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To start with, if I was to tell you the location of something

Â as being 55 meters away in the x direction and

Â 41 meters away in the y direction, where is this item located?

Â It all depends on where those coordinates are relative to.

Â Are they relative to me right now?

Â Or are they relative to some standardized location on the earth?

Â This distinction is very important.

Â Packed into that description I just gave you are two important concepts about

Â geographic data.

Â Number one, I gave you a unit of measure for our coordinates in meters.

Â Without units, distances are meaningless.

Â And number two, we need to have a reference point with a known location in

Â order to locate items with these coordinates.

Â This reference point is often called a datum and it's effectively a model

Â of the Earth's surface that coordinate systems can be built on.

Â Together, these two concepts form the basis for a coordinate system.

Â Not all data uses the same coordinate system.

Â Far from it.

Â And to be displayed on the same map,

Â your data doesn't need to be stored in the same coordinate system.

Â But the GIS software does need to convert your data

Â to the same coordinate system behind the scenes.

Â This will intuitively make sense because you have different reference points and

Â different coordinates.

Â How can you overlay them without some conversion to a common system.

Â Building on coordinate systems is the concept of projections.

Â The term projection is often used interchangeably with coordinate systems,

Â and you may hear me make that mistake occasionally.

Â But, in fact, they are different, and projections build on coordinate systems.

Â So to start with, what is a projection?

Â Projections help us display the earth on a flat surface like your screen or

Â a sheet of paper.

Â While it may not be intuitive at first,

Â we can't just flatten the Earth easily to fit on your screen.

Â To help illustrate this concept, let's try to imagine flattening a sphere.

Â Just like we would have to do to display the Earth on your screen.

Â Imagine an inflated spherical ball, just like a football, or a soccer ball for

Â you Americans.

Â Let's cut it down the side from top to bottom so

Â that the interior hollow part is exposed.

Â To make it even easier to visualize it we can cut it into completely into halves so

Â that we can set the cut side on the ground.

Â We now have half the ball sticking up off the ground but there's no

Â easy way to completely flatten it so the skin of the ball is against the ground.

Â If this ball was the Earth, we would need to stretch and distort it

Â in order to get it completely flat to display on your screen or on paper.

Â This set of stretches and distortions is what a projection is.

Â When working on a two-dimensional surface like computer screen,

Â we get some benefits out of working with projected data.

Â First, lengths and angles can be constant across the two dimensions

Â which we can't always say about our geographic coordinate system.

Â Think about the lines of longitude and how they converge at the poles.

Â The distance between the degree of longitude at the poles is very

Â different than the distance between the same degrees at the equator.

Â Projected coordinate systems have uniform distances and

Â map units regardless of location, as well.

Â This lets us identify locations by X, Y coordinates on a grid.

Â To bring this all together, to build a geographic coordinate system we

Â need an accurate model of the Earth's surface.

Â From there, coordinate systems are built.

Â And building on that are projected coordinate systems.

Â Now, we don't get this translation of our data to a 2D surface for

Â free, there are trade offs.

Â When we project geospatial data, you end up creating distortions.

Â Distortions can occur in the shape, the area, the distance, or

Â the direction of the data.

Â Different projections are created to optimize for

Â these distortions so some projections are good at preserving local shape.

Â These are called conformal projections.

Â Others preserve the area of the features.

Â These are called equal area projections and

Â still others preserve distances between points on the map.

Â These are called equidistant projections.

Â In practice a projection must restore at least one attribute.

Â Shape, area, distance, or direction.

Â Different projections also optimize these attributes for

Â different locations on the earth, and others do it for the entire earth.

Â The result is a large number of projections optimizing for

Â different attributes and different locations.

Â Depending on the work that you're doing,

Â you will find yourself needing different projections and coordinate systems.

Â So now let's take a look at some projections.

Â Before we do that, let's take a look at the earth on a sphere.

Â We'll start by looking at Africa, and

Â then we're going to compare it to the size of Greenland.

Â Since Africa is near the equator, it's shown closer to its true size in this

Â often undersized relative to the rest of the world on a world wide projection.

Â In contrast, Greenland is oversized by virtue of being

Â near the poles where more distortion occurs.

Â 5:04

So keep these size differences in mind as we look at the following map projections.

Â The first projection here is the equirectangular projection.

Â It results from simply taking the angular coordinates of the globe and

Â plotting them as if they are linear coordinates on a sheet of paper.

Â So as you get closer to the poles, you have more distortion as the meridians,

Â your lines of longitude, stay the same with the part

Â on the sheet of paper instead of converging as they do on the globe.

Â It's a common choice in GIS software for

Â displaying data stored in a geographic coordinate system.

Â I want you to note right here the size of Greenland relative to Africa as well.

Â Next up is the Mercator projection.

Â You've probably seen quite a bit of this projection as it's very

Â common on the Internet.

Â A variant of the Mercator projection is used to most mapping applications online.

Â Note that in this projection,

Â Greenland is about as big as Africa, much, much larger than it should be.

Â The distortion is the result of this projection preserving angles

Â in order to aid a navigation and sacrificing sizes as a result.

Â It's an instance of a conformal projection.

Â Now, take a look at the Mollweide projection.

Â It is an equal area projection,

Â which means that area is preserved within the map.

Â Note that Greenland is it's appropriate size relative to Africa, but

Â we've distorted the shape of all of these locations in order to get

Â the appropriate areas.

Â So far the projections we've looked at have been very simple and optimized for

Â displaying things around the world.

Â As a result the distortions that we have in our maps are greater,

Â the Universal Transverse Mercator or UTM for

Â short, projection, tries to account for this by creating a series of

Â nearly identical projections that optimize for each area of the planet.

Â If you want to understand more details of exactly how it does this,

Â you'll need to take the rest of the courses in the specialization, but for

Â now we'll show you enough to use it.

Â The UTM projection divides the earth into 120 zones, 60 in the north and

Â 60 in the south.

Â Each of these zones is 6 degrees of longitude wide.

Â Since this is just a rotated Mercator projection,

Â we're minimizing distortion by effectively making each location have the properties

Â that exist near the equator in the standard Mercator projection.

Â In doing this, we minimize the distortion of area that occurs in the Mercator

Â projection while getting the benefit of preserving angles.

Â You can still display locations that are outside of each zone, but

Â they become more distorted as they do in a Mercator map.

Â This projection is very useful, and I encourage you to look up

Â the zone that you are in as it could become a common part of your mapping.

Â