Did you know that the same black square can mean a factory on one historical map and a school on another? Students often look at two old maps and count matching shapes without first reading each legend. The result can be a confident but unsupported story about how a town changed.
A map legend or key translates a map’s drawn symbols into the features they represent. When comparing different editions, interpret each map with its own legend before comparing the underlying features. A shape has no universal meaning. If symbol definitions or map coverage changed, a change in the number of symbols is not necessarily a change in the real world.
The first diagnostic: what does each map actually promise?
Imagine that a project asks whether the number of schools increased between two decades. One pupil counts circles on the earlier map and squares on the later one. Another counts squares on both because squares seem easier to identify. Before choosing an answer, ask: What do circles and squares mean in each edition? Are the maps of the same district, at similar detail, and made for the same purpose?
A historical map is both a representation of a place and a historical document made by particular people. It may omit small places, simplify complex features, or emphasise what mattered to its creators. A symbol count is therefore evidence about what the map depicts, not an automatic census of everything that existed.
Four checks before making a comparison
- Legend: interpret each edition’s squares, circles, colours, line styles and abbreviations from the key printed with that edition.
- Date: distinguish the event or survey date from the publication and later reprint dates; the map may combine observations from different years.
- Coverage and scale: confirm that both editions show comparable areas and enough detail for the feature being counted.
- Selection rules: check whether a map marks all examples of a feature or only major ones, designated sites, or selected highlights.
If the legend is absent from a cropped image, find the complete sheet or accompanying catalogue before assigning a meaning. Do not copy the legend from another edition because it happens to show a similar coloured shape.
Worked example: two maps with different symbol dictionaries
Fictional classroom example: A town’s 1920 map uses circles for schools and black squares for mills. Within the study boundary, its legend identifies two school circles and three mill squares. A 1940 map uses black squares for schools and triangles for mills. Within the same drawn boundary, it displays five school squares and one mill triangle. These figures are invented solely for practice.
A student who counts black squares across both maps reports that a feature rose from three to five. But this compares mills in 1920 with schools in 1940; it is a category error, not a measured increase. After decoding each map with its correct legend, the school count depicted changes from two to five, an increase of 5 − 2 = three depicted schools. The mill count depicted changes from three to one, a decrease of two depicted mills.
Are there definitely three more actual schools? Not yet. If the 1920 map omitted small schools while the 1940 map included them, the comparison is not like-for-like. The careful conclusion is: “These two maps depict two and five schools respectively; whether that reflects the full historical change depends on consistent coverage and mapping conventions.”
An image can be accurate while your comparison is not
Consider a monochrome photocopy of a coloured map. What originally distinguished a red road from a blue watercourse may vanish into similar grey lines. A low-resolution scan can also turn a tiny hollow circle into a solid dot. Before inventing meaning from a blurry image, seek a clearer copy or an accompanying written key. Enlarging an image can make pixels larger without recovering detail that is missing.
Similarly, an updated map may redraw features with cleaner lines, change boundary shapes or use a newly classified category such as “educational institution” instead of “primary school.” Such changes can be meaningful information about the mapmaker’s classification, but should not be mistaken for direct physical changes in the district.
A five-stage evidence method
1. Create a separate legend card for each edition
Write the year, map title, source archive and symbol meaning. Resist the temptation to create a single combined card until you have decoded the maps separately. Use text labels as well as coloured marks so the exercise remains understandable if printed in black and white.
2. Match meanings, not shapes
Construct a little equivalence table: “1920 circle = school; 1940 square = school.” If the category definitions are not equivalent, leave the pair unmatched or add a qualification. In historical work, refusing a false match is better than manufacturing a neat time-series.
3. Fix a common comparison boundary
Identify the study area in both maps. If one includes a newly annexed suburb, count only the shared area first or clearly explain the territorial difference. Record whether the boundary itself changed rather than quietly treating two districts as identical.
4. Test mapping completeness
Use each map’s purpose, title and annotations. A visitor map may show prominent buildings rather than every institution; a survey map may use another inclusion rule. If possible, cross-check with a dated directory, records or another trustworthy source. A map symbol is a clue, not an exhaustive register.
5. Write a conclusion with a source boundary
Choose a verb that matches the evidence: “depicts,” “labels” or “marks” for what the map shows. Reserve “there were” or “increased in reality” for claims supported by additional sources and comparable definitions.
Practice ladder: guided, paired and independent
Guided: decode the key
A fictional 1910 map marks hospitals with stars; a 1930 revision uses stars for railway stations and crosses for hospitals. Which 1930 symbol should a student count to compare hospitals? Answer: crosses. Which symbol should be counted in 1910? Stars. Explain why matching star counts would be invalid.
Paired: challenge the boundary
One learner receives an older map of Central District. Another receives a newer map showing Central District plus a newly added suburb. Each counts libraries using their edition’s legend. Compare results, then highlight the shared area before drawing a conclusion. Discuss whether the change came from symbol meaning, geographic coverage or actual libraries.
Independent: write an evidence-safe caption
Choose two historical map images with accessible complete legends. Create a two-column symbol table, a sentence about differences in purpose or coverage, and a 70-word paragraph stating one supported comparison and one unresolved uncertainty. Mark each claim as map observation, external corroboration or interpretation.
What students often get wrong
- Same shape, same meaning: symbols are conventional and can change between map series or editions.
- A missing symbol proves absence: omission may reflect scale, purpose, damage or selection.
- Colour survives copying: greyscale scans and low resolution can obscure the distinction originally intended.
- Same place name, same boundary: the mapped jurisdiction or study area may have changed.
- Publication date equals observation date: survey, revision, printing and reproduction dates can differ.
What parents and teachers can do
A parent can ask, “Show me the legend for each map,” rather than asking only which map contains more squares. Teachers can deliberately present two maps whose symbols look alike but carry different labels and let learners explain the trap. A class progress rubric can award separate credit for correct decoding, fair comparison and cautious conclusion.
Observable progress begins when a learner stops guessing from symbol shape. It becomes independent mastery when that learner can annotate two legend cards, identify a broken comparison, and propose the missing evidence that would make the comparison defensible.
Limits: when a school exercise meets historical cartography
Old maps can contain copying errors, outdated boundaries, politically shaped classifications or uncertain dates. Some symbols have no surviving legend. A modern student may not be able to reconstruct their intended meanings conclusively. For serious historical research, seek archival catalogue records and specialist advice; describe uncertainties honestly.
The Library of Congress historical-map classroom lesson teaches observation of titles, legends, scales and mapmaker perspective. Its primary-source analysis guides give a practical observe–reflect–question routine. They support the method here; neither source verifies our invented town counts.
Frequently asked questions
Can the same symbol mean different things on two maps?
Yes. Symbols are defined by the edition’s own legend and conventions. Interpret them from that source, even if you have seen the same shape used differently elsewhere.
What if the maps have no visible key?
Find a complete image or source catalogue. If the meaning remains unknown, report it as unresolved instead of borrowing a legend from another map.
Does a change from two to five symbols prove three new buildings?
Only that the two maps depict different counts under their respective conventions. Proving a real change requires comparable boundaries, inclusion rules and ideally independent records.
Finish by comparing the things, not the ink
The underlying habit applies beyond history: a graph axis, a colour scale or a science diagram can change its visual language while the learner assumes it stayed fixed. See how to compare heat maps using different colour scales and how to separate historical originals from reproductions. Explore the Core Aim hub and eduKate Education Hub; the Clementi learning reference shows why diagnosis, explanation and independent checking belong together.