At the threshold of observation, some people convinced themselves they had seen something, and, once they had, thought they could construct maps of the 'canals'. But this relied on them having the idea in their head of what they thought they'd already seen.
canals of Mars, apparent systems of long, straight linear markings on the surface of Mars that are now known to be illusions caused by the chance alignment of craters and other natural surface features seen in telescopes near the limit of resolution. They were the subject of much controversy in the late 19th and early 20th centuries and influenced popular thinking about the possibility of life beyond Earth.
The Italian astronomer and statesman Giovanni Virginio Schiaparelli reported observing about 100 of these markings, beginning in 1877, and described them as canali (Italian: “channels”), a neutral term that implied nothing about their origin. Other observers had earlier noted similar markings, but Schiaparelli’s writings first drew wide attention to the subject. About the turn of the 20th century the American astronomer Percival Lowell became the champion of those who believed the markings to be bands of vegetation, kilometres wide, bordering irrigation ditches, or canals, dug by intelligent beings to carry water from the polar caps. Lowell and others described canal networks studded with dark intersections called oases and covering much of the surface of the planet. Occasionally the lines were perceived as doubled; i.e., two parallel lines became visible where only a single canal had been seen before.
Most astronomers could see no canals, and many doubted their reality. Experiments with untrained observers showed that disconnected features in diagrams or drawings might be perceived as straight-line networks when viewed at the proper distance. Telescopic photography through Earth’s atmosphere offered no solution, because the lines were barely discernible by the human eye and beyond the recording capability of the camera. The controversy was finally resolved only when close-up images of the Martian surface were taken from spacecraft, beginning with Mariner 4 (1965) and Mariners 6 and 7 (1969). These showed many craters and other features but nothing resembling networks of long linear channels, either natural or artificial.
http://www.britannica.com/EBchecked/topic/366397/canals-of-Mars
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