiles and use it. The habit is to read each point's distance and value off both axes before you write a sentence. Answer one says plant cover rises inland, from none at the strand line to complete cover at two hundred and fifty metres, with the biggest rise in the first one hundred and fifty metres. Answer two says cover rises inland because marram grass can survive salt spray and being buried by sand. Which answer describes the change, and why is the other off target? Answer one describes. It gives the trend, the distances and the figures. Answer two explains why, which the question never asked. One examiner's report on a dune question notes some students "described the reasons for the changes, such as salt tolerant plants, which was not required." The fix is to describe first, and to add reasons only when the question says explain. A full answer also names the change in plants, from sea couch grass near the sea to heather and scrub furthest inland. Some courses use dunes and eroded footpaths as their own transect examples. Planning a transect for your own fieldwork has a video of its own. Some courses also print simple geological cross-sections. They draw rock layers under the surface line, and you read which rock lies under which part of the land. What the rocks mean for the landscape has its own video. One course also uses a cross-section of the Earth, read the same way, while the Earth's layers themselves are taught with plate tectonics.

Four questions to finish, and each one is yours to answer first.

What goes on each axis of a cross-section?

Height up the vertical axis, and distance along the line on the horizontal axis.

Now a section the video has not drawn. A river crosses the line between two pencil marks for the eighty metre contour.

Where does the river sit on the drawing?

It sits in a small dip, a little lower than the ground either side.

Rivers lie in low points, so the line dips just below eighty metres between the two pencil marks.

Another: a path two centimetres deep at the edge, twelve in the middle. How does it vary?

The path gets deeper towards the middle, from two centimetres at the edge to twelve in the centre, with no reason given.

Now the last: when is a cross-section finished?

When every crossing is plotted, the points are joined in order, and the features are labelled.

That one drawing is what the council planning the path from Perchley to Penbury needed: the side view that shows the ninety metre climb the flat map hid.

If you could draw that section again from scratch, a thumbs-up says you could teach this one. If that felt fast, that's normal, so save it and come back to it.

Next in the chapter is Reading a landscape from an OS map: relief, settlement, land use and tourism. Hopefully that's next in your feed, but if not, search for ScholaFly G E oh two, oh five.

Every other video is on scholafly.com too, easy to search by video code or by keyword. Whatever you're revising, go there and have a look, or, watch the next video.