Five Maps That Changed History: From Eratosthenes' Earth-Radius Calculation to NASA's Landsat Imagery

The video starts with Eratosthenes' World Map, dated to around 194 BCE. The host says the map is revolutionary partly because of who made it, and describes Eratosthenes as one of the ancient Greeks considered one of the founding fathers of geography.

Eratosthenes' most famous achievement, the host says, was calculating the radius of the Earth to within 1% of the correct value, giving 6366 kilometers instead of the correct 6378. The calculation relied on basic geometry. Ancient Greek scholars already knew the Earth was round, so if one measured the angle of the sun in two cities at the same moment and knew the distance between them, the radius and circumference could be worked out.

Because one person cannot be in two places at once, Eratosthenes measured in one city one year and in the other city the next year at the same time. The distance between the cities was surveyed by a professional called a bematist, trained to walk in perfectly equal steps. The host notes that the measurements were not exactly precise, but the math mostly held up. The calculations also underpinned the coordinate system of parallels and meridians on his map, the same system that is known today as latitude and longitude.

The host explains that earlier maps were good for showing relative locations but were more conceptual and of little use for calculations or long-distance navigation. The introduction of a mathematically based grid system, such as parallels and meridians, is described as completely revolutionizing mapmaking and turning maps into a precise navigational tool.

The Cantino Planisphere dates to 1502 and is named for Alberto Cantino, who smuggled the map into Italy. The host calls it one of the first maps on record to depict North America clearly and distinctly as its own continent. Most earlier maps either left out the North American continent or labeled it incorrectly as part of Eastern Asia.

At the time, documents containing details about the so-called New World were closely guarded, because European countries wanted the information for themselves. The map was stolen at one point, but was later found hidden in a butcher shop. The host says the map and others like it mark the start of a transition between the worlds that existed before and after European powers colonized North America.

The map also marks a transition in mapmaking, being the first to include the equator, the tropics, and the Arctic circle. Half of it uses parallels and meridians, while the other half uses the portolan-chart model.

The host says the creators of the Cantino Planisphere combined the detailed coastal navigation of the portolan-chart model with the long-distance navigation ability of parallels and meridians. Portolan charts were detailed for coastal use but distorted farther out to sea, so the combination addressed that weakness.

The host introduces the Mercator projection as infamous, recounting that when asked their favorite map projection, few people would name it. The host, a marine geologist, names the Spilhaus projection of 1942 as a personal favorite.

The Mercator projection was created in 1569 by projecting a globe-shaped Earth onto a cylinder that was rolled out in all directions to make the map. This was useful for several reasons but had an unfortunate side effect: stretching the small areas around the poles to cover the same distance as the larger areas at the equator artificially inflates the size of countries nearer the poles. On a Mercator map, Greenland looks about the same size as the entire continent of Africa, though in reality Africa is about fourteen times larger than Greenland.

The host notes that the projection is the most widely used and that this is why people dislike it. People often equate size with importance, and the host says the Western world benefits most from the projection, which is a problem. Every line meets at a 90-degree angle, so a straight line drawn on the map keeps a constant direction, which makes navigation much easier. That is what it was made for, and its perfect angles also make it well suited to online zoom in computer programs, which is why it has lasted. The host concludes that it is great for many things but perhaps not the best choice for a classroom.

The Broad Street Cholera Map was created in 1854 during a cholera outbreak in London. In the 1800s, most disease outbreaks were blamed on breathing bad air, a view called the miasma theory. Hygiene and plumbing standards were basically nonexistent at the time, and because disease often appeared in the smelliest areas, people assumed the smell carried disease. Some doctors, including John Snow, had suspicions that this was not the case.

Snow tracked the source of the outbreak to one specific water pump on London's Broad Street and published his findings, including the map. The host says the story has become legend in geography: that he went door to door mapping the deaths, found the source, and single-handedly stopped the outbreak, saving London. The host says that story is usually told this way but that much of it may be exaggerated.

Some versions of the story say Snow asked the Board of Broad Street Parish to remove the pump, while others say he went and destroyed it himself. But Snow never claimed to have removed the pump himself. Even if the board did agree to remove it, the outbreak had already started slowing down on its own well before the pump was taken out.

The host also notes that Snow published the map, but there is no evidence he used it to identify the source of the outbreak. He was thinking geographically about tracking locations, which remains a big win for geographers, but the map itself probably was not how he figured it out.

It has been claimed that Snow was the first person to put disease outbreaks on a map. The host says, however, that there is a map of a yellow fever outbreak in New York City dated nearly 60 years before Snow made his map.

Despite the embellished elements of Snow's story, the host says his work had a huge impact. He showed how a deadly disease was being transmitted and showed that miasma theory did not explain how cholera spread. He also provided the alternative to miasma that eventually became accepted as scientific fact, and popularized a new way of looking at how disease spreads. The host says the study of epidemiology today has been profoundly shaped by his success, and jokes that it may be due to the myths about his research as much as the research itself.

The host's personal favorite is the NASA Landsat maps. The name combines 'land' and 'satellite,' and the host notes that anyone who has clicked around in Google Earth has used a Landsat map. These maps show the entirety of Earth's land surface from satellites orbiting more than 650 kilometers away. They were the first of their kind and changed not only mapping capabilities but the entire field of Earth and space science.

Landsat was launched in the 1970s as a joint effort between NASA and the United States Geological Survey, with the goal of creating the first visual long-term record of Earth's surface. The aim was not only to visualize the surface but to monitor changes over time. Each satellite crosses the same point on Earth at nearly the same time of day every 16 or 18 days, and once the satellites were in place, scientists had access to kinds of data they had never had before.

The host says the satellites can see through shallow water, which helps ships navigate. The maps are used for agriculture, including monitoring crop health and investigating water shortages. They are also used to combat wildfires, to plan the development of cities, and to study the melting of glaciers and icebergs.

The satellites have multi-spectral imaging, meaning they can see wavelengths of light that humans cannot. The host, a volcanologist, says this is important for the field, because when Landsat passes over a volcano it can visualize the heat coming from it. Before starting their PhD, the host worked as a lab tech for the Alaska Volcano Observatory, where images such as one of an eruption from Shishaldin volcano were vital to keeping people safe from volcanic eruptions.

The host responds to the thought that Alaska has no volcanoes by saying Alaska has over 130 of them, with one or two having erupted just about every year since 1900. As an Alaskan, the host points out that Alaska could be split in half and Texas would still be only the third largest state, with the two halves of Alaska ranking first and second.

The host adds that the misconception that Alaska is not that big is not the fault of the Mercator projection, but of lower 48 geographers who placed Alaska in a little corner next to Hawaii on national maps.

Landsat maps have helped discover new places. An island in the Arctic inhabited entirely by polar bears was discovered only because of Landsat in 1976, and it was promptly named Landsat Island. The host says Landsat was one of the first global efforts to create open-source maps that everyone could view free of charge. Anywhere in the world can be explored from home using Landsat imaging, which is how the host says it really changed the world.