Specialised Cartographic Skills: Isolines, Choropleths and Flow Lines

Flow Line Maps

Understanding how traffic moves through a city or how migrants travel globally requires more than just a list of numbers; it requires mapping movement. A flow line map shows movement between an origin and destination using proportional arrows.

These arrows display two variables simultaneously:

• Direction: Indicated by the arrowhead pointing to the destination.
• Volume: Indicated by the thickness of the arrow, which corresponds to a specific quantity scale.

Unlike desire lines (which are straight lines connecting origin to destination), flow lines should follow the actual route taken, such as a road network or a shipping lane. The width of the arrow must remain perfectly consistent along its entire length.

Worked Example: Calculating Flow Line Width

• Step 1: Identify the scale and the data. For example, the scale is $1\text{ mm} = 500\text{ vehicles}$ and the data value is $2500\text{ vehicles}$.

• Step 2: Substitute the values into the calculation.

• $2500 \div 500 = 5\text{ mm}$

• Step 3: Draw the line exactly $5\text{ mm}$ thick from the origin to the destination.

Isoline Maps, Contour, and Gradient

You can map a 3D mountain on a completely flat piece of paper just by drawing lines that never cross. An isoline is a line connecting points of equal value to represent continuous data (data found everywhere). Specific types include contours (height), isotherms (temperature), and isobars (air pressure). An isopleth is a special type of isoline map connecting equal values of averaged or calculated data, such as population density.

Isolines must follow strict rules: they must never cross or touch (except at vertical cliffs) and must form closed loops or extend to the edge of the map. They cannot stop abruptly.

Understanding Relief and Topography When reading maps with contours, you are studying the area's topography (the physical appearance and shape of the land surface) and its relief (the difference in height between the highest and lowest points).

To understand the exact height or 'value' on these maps, geographers use three main height indicators:

• Contours: Lines connecting points of equal height above sea level.
• Spot heights: A specific point on a map marked by a small dot with a number beside it, indicating the exact height of the land at that precise location.
• Triangulation stations: Shown as a small triangle with a dot in the middle, these indicate the exact height of a high point, such as a hilltop, measured accurately by surveyors.

Gradient and Spacing The proximity of isolines tells you the rate of change or gradient.

• Tightly packed lines indicate a steep gradient (rapid change).
• Widely spaced lines indicate a gentle gradient or flat land.
• The constant vertical distance between two adjacent lines is called the contour interval.

Interpolation is the process of estimating exactly where an isoline should be drawn between two known points. For example, if you must draw a '100' isoline between a point valued at 95 and a point valued at 115, you must draw the line proportionally closer to the 95.

Worked Example: Calculating Gradient

• Step 1: Identify the rise in height (Vertical Interval (VI)) and the horizontal distance (Horizontal Equivalent (HE)).

• Example: $\text{VI} = 50\text{ m}$, $\text{HE} = 1000\text{ m}$

• Step 2: Substitute into the formula.

• $50 \div 1000 = 0.05$

• Step 3: Express the final answer as an AQA-accepted format: a ratio ($1:20$), a fraction ($1/20$), or a percentage ($5\%$).