To compare maps fairly, convert each map to real ground distances or areas first, then compare. The skill appears in map interpretation questions and in change-over-time topics. It belongs to graphs, photographs and spatial data.
How do you compare two maps step by step?
- Record each map’s date and scale.
- Measure the feature in centimetres on each map.
- Convert to real distance: map cm × scale number, then divide by 100 000 for kilometres.
- For area, convert both sides to kilometres and multiply, because converting areas directly is error-prone.
- Compare the real values, then comment on reliability.
Worked example
All data is invented.
Map X is dated 1990 at 1:50 000. Map Y is dated 2020 at 1:25 000. Both show the same settlement.
On Map X, the built-up area measures 2.0 cm by 1.5 cm.
- 2.0 × 50 000 = 100 000 cm = 1.0 km
- 1.5 × 50 000 = 75 000 cm = 0.75 km
- Area = 1.0 × 0.75 = 0.75 km²
On Map Y, the built-up area measures 5.0 cm by 3.6 cm.
- 5.0 × 25 000 = 125 000 cm = 1.25 km
- 3.6 × 25 000 = 90 000 cm = 0.9 km
- Area = 1.25 × 0.9 = 1.125 km²
Change: 1.125 − 0.75 = 0.375 km². As a percentage of the original: 0.375 ÷ 0.75 = 0.5, which is a 50% increase.
Comment: The settlement area measured about 50% larger on the later map. Both maps were drawn by different surveys, so the edge of “built-up” may be defined differently, and the estimate has measurement error from reading centimetres.
What mistake do students make?
Mistaken answer: “The settlement is 2.0 cm on Map X and 5.0 cm on Map Y, so it is 2.5 times wider.”
The two maps use different scales, so the centimetre readings are not on the same footing. The real widths are 1.0 km and 1.25 km, so the settlement is 1.25 times wider, not 2.5 times.
The correction is to convert before comparing, every time.
Check yourself
Invented figures.
1. On a 1:10 000 map, a path measures 7.5 cm. What is its real length?
Show answer
7.5 × 10 000 = 75 000 cm = 0.75 km.
2. A real distance of 2.4 km is shown on a 1:50 000 map. How long is it on the map?
Show answer
2.4 km = 240 000 cm. 240 000 ÷ 50 000 = 4.8 cm.
3. Name one reason a difference between two maps from different years might not be real change.
Show answer
Different surveys may use different symbols or boundaries, or define a feature such as “built-up” differently. Scale also changes how much detail can be drawn. Either answer is acceptable.
Where this leads next
Scale also affects how large symbols appear, so continue with reading proportional symbols consistently. Use the map scale, contour and gradient practice tool for extra conversion practice. If you want a teacher to give you map pairs and check your working, see our online one-to-one Geography tuition.