Grid references, scale, distance and direction on an OS map

In the fictional land of Owlobia, a wind turbine is planned on a hill near the village of Hootley. How far will it stand from the nearest house? The developer says more than two kilometres. The villagers say barely one kilometre, close enough to hear it turning. Both sides are holding the same map, with the same four and a half centimetres measured between the turbine and the house. One of them has read the scale wrong, and the planning decision depends on which one. By the end of this video, you can settle it with a ruler.

An OS map is a map made by the Ordnance Survey, Britain's national mapping agency. Every map in this chapter is drawn in its style. Across every OS map runs a grid of thin blue lines, and each line has a number printed at the edge of the map. On both scales you meet in the exam, those lines are one kilometre apart. That makes every grid square one kilometre across and one kilometre up, an area of one square kilometre. The lines that run up the map are called eastings, because their numbers rise as you go east. You read their numbers along the bottom or the top of the map. The lines that run across the map are called northings, because their numbers rise as you go north. You read their numbers up the sides. A grid reference is a set of figures that tells you where something is, using those numbered lines. It always gives the easting first and the northing second. You may have heard it as along the corridor, then up the stairs. Along the bottom for the easting, then up the side for the northing. A four-figure grid reference names a whole square. It uses the two lines that meet at the square's bottom-left corner, which is its south-west corner. Try it on one square. A youth hostel stands in the square whose south-west corner is where northing seventy-one meets easting thirty-eight. What are the four figures for the square the hostel stands in? It's three eight seven one. The easting of the left-hand line comes first, then the northing of the line along its south edge. A whole square is a kilometre across, which is far too big to pin down one building. For a single point, you need a six-figure grid reference. Picture the square cut into tenths, ten across and ten up, like a small grid laid inside the big one. Count the tenths east from the easting, and that count becomes the third figure. Count the tenths north from the northing, and that count becomes the sixth. Each tenth of a kilometre is one hundred metres. A six-figure reference pins a point to within one hundred metres. Quick check on the counting. A point sits exactly halfway across the square that starts at easting thirty-eight. What are the first three figures of that point's reference? The first three figures are three eight five. Halfway across is five tenths, so the five follows the thirty-eight. Now back to the hostel. It sits about seven tenths of the way across its square to the east, and about two tenths of the way up to the north. Option A reads seven one two, three eight seven. Option B reads three eight seven, seven one two. Option C reads three nine seven, seven one two. Which one is the six-figure reference for the hostel? It's B. Thirty-eight plus seven tenths gives three eight seven, and seventy-one plus two tenths gives seven one two. Now a harder one: why is it B and not C? Option C took its easting from the line on the right of the square, thirty-nine. The count always starts from the line on the left, the west side, and runs east from there. Option A puts the northing first. Read the usual way, eastings first, it sends you to a different place altogether. There is one more slip with six figures, and an examiner's report records it plainly: "Some candidates are still giving four-figure grid references when a six-figure reference is needed." The fix is to read the question for the word four or six before you start. A square gets four figures, and a point gets six. Grid references work in reverse too. Given six figures, you go to that exact point and name only what is there. Say six figures land on a small farm symbol. One square to the north, the bold name of a village is printed, and a student writes that village as the answer. Is the student right, and why? No. It is only the symbol under the point, and nothing else. That symbol is the farm. The village name is close by, but it is not at the point. One examiner's report describes students who named a well-known place nearby. The report's reason it was wrong: "although it is shown on the map, it is not located at this grid reference." Instead, put your finger on the point the figures give you and read the symbol under it, even when a bold name sits a square away. On some courses the same habit, across first and then up, plots a point on a graph. The video on latitude and longitude takes it on to plotting a storm.

Scale tells you how much the real land has been shrunk to fit on the paper. A scale of one to fifty thousand means one centimetre on the map stands for fifty thousand centimetres on the ground. Fifty thousand centimetres is five hundred metres. Two centimetres on the map makes one kilometre, which is why a grid square on this map is two centimetres across. Now predict it: at one to twenty-five thousand, how many centimetres make a kilometre? It takes four centimetres to make a kilometre. One centimetre stands for twenty-five thousand centimetres, which is two hundred and fifty metres, and four of those make a kilometre. A grid square on this map is four centimetres across. It is the same kilometre, drawn twice as big, with room for more detail. Before converting anything, check which scale your map is. The scale is printed on it, along with a scale line, a ruler marked in kilometres that does the conversion for you. Straight-line distance comes first. Lay a ruler from the centre of one symbol to the centre of the other, then convert. On a one to fifty thousand map, the straight line from the church in Perchley to the railway station in Gradeley measures six point four centimetres. How far apart are the church and the station, in kilometres? It's three point two kilometres. On this scale, one centimetre is nought point five kilometres. Six point four times nought point five gives three point two, and the unit is kilometres. Two answers go wrong right here. Six point four kilometres forgets to convert at all. One point six kilometres divides by four, the conversion for the other scale. One examiner's report on a map distance question puts it in a sentence: "Where candidates had stated an incorrect answer, they had measured correctly but had been unable to complete the calculation successfully." The habit that fixes it is writing the conversion as its own line of working, with the unit, every time. A road or a river rarely runs straight, so a ruler alone cannot follow it. Lay the straight edge of a piece of paper along the start of the route, and mark the start with a pencil. At each bend, hold the paper still with your pencil point and turn it to follow the next stretch, marking as you go. It works the way a tailor's tape follows a curve. At the end, lay the marked edge against the scale line and read off the total. The road from the church to the station winds, and the paper laid round its bends measures nine point three centimetres. The question wants one decimal place. Have a go at this one before I show you. It comes to four point seven kilometres. Nine point three times nought point five is four point six five. Rounded to one decimal place, that becomes four point seven. Leaving it as four point six five skips the rounding the question asked for. The scale also answers questions about size. A spit, a ridge of sand growing out from the coast, is better described as about one and a half kilometres long than as large or wide. Now the turbine near Hootley. The map is a one to twenty-five thousand extract, and the turbine is four and a half centimetres from the nearest house. Who has read the scale right, the developer or the villagers? The villagers have read it right. On this scale four centimetres is one kilometre. The last half centimetre adds a hundred and twenty-five metres, so the turbine is about one point one kilometres away. The developer read it as a one to fifty thousand map, where four and a half centimetres would be two and a quarter kilometres. Same ruler, wrong scale. If this is going well, you are in good company. The report that recorded those conversion slips, on a question measuring across a reservoir, also says most students got the distance right, between thirteen hundred and fifteen hundred metres.

Direction on an OS map starts from one fact: north is the top of the map. The exam uses eight compass points. There are the four main ones, and the four that sit halfway between them, such as north-east. A direction is always from one place to another. The reservoir is north-west of the village means you stand at the village and face north-west. Now turn it round: which way is the village from the reservoir? The village is south-east of the reservoir. Standing at the reservoir, you face the opposite way, and the opposite of north-west is south-east. Some questions use a bearing instead, measured in degrees clockwise from north and written with three figures, so oh four five degrees is north-east. Describing a location takes more than a direction. A strong description gives a direction, a distance and a named feature. On a one to twenty-five thousand map of the land around Hootley, a reservoir fills a valley between two hills in the north-west of the map. The main road runs just to its east. Answer one says the reservoir is above the village. Answer two says the reservoir is about two kilometres north-west of Hootley, just west of the main road, in a valley between two hills. Which answer is stronger, and why? Answer two is stronger. It gives a compass direction, a distance from the village and named features, three different kinds of information. Above could mean north or could mean higher up, so it pins down nothing. A bare grid reference falls short too, because it says where the reservoir is but not what it is near. One examiner's report on a location question says where the weaker answers stopped: "Those who gained 1 mark usually restricted their answers to directions only." The fix is never to stop at a direction. Add a distance, a named feature, or both. The same reservoir has an area to estimate. Each grid square is one square kilometre on both scales, so area on these maps is counted in squares. The reservoir fills three whole squares. It also covers small corners of four more squares, each about one eighth full. Count the whole squares first. Then add the part squares together, putting the small parts side by side until they roughly make whole ones. Roughly what area does the reservoir cover, in square kilometres? About three and a half square kilometres. The whole squares give three. Four corners at about one eighth each add up to four eighths, which is half a square. That brings the total to about three and a half. One examiner's report on an area question says many students "did not understand how to calculate area on a map". The habit that earns it is counting whole squares, adding the parts, and writing the unit, square kilometres. Some courses list this skill by name, but it has turned up on other courses' papers too. What the symbols on the map stand for is a separate skill, and the video on reading the key covers it.

Time to bring it back, and this time you do the answering.

Which comes first in a grid reference, the easting or the northing?

The easting comes first, the number along the bottom, then the northing up the side.

Now, easting forty-two plus three tenths, northing sixteen plus nine tenths. Six figures?

Four two three, one six nine. Forty-two with three tenths, then sixteen with nine tenths.

Another one: a lake is three centimetres across on a one to fifty thousand map. How wide?

One and a half kilometres, because each centimetre stands for half a kilometre.

Now the other scale: the same three centimetres at one to twenty-five thousand?

Seven hundred and fifty metres, which is three quarters of a kilometre.

Now switch to location: what three things make a strong description?

A compass direction, a distance and a named feature, never a bare grid reference.

That is how the villagers near Hootley could show the turbine was barely a kilometre from their homes: a ruler, the right scale, and a line of working.

If grid references and scale feel solid now, give this video a thumbs-up, and this one's done. If they have not landed yet, save it and come back, because the rest of the chapter keeps using this, so it comes back round.

Next in the chapter is OS map symbols: reading the key and spotting features. Hopefully that's next in your feed, but if not, search for ScholaFly G E oh two, oh two.

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.