Contours, spot heights, gradient and isoline maps

Two footpaths climb the same hill to the same summit. With a pushchair, which would you take? The hill is in the Talon Hills, in the fictional land of Owlobia. On the map, one path is one point two kilometres long and the other is three kilometres. A family with a pushchair takes the short one, and halfway up they meet a slope they cannot push up. The map had warned them before they set off. Along the short path, the brown lines are packed tightly together. Those brown lines are what this video teaches you to read.

A contour is a brown line on the map joining points of equal height above sea level. If you walked along one contour on the ground, you would never go up or down, like following a level path round a hillside. Contours are drawn at a regular step in height, called the contour interval, and the map prints it. It is commonly ten metres on a one to fifty thousand map and five metres on a one to twenty-five thousand map. Check the interval on the map in front of you, because the height of every unlabelled contour depends on it. A spot height is a dot with its height in metres beside it. A triangulation pillar is a small triangle with its height, often on a hilltop. Both give an exact height between contours. To compare two places, read both spot heights and name the higher one. The numbers settle it. Picture two slopes on one map. On the first, the contours are packed close together. On the second, they are spread far apart. Which slope is steeper, the first or the second? The first slope is the steeper one. Each contour is one more step up in height. When they are close together, you climb a lot of height in a short distance across the map. Contours far apart mean a gentle slope, and no contours at all means flat land. Spacing tells you how steep, but not the direction of the slope. For that, read the values, because the numbers rise towards the high ground. That settles the two paths in the Talon Hills. Both climb to the same summit, but the short path does it in less distance, which is why the contours it crosses are packed so tight. Now two other places on one map. A plateau, a high and flat-topped area, sits at four hundred metres with its contours spread out. A river bank at forty metres has its contours crowded together. Option A says the river bank is higher, because its contours are crowded. Option B says the plateau is higher, but the river bank is steeper. Option C says the plateau is both higher and steeper. Which statement is right? It is B, the second of the three statements. The numbers put the plateau higher, and the spacing makes the river bank steeper. Close contours mean steep ground, not high ground. Contour patterns also draw the shapes of the land, and each shape has a pattern of its own. A hill shows as closed rings, with the highest value in the middle. A valley shows as contours bent into V shapes whose points aim uphill, usually with a stream running along the middle. A spur is a tongue of high land sticking out into lower ground. Its contours bend into V shapes whose points aim downhill. A ridge is a long, narrow band of high ground. A plateau has few contours on its flat top and steep slopes round its edges. A cliff or crag shows where contours bunch so tightly they almost merge, often with a rock symbol drawn on top. Try one. A set of contours bends into V shapes. The values read one hundred, one hundred and ten and one hundred and twenty metres, and the points of the Vs aim towards the hundred. Is that a valley or a spur? It's a spur, high land sticking out. The Vs point towards the lower values, which means they point downhill. Valley Vs would point the other way, up into the hills. When a question asks about relief, it means the height and shape of the land, read from contours and spot heights. Here is a square to describe. In grid square six two four three, the contours run from one hundred and fifty to three hundred metres, and their values fall from west to east. They are packed close in the west of the square and widely spaced in the east. A spot height of three hundred and five metres sits in the north-west corner. Describe the relief of that square in two sentences, using the heights and the spacing. Try it on paper before I show you the answer. The land is highest in the north-west, reaching three hundred and five metres, and falls towards the east to about one hundred and fifty metres. The slope is steep in the west, where the contours are close together, and gentle in the east, where they are widely spaced. That answer gives heights with figures, the direction of slope, and steep against gentle, all from the contours. An answer that says the square has a wood and a road in it is not about relief at all. One examiner's report shows an answer about other features scoring nothing: "This response was not awarded a mark as the candidate has not written about a feature of the relief." The same report's advice is one line: "You will need to be able to interpret contour lines on an OS map." The fix is to call them contours by name and to back every point with a contour value, a spot height or the spacing.

Contours also let you put numbers on a slope. On a one to twenty-five thousand map, a spot height of three hundred and twelve metres marks a hilltop. A bridge one and a half kilometres due east of it stands beside a spot height of eighty-seven metres. A height difference is the higher height minus the lower one. The answer is the gap between them, not either height on its own. What is the difference in height between the hilltop and the bridge? It comes to two hundred and twenty-five metres. Three hundred and twelve take away eighty-seven is two hundred and twenty-five, and the unit is metres. One examiner's report on a height difference question says the most common wrong answer was the end height on its own. Students had read the diagram correctly, but had not subtracted the starting height. The habit that fixes it is writing the subtraction out, higher minus lower, every time. Gradient is how steep a slope is on average. It is the height difference divided by the horizontal distance, with both in metres. The horizontal distance is the distance across the map, which comes from the scale. The answer can be a percentage, or written as one in something. Why must the horizontal distance be in metres as well? Because the height is already in metres. Dividing metres by kilometres would give a number a thousand times too big. Back to the hilltop and the bridge. The height difference is two hundred and twenty-five metres, and one and a half kilometres across is one thousand five hundred metres. Use the gradient rule from a moment ago, then turn the decimal into a percentage. What is the gradient between them, as a decimal and a percentage? Two hundred and twenty-five divided by one thousand five hundred is nought point one five, which is fifteen per cent. As one in something, divide the other way. One thousand five hundred divided by two hundred and twenty-five is about six point seven, so the gradient is about one in six point seven. That means the land rises one metre for every six point seven metres you travel across. Gradient, once more, is the height difference over the horizontal distance.

The same kind of line turns up on maps of things you cannot see, such as air pressure. An isoline is any line joining places with the same value. A contour is one kind, for height. An isobar joins places with the same air pressure, measured in millibars. Close isobars mean strong winds, the way close contours mean steep ground. Now: isobars crowd over the north of a map and spread over the south. Where is it windier? The north is windier, because that is where the pressure changes fastest over a short distance. Isotherms join places with the same temperature. Other isolines show wind speed, rainfall, or the depth of the sea. Some courses name weather charts directly, and some set completing an isoline on the paper. Reading a whole weather chart has its own video in the weather chapter. To complete an isoline, first find where its value falls between each pair of neighbouring readings, in proportion. A value of fifteen sits halfway between readings of ten and twenty, because it is five above one and five below the other. Then join those points with one smooth curve. It never crosses another isoline, and it runs to the edge of the map or closes into a loop. Six weather stations sit in three pairs, running from the north of the map to the south, each pair from west to east. The north pair reads one thousand and four and one thousand and twelve millibars. The middle pair reads one thousand and six and one thousand and ten. The south pair reads one thousand and seven and one thousand and nine. Where does the one thousand and eight isobar cross each pair? Halfway between the two stations, in every pair. One thousand and eight is the middle value each time: four either side in the north, two either side in the middle, and one either side in the south. Mark those three points, then join them with one smooth curve from the north edge of the map to the south edge. One examiner's report on completing an isoline of wind speed, measured in knots, records the two failures: "Some left it blank, and some joined the two parts of the 20-knot line using a straight line." Instead, find every in-between point first, then draw one smooth curve through them all. Isoline maps are well within reach. One report calls a question on reading temperature isolines across the UK "A high scoring question".

Now the questions are yours, with each answer straight after.

Crowded contours on low ground near a river: steep, high, or both?

Steep, but not high. The spacing gives steepness, and the low values say it is low ground.

Now an isobar the video has not drawn yet, with a value of one thousand and eight.

Where does it sit between one thousand and six and one thousand and fourteen?

A quarter of the way along, from the lower one.

It is two above the lower reading across a gap of eight, and two eighths is one quarter.

Another: hilltop four hundred and five metres, lake one hundred and thirty. Difference?

Two hundred and seventy-five metres, from four hundred and five take away one hundred and thirty.

Last one, back in the Talon Hills. Picture the short path climbing three hundred metres in its one point two kilometres, an invented figure for practice.

What is its average gradient, as one in something?

It's one in four, a steep pull for a pushchair.

Twelve hundred metres across divided by three hundred metres up gives four.

Spread the same climb over the long path's three kilometres and it eases to one in ten, which is why its contours sit far apart, and why the pushchair belonged on it.

If contours and gradient make sense now, a thumbs-up says that's in the bag. If they don't yet, save it, then give it a day and look again.

Next in the chapter is Cross sections and transects: drawing the shape of the land. Hopefully that's next in your feed, but if not, search for ScholaFly G E oh two, oh four.

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.