What does this number measure?
The government source reports more than 21,000 km of combined sections. The rounded 21,000 km reference compares that sum, not a single intact continuous wall, a straight-line distance or an area. The illustration shows a representative section of Ming masonry; the separate ruler indicates the cumulative length. The drawing is not a complete map of the network.
Compare the stated dimensions
The comparison uses the number in each fact sheet: altitude, height, diameter, length or distance, depending on the reference. Two values in metres do not necessarily describe the same property.
Source and limitations
Rounded reference: the source reports more than 21,000 km of sections from several periods. The illustrated section is representative and not at geographical scale; only the separate ruler carries the cumulative length.
The horizontal ruler compares approximately 21,000 km of combined sections. The section of wall is representative and not at geographical scale; its width and the height of its towers do not represent those 21,000 km. This is neither a single intact continuous wall nor a map of the complete network.