The initial stages of studying the Earth's shape

    Because the earth's not quite round, we need to know just what shape it is, so we can make accurate maps.
    There's an old saying that goes something like, "You can't tell where you're going unless you know where you've been". After all, if you don't know where you are, how can you figure out which way to go, much less find your way home?
    Maps are just like pictures. Have you ever heard the phrase, "A picture is worth a thousand words"? Well, a good map can help us to make sense of all kinds of information and these days, any thing we can do to help us sort out the mountains of information we're faced with is a good thing! Pictures, or maps, make these large amounts of information much easier to understand.
    Maps play a big role in our life. For example – when you invite a friend over for the first time, you'll probably need to give them directions so they can find your house. You might tell them to follow a certain street, go past the convenience store and turn right at the playground to get to your house. In other words, you use "landmarks" to describe to your friend where you live. Landmarks are places, buildings, roads etc. That are easy to identify. They give your friend a helping hand to find your house. If you make a picture of the landmarks and how to use them to find your house, well, you've made a map. A map is easy for you to draw, because you know the turns and landmarks near your house and it's easy for your friend to follow, because they can just follow the pictures!
    Today, we often use convenience stores, schools or playgrounds as local landmarks. But what if your friend was from out of town and didn't know how to get to any of the local landmarks, what then? You say we need better maps? You say the maps need to cover a larger area, have more detail, more "landmarks"? Sounds to me like you're getting the picture.
    In order to make larger, better maps, we need to use something called a spatial reference system. A spatial reference system defines the way that any location (trees, houses, roads, buildings etc.) can have its own unique address. Well, for us to have a good spatial reference system, we really need to need to know about the shape of the earth. One thing always leads to another.
   Truth is, we (mankind) have been making maps and studying the size and shape of the earth for hundreds of years. To begin with, we were concerned with the area around our homes, and later, as we traveled farther from home, our interest grew to include larger and larger areas. The size of the world we lived on began to be of interest.
The early Greek mathematicians and thinkers such as Homer, Platon and Pythagoras all had ideas on the shape and size of the earth. While a few thought the earth to be flat (One guy even thought the earth was rectangular in shape!) most agreed that the earth was round or "spherical".
   Support for the "round earth" theory came from the sailors of ancient Greece. They noticed that as they approached their home port they could see only the high points. As they got closer, the land appeared to "rise" from the sea.
   The Greeks made many estimates of the size of the earth. These estimates were pretty good for the times, but were guesses all the same.
   Eventually, one curious fellow had an idea about how he could not only prove the earth was round, but measure the size as well.
   His name was Eratosthenes. He was a Greek mathematician and while he was living and working in Egypt he came up with an idea to make a more accurate measurement of the size of the earth.
   Eratosthenes wondered if a second stick stuck in the sand in a different place would cast a shadow. If the second stick cast a shadow, then the earth could not be flat (as some presumed) but must be round. Knowing what he did about mathematics (mathematics does come in handy every now and again!), he figured out that if he could measure the distance between the two sticks, and the shadow cast at the second stick, he could work out the size of the earth.
   Eratosthenes raced off to Alexandria, 800 km away and placed a second stick in the sand. Sure enough, this time the sun cast a shadow.
   Eratosthenes measured the shadow and using the distance between the two sticks figured the earth's circumference to be 40,234 km (25,000 miles). The currently accepted value for the earth's circumference at the Equator is 40,074 km (24,901 miles). A difference of only 160 km (99 miles)!
   Eratosthenes lived in a place called Syene (now called Aswan). He noticed that on the longest day of the year (the summer solstice) when the sun reached it's very highest point in the sky, that a stick placed straight up and down in the sand did not cast a shadow. In fact, he looked down into a dry well and noticed that the midday sun shone straight to the bottom, and the walls of the well were in sunlight. It was as if the sun was directly overhead.
   Measuring "The Bulge".
   Here's an example of what we mean. Let's pretend that we have a special tape measure. This special tape measure can be stretched directly - between any 2 points on earth. It can go right through the centre of the earth so we can measure the shortest distance between any 2 points. If you have a globe, now would be a good time to get it out and have a look.
   Now, let's measure the straight – line distance between 2 points on the equator. The equator is an imaginary line that runs around the earth half way between the North and South poles. Let's measure the distance between two points on this imaginary line that are on the "opposite sides of the earth" from each other.
   Now, let's also measure the distance between the North pole and the South pole - again, we stretch our special tape measure right through the centre of the earth. If the earth were round, the distances would be the same right? Well, let's check the tape. This time, our measurement is 12,713,505 metres, or about 12,713.5 km.
   Almost the same, but not quite. Turns out the straight – line, pole to pole distance is shorter than the distance between the two points along the equator. So, the Earth's not as round as we think is it?
   Now you might think at this point, "But what about all the hills, valleys, the mountain ranges, and the deep-sea trenches in the ocean. Surely they must contribute to the shape of the earth". Now you might think at this point, "But what about all the hills, valleys, the mountain ranges, and the deep-sea trenches in the ocean. Surely they must contribute to the shape of the earth". Sure, we measure these features and draw them on maps, but in Geodesy, their contribution to the shape of the earth is pretty small.
   To illustrate, let's suppose we shrunk the earth down to the size of a golf ball. The "topographic" features, the mountains and valleys of the earth would be less significant than the dimples on the golf ball. A second example might be to look at a globe that shows the mountains in 3D (see a relief globe). Find Mount Everest in Nepal and rotate the globe so you are looking at the side of Mount Everest. It doesn't rise too far above the rest of the globe does it?
   Finally, let's look at a scale drawing of earth. In the previous section, we measured the equatorial axis to be about 12756 km. The earth's topographic features range from about 9 km above this surface (Mount Everest) and to about 12 km below this surface (Mariana Trench). The thickness of the line drawn to show the circumference, including our "topographic" features would change less than 0.2 mm. That's less than a pencil line thickness.