The scale bar lies everywhere except the equator
That little "200 km" ruler is only correct on the equator — everywhere else the map stretches distance, too.
By W. Viljoen · Published 21 June 2026
Gabon,
Libya,
Slovenia,
Sweden- Gabon261,718 km²
- Libya1,630,168 km² · 1.3× on the map
- Slovenia20,322 km² · 2.1× on the map
- Sweden444,486 km² · 4.3× on the map
Both panels are drawn to the same scale. On the left, each country is the size the Mercator map gives it; on the right, its true size with the latitude distortion removed.The ruler only works on one line
Every map prints a little scale bar — "200 km" next to a short line — as if one
ruler measured the whole world. On Mercator it doesn't. The bar is calibrated for the
equator, and the further north or south you read it, the more it under-measures. The
four highlighted countries climb that ladder, from a scale bar that's spot-on to one
that's off by more than double.
The numbers
How much the map stretches distance at each country's centre (the scale-bar error):
- Gabon (on the equator): 1.00× — the ruler is honest here.
- Libya (~30° N): ~1.15× longer than the bar claims.
- Slovenia (~46° N): ~1.44×.
- Sweden (~61° N): ~2.06× — over double.
These are smaller than the area numbers you see while dragging, and that's the point:
distance is stretched by sec(latitude), while area is stretched by sec²(latitude)
— the square. So Sweden's distances are ~2× off while its area is ~4× off.
Why the map lies
To keep compass bearings straight, Mercator stretches the map east–west by
sec(latitude), and stretches it north–south by the same factor to keep shapes from
skewing. A length therefore grows by sec(latitude) and an area — being two lengths
multiplied — by sec²(latitude). The scale bar only knows one number, so it can only
ever be right on the one line where that factor is 1: the equator.
Try it
Drag any highlighted country toward the equator and the readout falls to 1.0× —
where the scale bar finally tells the truth. Remember the readout shows the area
tax: take its square root for the scale-bar error. When Sweden reads 4× too big, its
distances — and that little ruler — are off by 2×.