GIS: reading layered maps on a screen

Can a flood map on a screen tell a family whether their own house will flood? Type an area of England into the Environment Agency's online flood service, and the map fills with coloured layers. Each layer shows long-term flood risk from one source: rivers and the sea, surface water, reservoirs, and groundwater where there is data. A family might check it before buying a house, and it looks as if it should answer their question. It does not. The service itself says it does not tell you how likely it is that an individual property will flood. This video is about what a layered map on a screen can tell you, and what it cannot. Why the family's question goes unanswered comes once the layers make sense.

Picture a blank map of a town. Drop on a layer of roads, then a layer for the river, then the buildings, then a shaded zone at risk of flooding. Now switch the flood zone off. The roads, the river and the buildings stay where they were. That stack is a GIS, short for geographical information system. A GIS is a digital system for capturing, storing, checking and displaying data related to positions on the Earth's surface. Each set of data is a layer on one base map, and each layer can be switched on or off, like sheets of tracing paper laid over a map. A layer can hold points, like schools, shops or earthquakes. It can hold lines, like roads, rivers or a storm's track. It can hold areas, like flood zones, or census areas shaded by a value. Or it can hold an image, such as a satellite photograph. Now a quick one. A GIS shows three layers: hospitals, a motorway, and a shaded zone of high air pollution. Of hospitals, a motorway and a pollution zone, which layer is a line? The motorway is the line. The hospitals are points, and the pollution zone is an area. That is why one screenshot can show a river, roads and flood risk at once. It is a stack of data layers, never a single map or a single photo.

In the exam you never use a GIS. You read a screenshot of one, so the reading comes first. Read the title and the key. Check which layers are switched on, find the scale bar and north, and look for the date of the data. On a screenshot with no date, what can you not tell about its data? How recent it is. Without a date, you cannot tell whether the map still shows the place as it is now. Then describe it like any map, with the trend, the evidence and the anomaly, using evidence only the screen can give you. Picture a GIS screenshot of the valley of the River Tawny, in the fictional land of Owlobia. It has three layers: land at high risk of flooding from the river, buildings, and roads. The shading follows the river for six kilometres, from the west edge of the screen to the east edge. It is widest, about one kilometre, where a smaller river, a tributary, joins in the middle. The main road runs along higher ground, five hundred metres north of the river. The village of Featherby sits on the valley floor. Your task is to describe that distribution, with the trend first and the evidence after it. Write your three-move description before you hear mine. The high-risk land forms a narrow strip along the river, running west to east right across the area. It is widest, about one kilometre, in the middle, where the tributary joins the river. This map has no exception to the strip, so the last point says where the risk is absent. The main road, on higher ground about five hundred metres north of the river, is outside it. Reading two layers together answers a second part, which asks for one place where people are at risk. That place is Featherby. The buildings layer puts the village on the valley floor, and the flood layer shades that same ground. One examiner's report on a question built on a GIS image says few students made much use of the image itself. Some improved their answers by using things like the main roads and the direction they follow. So quote what only the screen shows: a road, a distance from the scale bar, a shaded area.

Interpreting means reading one layer against another, to see where two patterns match, like the flood shading sitting on the low valley floor. Some courses go further and ask you to analyse, which means suggesting reasons for a pattern, each one tied to something the layers show. Picture a GIS screenshot of Owlton, a city in Owlobia, with three layers: the River Tawny, the ring road, and areas shaded by average household income, with invented figures. The area called Riverside lies beside the river, next to the old docks in the city centre. The area called Hillcrest lies on the western edge of the city, just inside the ring road. A first part asks for the area with the lowest income, and the key answers it. Riverside is in the lowest class, and Hillcrest is in the highest. A second part asks you to suggest reasons for the difference between Riverside and Hillcrest. Start each reason from a layer on the screen. Pick one layer and build a reason from it before I give mine. Riverside is by the river, next to the old docks, where many dock and factory jobs have gone. So more people there may be out of work, or in lower-paid jobs. Hillcrest is on the edge of the city, by the ring road, where housing is likely to be newer and larger. So it costs more, and only higher-income households can afford to live there. Each reason starts from a layer, the river and docks, or the ring road and the city edge. Suggest allows a reasoned idea, never a certainty.

Two dated images of the same place show change, and some courses also ask you to describe it. Picture two GIS images of the town of Talonford, from two thousand and from twenty twenty-four, with built-up land shaded and a five kilometre scale bar. In two thousand, the town covered the land around a river crossing. By twenty twenty-four, new building stretches north-east along the main road to the airport, and south across the river. To the west, where the Talon Hills are, there is no new building. Measured against the scale bar, the north-east spread is about eight kilometres, and the spread south is about three. Here are two first sentences. Answer A says the town got bigger. Answer B says the town grew most to the north-east, spreading about eight kilometres along the main road towards the airport. Which of the two earns the credit, and what makes it work? Sentence B earns the credit. It gives where the growth is, the direction it spread, and how far, measured with the scale. One examiner's report on a question like this says the students who gained no credit wrote very generally, with comments like it got bigger or it has grown. The same report says those who referred to an area of growth, a direction, and the extent using the scale gained full marks quickly. The full answer has three points. The town grew about eight kilometres north-east along the main road. It spread about three kilometres south, across the river. There was almost no growth to the west, where the Talon Hills are. A second slip sits close by. One examiner's report on a forest map notes that, rather than identifying changes in the pattern shown, some students wrote about events affecting the forest, such as a promise of money from another country. Describe what the images show changing, where, in what direction and how far, and leave out what caused it.

A GIS is only as good as the data in its layers, and only as recent as that data. Shaded areas have a second limit. Each one shows a single value for a whole area, which hides the variety inside it. Try it on a fresh case. A census area on a GIS is shaded in the highest class for average household income. Does that shading tell you what one family in the area earns? No. It is an average across the whole area, so one family could earn far more, or far less. That is the answer to the family checking the flood service. It maps the risk across areas, not single houses, so it cannot say whether their own house will flood. GIS screens turn up again in later topics, from a storm's track to the coast. Each one is read the way this video reads a screen, and taught with its own topic.

Before you go, check yourself on what this video built.

In one sentence, what is a GIS? A digital system that stacks layers of data tied to places on the Earth's surface, each one switched on or off.

Now a new screen. One layer shows bus stops, and another shades areas by the share of people over seventy-five. The bus stops are thinnest where older people are most common. Reading those two layers together, what might they suggest? That getting about without a car may be harder for them. It stays a suggestion, because two layers show a match, never a cause.

One more, on change between two dated images. What three things does each point of a growth answer say? Where it grew, which direction it spread, and how far, measured with the scale bar.

And the family with the flood map now knows what it can give them: the risk for their area, layer by layer, from rivers and the sea, surface water, reservoirs and groundwater. Whether their own house will flood is a question the service itself says it does not answer.

If this one's yours now, give it a thumbs-up so you can skip it next time. If not, save it; the census video reads maps the same way, so all of this comes back round.

Next in the chapter is Using online census data for a local area. Hopefully that's next in your feed, but if not, search for ScholaFly G E oh three, 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.