Geophysical maps are often colour coded in spectral order, that is, with violet, blue, green, yellow, orange, and red representing successively higher values of the mapped data. Viewing such maps with both eyes through a large convex lens can aid interpretation by making the colours appear to lie at successively higher levels in spectral order: red appears highest above the map plane and violet lowest. This colour stereoscopy (induced by chromatic aberration in the lens) will be in spectral order if saturated colours are separated by black contours. However, white separating uniform, saturated colours causes green to appear highest, with blue, red, and then yellow below. Fortunately, an overlay of fine black dots can restore spectral order to lens-induced colour stereoscopy in maps lacking black contours. Hence, I recommend that colour-coded maps use spectral order with red representing the highest values of mapped data and that an array of just visible black dots be printed over the colours if black contours are absent. Then viewing through a large magnifying glass will provide effortless perception of the map's implied third dimension.

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