Geographic skills: maps, graphs and data

What geographic skills are

Geographic skills are the techniques geographers use to locate places, read and draw maps and graphs, summarise data with simple calculations and judge how far a source can be trusted. They are tools for thinking about place, process and pattern: a choropleth map shows where a variable is high or low, a cross-section shows the shape of a valley, and a mean or a percentage change turns a column of numbers into a statement that can be compared with another place.

Skills run through every part of the course. In Paper 1 the structured question on each option is built on a stimulus, often a topographic map extract, a graph or a table, and its first parts ask you to read values, grid references, directions and differences from it. In Paper 2, Section A questions open with a figure to describe, and Section B is built on an infographic. The fieldwork report depends on the same skills in reverse: there you collect the data, choose the graphs and maps, and draw them yourself.

Locating places

Latitude and longitude

Any point on the Earth’s surface can be located by latitude and longitude. Latitude is the angle north or south of the Equator (0°), up to 90° N or 90° S at the poles; lines of latitude (parallels) run east–west. Longitude is the angle east or west of the Prime Meridian through Greenwich (0°), up to 180° E or W; lines of longitude (meridians) run north–south. Degrees are divided into 60 minutes, so 51° 30′ N is halfway between 51° N and 52° N.

A location is written latitude first: Nairobi lies at about 1° 17′ S, 36° 49′ E. Latitude matters in many explanations, because it controls the angle of the Sun and therefore temperature, and the position of the major pressure belts. On world maps in the exam you may be asked to state the latitude of a feature or to describe a distribution by latitude ("most tropical cyclones form between 5° and 20° north and south of the Equator").

Direction and bearings

Direction is given as one of the eight points of the compass (N, NE, E, SE, S, SW, W, NW), or of sixteen for more precision (NNE, ENE and so on). A bearing is more exact: the angle measured clockwise from north, written with three figures (045° is north-east, 270° is west). Directions are always given from one place to another: if a church is south-west of a hill, the hill is north-east of the church.

Scale and distance

A map scale is the ratio between a distance on the map and the same distance on the ground. It can be shown as a representative fraction (1:50,000 means 1 cm on the map represents 50,000 cm, or 500 m, on the ground), as a scale bar, or as a statement ("2 cm to 1 km"). A large-scale map (1:10,000, 1:25,000) shows a small area in detail; a small-scale map (1:1,000,000 or smaller) shows a large area with little detail.

Grid references

Topographic maps such as the British Ordnance Survey maps are covered by a grid of numbered lines 1 km apart. The lines running up the map are eastings, numbered from west to east along the bottom; the lines across the map are northings, numbered from south to north up the side. A grid reference always gives the easting first and the northing second.

• Four-figure grid reference (an area reference): the easting and then the northing of the line through the south-west corner of a 1 km square. It identifies the whole square.

• Six-figure grid reference: the four-figure reference with tenths added in each direction, which locates a point to the nearest 100 m. Divide the square into ten parts by eye (or with a romer) along the easting, then up the northing.

The map extract shows the standard features of a topographic map: a 1 km grid with eastings 32–36 and northings 51–55, contour lines at a 10 m interval, spot heights and a trig point, a river, a road, buildings and a key. The village of Ashby lies in grid square 3452, because the south-west corner of the square containing most of its buildings is where easting 34 meets northing 52.

Height, relief and gradient

Relief (the shape and height of the land) is shown by contour lines, which join points of equal height above sea level, and by spot heights, which give the exact height of a single point. The contour interval is the vertical distance between adjacent contours, usually 5 m or 10 m on 1:25,000 and 1:50,000 maps; every fifth contour (an index contour) is drawn thicker and labelled.

Gradient is the steepness of a slope: the vertical rise divided by the horizontal distance, both in the same units. It can be written as a ratio ("1 in 20"), a percentage or an angle.

Drawing a cross-section

A cross-section (cross-profile) is a side view of the land surface along a straight line between two points. It turns the contour pattern into a shape that can be described and compared, and it is the standard way to show a valley, a coastal slope or the position of settlements on relief.

1. Lay the straight edge of a strip of paper along the line between the two points (A and B) and mark both ends.

2. Mark on the strip every place where a contour crosses the edge, and write its height beside the mark. Mark rivers, roads and settlements too.

3. Draw a horizontal axis the same length as the strip (the map’s scale) and a vertical axis for height, with a scale chosen so that the section is readable.

4. Place the strip along the horizontal axis and plot each height above its mark. Join the points with a smooth curve, not straight lines; rivers sit at the lowest points and hilltops are rounded above the highest contour.

5. Label A and B, the features crossed, both axes with units, and state the vertical exaggeration.

The vertical scale is almost always larger than the horizontal one; otherwise most landscapes would look flat. The vertical exaggeration is the vertical scale divided by the horizontal scale. If 1 cm represents 25,000 cm (250 m) horizontally and 20 m vertically, the vertical exaggeration is 250 ÷ 20 = 12.5.

Thematic maps

Thematic maps show the distribution of one variable. Each type suits a particular kind of data, and choosing the wrong one gives a misleading picture. The key question is whether the data are absolute values (a number of people, visitors or migrants), relative values (a density, a rate, a percentage), or a quantity that varies continuously over space (temperature, rainfall, height).

Choropleth maps

A choropleth map shades each area (a country, region or district) according to the class its value falls into, in graded tones of one colour from light (low) to dark (high). It shows relative data well: population per km², the percentage of people over 65, or GDP per capita. It should not be used for absolute numbers, because a large area will look important simply because it is large.

To construct a choropleth map, decide on four to six classes that cover the data without gaps or overlaps (under 50, 50–99, 100–199 and so on), give each a tone that becomes darker as values rise, shade each area, and add a key with the class intervals and a "No data" class where values are missing. Class boundaries can be equal steps, steps that double, or natural breaks in the data; changing them can make the same data look quite different, which is why the key matters.

Isoline maps

An isoline map joins places of equal value of a quantity that changes gradually over space. Contours are isolines of height; isotherms join equal temperatures, isohyets equal rainfall and isobars equal pressure. Isolines are drawn by interpolation: estimating values between measured points and threading a line through where the chosen value would fall. They never cross, and close spacing shows a steep gradient in the variable.

The isoline map shows an urban heat island, a common fieldwork and exam topic: temperatures rise towards the central business district and fall towards the rural edge, and the intensity of the heat island is the difference between the two, here 18.4 − 13.3 = 5.1 °C. A place halfway between the 16 °C and 17 °C isotherms is estimated at 16.5 °C.

Proportional symbol maps

A proportional symbol map places a symbol, usually a circle, at each location and draws it with its area proportional to the value there. Because the area of a circle is πr², the radius must be proportional to the square root of the value: a town with four times the visitors of another has a circle twice as wide, not four times as wide.

Dot maps

A dot map shows a distribution by placing a dot for each feature (a shop, a well, a case of a disease) or for a fixed number of them (one dot for 1,000 people). It shows clustering and spacing directly, which makes it the starting point for the nearest neighbour index, a measure of whether the pattern is clustered, random or regular.

To construct a dot map, choose what one dot represents, place each dot where the features occur rather than in the middle of an area, and state the dot value in the key. Its main weakness is that dots merge where they are dense, so the value per dot must be chosen with the densest area in mind.

Flow-line maps

A flow-line map shows movement between places, such as migration, trade, commuting or traffic. The direction of each arrow shows the route, and its width is proportional to the volume of the flow, read against a key of reference widths. Flow lines can follow real routes or be drawn as straight lines from origin to destination. To construct one, choose a width scale (for example 3 mm for every 1,000 migrants), draw each arrow from origin to destination at its width, and show two or three reference widths in the key.

Topological maps, mental maps and sketch maps

A topological map keeps only the connections between places and their order, not true distances or shapes; the London Underground map is the classic example, and network diagrams of air routes or shipping lanes work in the same way. Lines are straightened, stations are spaced evenly and interchanges are emphasised, so the map is easy to follow but cannot be used to measure distance or direction.

To read a topological map, follow the lines and count stops or links, not kilometres: the number of connections a place has measures its accessibility within the network. A mental map is the map of an area that a person carries in their head, often drawn in fieldwork to compare how different groups perceive a city. A sketch map is a simple map drawn by the geographer to locate the features in an answer or a report.

Graphs and diagrams

A graph is the right choice when it makes a pattern easier to see than a table of the same numbers. The type depends on the data: time series suit line graphs, categories suit bar graphs, parts of a whole suit compound bars or pie charts, and a relationship between two variables suits a scatter graph. Several geographical graphs (triangular graphs, rose diagrams, Lorenz curves, population pyramids) exist for one particular kind of data.

Line, bar and compound bar graphs

Line graphs show continuous change, such as population over time or discharge over a storm. Bar graphs compare separate categories, such as rainfall by month or visitors by country of origin. Both need a title, labelled axes with units, a scale that starts at zero on bar graphs, and a key when there is more than one series.

A climate graph combines the two on one set of axes: monthly rainfall as bars against the left-hand axis and mean monthly temperature as a line against the right-hand axis. Rainfall is drawn as bars because each month’s total is a separate quantity; temperature is a line because it changes continuously through the year.

A compound bar graph divides each bar into the components of a total, so it shows both the total and its make-up. Drawn as percentages, every bar reaches 100% and only the shares change. It is widely used in fieldwork for land use, the types of shops in a street, or the modes of transport people use.

The compound bar graph summarises a land-use transect from the city centre. Its pattern matches the idea of a central commercial core surrounded by residential zones: the share of commercial land falls steadily with distance, and residential land takes over between sites 2 and 3. A pie chart for each site would show the same data but make the change between sites much harder to compare.

Pie charts

A pie chart shows how one total is divided between categories, each drawn as a sector whose angle is proportional to its share. It suits a single place or year with a few categories, such as the transport used by shoppers or the land use of one survey area. Sectors are drawn clockwise from the top, largest first, with "other" last, and labelled with percentages.

Scatter graphs

A scatter graph plots two variables measured at the same places, such as distance downstream and channel width, or GNI per capita and life expectancy. The independent variable, the one thought to cause the change, goes on the x-axis. If the points form a band, a best-fit line can be drawn by eye through the middle of them, with about as many points above as below; it does not have to pass through the origin or through any point.

The pattern describes the correlation: positive if both variables rise together, negative if one falls as the other rises, and none if there is no trend. A tight band is a strong correlation, a loose cloud a weak one. Points far from the line are anomalies and need explaining. Spearman’s rank correlation coefficient measures the strength of the relationship and tests whether it could have arisen by chance.

Triangular graphs

A triangular graph plots data made of three percentages that add up to 100%, such as employment in the primary sector, secondary sector and tertiary sector, or the sand, silt and clay in a soil. Each side of the triangle is a 0–100% axis for one component, and each point represents one place.

Reading a triangular graph is easiest one axis at a time. For each component, find the gridlines that run parallel to the side opposite that component’s 100% corner, follow the line through the point to the axis, and read the value in the direction of the arrow. The three readings must add up to 100%, which is a quick check that each has been read correctly.

Logarithmic graphs

A logarithmic scale has equal distances for each tenfold increase (10, 100, 1,000, 10,000), so data with a very wide range fit on one graph: villages and megacities, or the GNI per capita of the poorest and richest countries. Between the main gridlines, the minor lines for 2, 3, 4 … 9 get closer together. A semi-logarithmic graph has one log axis; a log-log graph has two.

The most useful property of a logarithmic graph is that the slope shows the rate of change, not the absolute change. On a normal axis, the city would appear to grow much faster than the town because it adds more people each year; on a log axis, the two lines are parallel from 1950 to 1990 because both grew at the same percentage rate.

Rose diagrams

A rose diagram shows how often something comes from or points in each compass direction. Each petal points in one direction and its length is proportional to the frequency; rings show the scale, and calm days (for wind) are written in the centre circle. Rose diagrams are used for wind direction, which controls wave approach and longshore drift, for the orientation of cirques or drumlins, and for the direction of pebbles’ long axes in a till deposit.

Bipolar graphs

A bipolar graph plots scores on a scale between two opposite descriptions, typically from −3 (poor) to +3 (good). It is the usual way to present an environmental quality survey, in which each site is scored on criteria such as litter, noise and building condition. Lines for different sites can be compared criterion by criterion, and the scores can be totalled to give each site an overall index.

Population pyramids

A population pyramid is a pair of horizontal bar graphs, males on the left and females on the right, showing the share of the population in each five-year age group. Its shape summarises a country’s demographic history: a wide base means high fertility, a narrow base low fertility, a wide top an ageing population, and an indentation or bulge a past event such as a war, a baby boom or a wave of migration.

When describing a pyramid, start with the overall shape, then quantify the base and top ("about 9% of the population in each sex is aged 0–4"), then any irregularities and any difference between males and females in the oldest groups, where women usually outnumber men. A pyramid also gives the data for the dependency ratio, the number of people aged 0–14 and 65 and over for every 100 aged 15–64.

Development diamonds

A development diamond compares places on four indicators at once. Each axis runs outwards from a centre, with each indicator scaled from 0 to 100 so that outward always means "more developed" (for an indicator such as infant mortality, the scale is reversed). The larger and more regular the diamond, the higher and more balanced the level of development.

The shape of a diamond is as informative as its size. A diamond stretched along one axis shows uneven development: a country with high income but low literacy, for example, has used its wealth unevenly. Like any composite measure, the diamond depends on which four indicators are chosen and how each is scaled.

Lorenz curves and the Gini coefficient

A Lorenz curve shows how evenly income, wealth, land or water is shared. Both axes are cumulative percentages: of the population (poorest first) along the bottom, and of the income they receive up the side. If everyone received the same, the curve would follow the diagonal line of perfect equality; the further it bows below the line, the more unequal the distribution.

The Gini coefficient turns the curve into one number: the area A between the line of equality and the curve, divided by the whole area under the line (A + B). It runs from 0 (perfect equality) to 1 (one person has everything), and is often written on a scale of 0–100. National income Gini coefficients range from about 0.24 in the most equal countries, such as Slovakia and Slovenia, to about 0.6 in the most unequal, in southern Africa; the figure for a country varies with the survey year and with whether income or consumption is measured.

Photographs, satellite images and field sketches

Photographs and satellite images are stimulus material in their own right. To describe a ground-level photograph, divide it into foreground, middle ground and background, and describe what is in each with its position ("in the left foreground, terraced fields on a steep slope"). For an aerial or satellite image, give the scale or approximate size, the orientation and the land uses or landforms visible, using compass directions.

A field sketch is a simplified drawing of a view made in the field. It shows only what is relevant to the investigation, with labels (names) and annotations (explanations), a title, the direction of view and the date. A field sketch can be better evidence than a photograph because it forces the geographer to select and interpret features rather than record everything.

Calculations that summarise data

Simple calculations turn raw data into figures that can be compared between places or over time. They appear in structured questions ("State the difference between…", "Estimate…", "Determine…") and throughout fieldwork. Show each step with its unit; a correct method with a slip in the arithmetic can still earn credit, while a bare wrong number earns nothing.

Averages and measures of spread

The three averages describe the typical value of a data set in different ways. The mean is the total divided by the number of values; it uses every value, so a single extreme value pulls it up or down. The median is the middle value when the data are in order, and is not affected by extremes. The mode is the most frequent value, useful for categories ("the modal shop type is a café").

Measures of spread show how far the values vary. The range is the largest value minus the smallest. The interquartile range is the upper quartile minus the lower quartile: the spread of the middle half of the data, which ignores the extremes. Two sites with the same mean can have very different spreads, so a description of data gives both an average and a measure of spread.

Percentages and percentage change

A percentage expresses a part as a share of 100, which allows places of different size to be compared. Percentage change compares a new value with the original one, and is always divided by the original value.

Densities and ratios

A density is an amount per unit of area. Population density is the number of people per km²; drainage density is the total length of streams per km² of a drainage basin. Densities allow areas of different size to be compared, which is why they suit choropleth maps.

A ratio compares two quantities. The sex ratio is the number of males per 100 females (1,060 males to 1,000 females gives 106). The dependency ratio compares the population of dependent age with the population of working age.

Measuring concentration and dispersion

Two measures describe how features are spread across space. The nearest neighbour index ($R_n$) describes the pattern of points on a map; the location quotient describes whether an activity is concentrated in one area compared with a larger region.

Here $\bar{d}$ is the mean distance from each point to its nearest neighbour, n the number of points and A the area, in matching units (km and km²). $R_n=0$ means all points are clustered at one place, about 1 means a random pattern, and 2.15 means a perfectly regular (uniform) pattern, as in a triangular grid.

The location quotient compares an activity’s share of the total in an area with its share in a larger reference area, usually the country. It is most often calculated for employment.

Here $e_i$ is employment in activity i in the area and $e$ is total employment in the area; $E_i$ and $E$ are the same figures for the reference area. LQ > 1 means the activity is more concentrated in the area than in the country, LQ < 1 that it is under-represented, and LQ = 1 that its share is the same.

Testing relationships and differences

Two statistical tests are used most in fieldwork, and their results can appear in a stimulus to be interpreted. Spearman’s rank correlation coefficient ($r_s$) measures the strength and direction of the relationship between two variables measured at the same sites, from +1 (perfect positive) through 0 (none) to −1 (perfect negative). Each variable is ranked (1 = highest; tied values share the mean of their ranks) and d is the difference between the two ranks of each pair.

The value is then compared with a critical value for the number of pairs, n, at a chosen significance level, usually 0.05 (95% confidence) or 0.01 (99% confidence). If the value, ignoring its sign, is at least the critical value, the null hypothesis of no relationship is rejected. The chi-squared test does the same job for counts in categories, comparing observed frequencies (O) with those expected (E) if there were no difference.

Describing patterns, trends and anomalies

Many questions begin with "Describe…" a map, graph or table. A description says what the figure shows, not why. The most reliable structure is TEA: the Trend or general pattern, Evidence with figures and units from the stimulus, and the Anomaly or exception.

Useful vocabulary makes descriptions precise: "increased steadily", "rose sharply after 1990", "fluctuated between", "peaked at", "a strong negative relationship", "concentrated in the north-east", "a linear pattern along the coast", "evenly spaced". Avoid vague words such as "a lot", "big" or "goes up and down" without figures.

Evaluating sources

Geographers use maps, statistics, photographs, reports, news articles and interviews made by other people, and each has to be judged before it is trusted. Three questions apply to any source: is it accurate, is it relevant to the question, and is it free from bias?

A photograph shows one view at one moment, chosen by the photographer; an interview gives one person’s perception, which is valid evidence of that perception but not of the facts. Opinions, values and perceptions are legitimate geographical data when they are identified as such and compared with measured evidence.

Exam practice

Skills are tested in every paper, but most directly in the structured questions on Paper 1 options, where a topographic map extract is a common stimulus, and in Paper 2 Sections A and B. This structured question follows the Paper 1 format and uses the Ashby map extract.

Skills checklist

• To do:

  I can give four- and six-figure grid references, directions, bearings and distances from a map, and convert between map and ground distances at 1:25,000 and 1:50,000.

• To do:

  I can read relief from contours and spot heights, determine a gradient and draw a labelled cross-section with its vertical exaggeration.

• To do:

  I can choose, read and construct choropleth, isoline, proportional symbol, dot and flow-line maps, and explain why each suits its data.

• To do:

  I can read and construct line, bar, compound bar, scatter, triangular, logarithmic, rose and bipolar graphs.

• To do:

  I can describe population pyramids and development diamonds, and construct a Lorenz curve and determine a Gini coefficient.

• To do:

  I can determine the mean, median, mode, range, interquartile range, percentage change, densities and ratios, with units.

• To do:

  I can determine and interpret a nearest neighbour index and a location quotient.

• To do:

  I can describe a pattern or trend with figures and an anomaly, and evaluate a source for accuracy, relevance and bias.