Maps and Grid Reference System
Senior 4 · Geography
Maps and Grid Reference System
Lesson 2 of 3
25 min
Rwanda's fifth Population and Housing Census counted 13,246,394 residents on 15 August 2022 (NISR). Figures 2 and 3 show its district results in two different ways. Compare them before reading on: which map tells you where people are most crowded, and which tells you where the most people are?
Figure 2: a choropleth map. Data: NISR, Fifth Population and Housing Census 2022, Table 1. District boundaries: geoBoundaries (Open Data Rwanda), CC BY 4.0. Lake Kivu and borders: Natural Earth.
Figure 3: a dot map of the same census. Each district has one dot per 40,000 residents (Gasabo 22, Nyaruguru 8), but the dots are scattered at random within each district, not at the true places where people live.
The choropleth shades each district by its density, so it shows crowding: the three districts of the City of Kigali (Kicukiro 2,944, Nyarugenge 2,830 and Gasabo 2,056 people per km²) stand out, then Rubavu (1,614) and Musanze (1,157), while Kayonza (338) is the least dense district. The dot map shows numbers: Gasabo, with 879,505 residents, has the most dots, but large rural districts such as Nyagatare (653,861) also have many. All figures are from NISR's 2022 census report.
Definition
Topographic map: a general-purpose map that shows many kinds of features of an area together: relief (by contours and spot heights), drainage, vegetation, settlements, roads and other communications, usually at a medium or large scale with a grid.
Thematic map: a map that shows one theme or topic, such as population density, rainfall or soil type. Example: Figure 2 is a thematic map of population density.
| Type | How it shows data | Best for | Example in Rwanda |
|---|---|---|---|
| Choropleth | Areas shaded by class; darker means more | Rates and densities for administrative areas | Population density by district (Figure 2) |
| Dot map | Each dot stands for a fixed quantity | Distribution of counts: people, cattle, farms | Population by district (Figure 3) |
| Isopleth (isoline) map | Lines joining places of equal value | Values that change continuously: rainfall, temperature, height | Isohyets of mean annual rainfall; contours on a topographic map |
| Proportional symbol map | Circles or squares whose area is proportional to the value | Totals at points: town populations, factory output | Population of towns |
| Flow map | Lines or arrows whose width shows the amount moving | Movement: migration, traffic, trade | Goods moving along the Northern and Central Corridors |
| Located graph | Small graphs placed on the map | Comparing places over time | Climate graphs placed at weather stations |
Other maps are named by their use: atlas maps (small-scale maps of countries and regions), cadastral maps (land plots and ownership, used in land registration), weather maps, road maps and tourist maps.
Warning
A choropleth shows averages for whole areas. Nyamagabe's density (441 per km²) is spread evenly over the district on Figure 2, yet much of the district is forest where almost nobody lives, and its towns are far denser. A choropleth also makes the change appear sudden at every boundary. Use it for rates and densities, never for raw totals: a large district would look important just because it is large.
Worked example 1: checking a density
Problem. Rwanda's total surface area is 26,338 km² (NISR, Statistical Yearbook 2019). Calculate the national population density in 2022 and compare it with the figure in the census report.
Solution.
This matches the 503 people per km² given by NISR for August 2022.
The second way to classify maps is by their scale. A large-scale map has a small denominator (1:2 500, 1:25 000): a small area appears large, with much detail. A small-scale map has a large denominator (1:1 000 000 or smaller): a large area appears small, with little detail.
| Class | Typical RF | Area shown | Detail | Examples |
|---|---|---|---|---|
| Plan | 1:2 500 or larger, e.g. 1:500 | A building, plot or school compound | Individual rooms, walls, plots | Building plans, cadastral plans |
| Large scale | larger than 1:50 000, e.g. 1:10 000, 1:25 000 | A town or a few sectors | Individual buildings, paths, field boundaries | Town maps |
| Medium scale | 1:50 000 to 1:250 000 | A district or a region | Contours, roads, villages, rivers | Topographic sheets |
| Small scale | smaller than 1:250 000, e.g. 1:1 000 000 | A country, continent or the world | Main towns, main rivers, borders | Atlas maps, wall maps |
Warning
The boundaries between classes vary between books. Some put 1:50 000 in the large-scale class. What never changes is the rule: the smaller the denominator, the larger the scale. 1:25 000 is a larger scale than 1:250 000, even though 250 000 is the bigger number.
Worked example 2: how much ground does a sheet cover?
Problem. A map sheet has a map face 40 cm by 40 cm. What area of ground does it show at 1:50 000, and at 1:1 000 000?
Solution. At 1:50 000, 40 cm represents km, so the sheet covers . At 1:1 000 000, 40 cm represents km, so the sheet covers , about six times Rwanda's area. The ratio of the two areas is
The small-scale sheet covers 400 times more ground, so it can show only a small fraction of the detail.
Tip
Choosing a map for a job. To find your way along footpaths, use a large-scale map. To plan a journey from Kigali to Rusizi, use a medium- or small-scale road map. To compare districts, use a thematic map. To show where land changes height, use contours on a topographic map.
Marcos Luna: "Understanding Map Scale". A clear explanation of representative fractions and of why a "large-scale" map shows a small area. Note his rule for telling large from small scale.
Practice
Key points
Sources and credits
Data: Fifth Rwanda Population and Housing Census, 2022: Main Indicators Report, National Institute of Statistics of Rwanda (NISR); Statistical Yearbook 2019, NISR (surface area).
Figures 2 and 3 were drawn by Ganzaa from these data. District boundaries: geoBoundaries (William & Mary geoLab), from Open Data Rwanda, CC BY 4.0. Lake Kivu and neighbouring countries: Natural Earth, public domain.
Map: World Factbook (1990) Rwanda, US Central Intelligence Agency, public domain, via Wikimedia Commons.
Video: "Understanding Map Scale", Marcos Luna, embedded from YouTube. The rest of the lesson was written by Ganzaa.
Quick check
Which scale is the largest?
