IB Geography G.5.4 Representing Urban Data Notes

Choose a Graphic for the Geographic Pattern You Need to Show

1. A graphic is useful only when its form matches whether the evidence concerns location, movement, change through time or variation with distance.
2. The title, legend, units, scale and source must allow the reader to interpret the pattern without guessing.
3. The same dataset can support different conclusions when mapped as totals, percentages or rates.
4. Good analysis describes concentration, direction, connection, gradient and change rather than listing colours or individual values.

Choropleth Maps Show Rates Across Defined Areas

1. A choropleth map shades administrative units according to a rate, ratio or percentage such as deprivation or population density.
2. Class intervals should be logical and stated because changing them can make the same distribution appear more or less unequal.
3. Sequential colour schemes suit low-to-high values, while diverging schemes suit values above and below a meaningful midpoint.
4. Raw totals should not normally be mapped this way because large administrative units may appear important simply because they contain more people.
5. The map also hides variation within each unit, so neighbourhood inequality may disappear at borough or city scale.

Proportional Symbols Compare Magnitudes at Locations

1. A proportional-symbol map varies symbol area to show quantities such as city population, passenger numbers or emissions.
2. Symbols can be placed at city centroids or exact sites, allowing totals to be compared without shading the whole administrative unit.
3. The area of each symbol must be proportional to the value because scaling the radius directly exaggerates large observations.
4. Overlapping symbols can conceal smaller places in dense urban regions, so transparency, insets or careful ordering may be needed.
5. A proportional-symbol map shows how much occurs at a place but does not by itself show the direction of movement.

Flow Lines and Desire Lines Represent Movement

1. A flow-line map uses arrows or bands to show movement between origins and destinations.
2. Line width should represent volume, while arrow direction shows where the flow is going.
3. Flow lines can represent commuting, migration, freight, investment or information and reveal major nodes and corridors.
4. A desire line is a straight origin-to-destination connection that shows the relationship independently of the actual road or route.
5. Desire lines are useful for showing travel demand, but they should not be read as the physical path taken.
6. Dense networks can become unreadable, so minor flows may need to be grouped or filtered using a stated threshold.

Line and Compound Bar Graphs Show Change Through Time

1. A line graph is suited to continuous time series such as urbanisation rate, population, traffic or land value.
2. The gradient shows the rate of change, while turning points identify acceleration, slowing or reversal.
3. A truncated vertical axis can exaggerate small differences, so the scale must be checked before comparing slopes.
4. A compound bar graph shows how a total is divided into categories, such as land use or employment sectors, at several dates.
5. It reveals both the changing total and the changing composition, but narrow segments are difficult to compare across time.
6. Percentages are preferable when composition is the focus, while absolute values are needed when the total magnitude also matters.

Cross-Profiles and Bid-Rent Curves Show Change with Distance

1. A cross-profile plots a variable along a route or transect and is useful for showing the urban heat island from rural edge to city centre.
2. Repeated measurements should use similar weather and time conditions because heat-island intensity changes between day and night.
3. Local parks, water bodies, building density and industrial sites can produce peaks and dips within the overall gradient.
4. A bid-rent curve represents how the amount different land users are willing to pay generally changes with accessibility to the centre.
5. The curve is a model rather than a direct map, and polycentric cities may have several peaks instead of one central maximum.
6. Both techniques make distance explicit, but a measured profile and a theoretical model should not be interpreted in the same way.

Graphic Choice Should Follow the Question

1. Use a choropleth map for a rate by area and proportional symbols for a total at a location.
2. Use flow lines for movement direction and volume, and use desire lines when only the origin-destination relationship matters.
3. Use a line graph for temporal trend and a compound bar graph for changing composition.
4. Use a cross-profile for an observed gradient with distance and a bid-rent curve for a theoretical relationship.
5. Combine two graphics only when each adds a distinct dimension, such as a map for spatial pattern and a line graph for temporal change.
6. Every conclusion should be limited by the data's date, spatial unit, classification and collection method.