PSLE Science Reality Lab Vol No.083 | “Both Maps Are Red” — Does Red Mean the Same Value on Both Maps?

PSLE-SCI-REALITY-0083

Wait, What? The same red can mean 30 on one map and 80 on another.

You are shown two scientific maps side by side. Both have dark-red patches. The caption says, “The red regions are equally high in both maps.” That sounds reasonable: same colour, same meaning.

Then you read the legends. On Map A, dark red means 25–30 units. On Map B, dark red means 70–80 units. The colour looks the same because each panel was coloured using its own scale.

Reality Lab Vol No.083 teaches one transfer job: when scientific maps, heatmaps or images sit side by side, never compare colours across panels until you have checked whether the numerical legends are the same.

Quick Answer

Read Map A’s legend. What number range does each colour represent?
Read Map B’s legend separately. Do not assume it matches.
Check the endpoints and intervals. Is red 20–30 on one map but 60–80 on the other?
Compare numbers, not colour impressions.
Check units and measured quantity. Equal colours are meaningless if the maps measure different things.
Limit the claim. A colour comparison is valid only when the scales are aligned or explicitly converted.

Reality Lab habit: Colour is a code. The legend tells you what the code means.

The Owned Learner Job — and the Boundary

This page does not own general map reading, colour vision, graph scales or satellite science. It applies existing PSLE Science representation skills to one communication object: two or more panels whose colours look directly comparable even though each panel may use a different numerical colour scale.

Vol No.032 asks how one map’s bin boundaries shape its pattern. Vol No.034 asks what false colour represents. Vol No.083 asks a different question: can the same colour be compared across separate panels?

Original Reality Lab Case: Two Surface-Temperature Maps

This is an original composite case made for teaching. It does not describe a real location or organisation.

Two trays are photographed with a sensor after different treatments. A display converts the readings into colour maps.

Panel	Blue end of legend	Red end of legend	Value at dark-red patch
Map A	20 units	30 units	about 29 units
Map B	50 units	80 units	about 77 units

Both panels contain dark red. But the red encodes very different values. A viewer who compares hue without reading the legends would collapse two separate number systems into one.

Observed, Claimed and Inferred

Layer	What can be said
Observed	Both panels contain a dark-red region.
Claimed	The dark-red regions have similar measured values.
Inferred	The claim assumes the same red corresponds to the same numerical range in both legends.

Why Software Often Rescales Panels

A computer may assign the coldest or lowest value in each panel to blue and the hottest or highest to red. This can help viewers see variation within each panel. But it can make cross-panel comparison dangerous.

Imagine one tray ranges from 20 to 30, while another ranges from 50 to 80. If each gets its own full blue-to-red palette, both will contain blue, yellow and red even though their numerical ranges barely overlap—or do not overlap at all.

Representation Check 1: One Legend or Two?

If one shared legend clearly applies to both panels, colour comparison may be meaningful. If each panel has its own legend, treat colours as local codes until proven otherwise.

Representation Check 2: Same Minimum and Maximum?

Two legends can use the same colour order but different endpoints. Red could mean “near 30” on one and “near 80” on another. Compare the printed numbers.

Representation Check 3: Same Bin Boundaries?

Even if both legends run from 0 to 100, the colour bands might be divided differently. One map might use equal 20-unit steps. Another might place extra boundaries near values of special interest. The same colour name may still cover different numerical intervals.

Representation Check 4: Same Quantity and Unit?

A red patch on a temperature map cannot be compared directly with a red patch on a moisture map. The visual palette may be identical while the scientific quantity is completely different. Always read the title, unit and legend together.

Source and Provenance Check

Ask how the colour image was produced. Was it directly measured at every pixel, interpolated from fewer measurements, calculated from another signal, or automatically stretched for display? This does not make the image untrustworthy. It tells you what kind of representation you are evaluating.

A good scientific figure makes scale choices visible enough that another reader can reconstruct what the colours mean.

Comparison Check: Put Both Panels on a Common Scale

If you need a direct visual comparison, the strongest representation often uses one shared minimum, maximum and colour mapping. For example, both panels might use 0 = blue and 100 = red.

Then a given colour has the same numerical meaning in both. But note the trade-off: a very wide shared scale can make small within-panel differences harder to see. Scientific communication choices help one question while sometimes making another harder.

Worked Case 1: Same Red, Different Values

Panel X uses 0–20. Panel Y uses 0–100. A red point is 19 on X and 95 on Y. Same appearance, very different magnitude.

Worked Case 2: Same Scale, Valid Colour Comparison

Two maps both use one shared legend: blue = 0–10, green = 10–20, yellow = 20–30, red = 30–40 units. A red region on either map falls in the same 30–40 interval. Colour comparison is now supported at the resolution of those bins.

Worked Case 3: Same Colours, Different Units

Map A uses centimetres. Map B uses grams. Both use blue-to-red. Comparing “redness” as though it represented one common quantity is scientifically meaningless.

Worked Case 4: Independent Auto-Scaling Hides Change

A before panel ranges from 10 to 20. An after panel ranges from 30 to 40. Both are independently scaled, so each contains the same spread of colours. Looking only at colour patterns might make the overall level seem unchanged even though every after value is higher.

Worked Case 5: Shared Scale Hides Fine Detail

Two maps share a 0–100 scale, but one map’s values all lie between 48 and 52. Most of that map may look nearly the same colour even though small differences matter to the question. A common scale is good for cross-panel magnitude comparison, but may hide local variation.

Method and Variable Check

Even perfectly matched legends do not make two panels scientifically comparable if the measurements were collected under different conditions. Check time, instrument, distance, calibration, sample preparation and other relevant variables.

Alternative Explanations to Keep Alive

The numerical values genuinely differ.
The panels use different colour limits.
One panel uses different bin boundaries.
The panels measure different quantities or units.
Automatic image stretching changes display contrast.
One map includes interpolation or smoothing that the other does not.

What Evidence Would Strengthen a Cross-Panel Comparison?

A single shared legend.
Identical units and measured quantity.
Matching minimum, maximum and interval boundaries.
Clear statement of any transformations or normalisation.
Comparable measurement methods and conditions.
Access to underlying numerical values when exact comparison matters.

What Would Weaken It?

Each panel has its own unexplained legend.
Colour endpoints differ.
The legends are cropped out of the shared image.
Only screenshots are shown, without units or values.
One image has been contrast-stretched differently.
The caption makes numerical claims from colour alone.

How Far Can the Conclusion Travel?

If both panels use the same scale and method, you can compare their represented values within the resolution of the legend. You still cannot automatically claim why one region is higher, whether the pattern will persist, or whether every location inside a colour band has exactly the same value.

Tempting Reasoning That Fails

“Red means high, so red equals red.” High is relative to the legend.
“The colour palettes look identical, so the scales are identical.” Palettes can be reused with different endpoints.
“The after image has more red, so the measured quantity definitely increased.” Check whether the scale changed.
“A shared legend makes the whole experiment fair.” It only repairs one representation problem.
“The exact colour tells me the exact value.” A colour band may represent a range.

PSLE-Style Transfer Case

Two diagrams show the amount of a substance in different parts of two samples. Both use pale-to-dark shading. Sample A’s legend runs from 0 to 20 units. Sample B’s legend runs from 0 to 60 units. A student says two equally dark regions contain the same amount.

Evaluation: The conclusion is not justified from shade alone because the legends use different numerical ranges. The learner must read the value range represented by the dark shade in each diagram before comparing the two regions.

Explained Practice

Practice A: Map A and B share one legend. Can identical colours be compared numerically? Yes, within the legend’s defined ranges and assuming the same quantity and units.

Practice B: Two maps both use red, but A’s maximum is 10 and B’s maximum is 100. What must you not do? Treat the two red regions as equal values.

Practice C: Two maps use identical 0–50 scales, but one measures temperature and one measures moisture. Can colour magnitude be compared as one quantity? No.

Delayed Independent Return: The L-E-G-E-N-D Check

L — Look: Are there separate legends?
E — Endpoints: Do minimum and maximum values match?
G — Gaps: Are interval boundaries the same?
E — Exact quantity: Same variable and units?
N — Numbers: Compare numerical values, not just hue.
D — Design: Were measurement and display methods comparable?

Parent and Tutor Teaching Guide

Draw two five-box strips and colour both from pale to dark. Under the first, label the scale 0, 5, 10, 15, 20. Under the second, label it 0, 25, 50, 75, 100. Ask whether the darkest box means the same number. Then replace both with one shared 0–100 legend.

The aim is not to make children suspicious of colour. It is to make them disciplined about decoding a representation before reasoning from it.

Authoritative Sources

SEAB’s current assessment objectives explicitly include interpreting and analysing information and evaluating observations, information and methods. MOE also expects learners to work with multiple forms of scientific representation and to practise healthy scepticism. A colour map is useful evidence only after its code has been decoded.

The Quiet Return

Colour catches your eye before numbers do.

That is exactly why a careful scientist reads the legend before trusting the impression.

Same colour is not the same evidence until the scale says it is.

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