You're probably already familiar with longitude and latitude, but I'd like to go over it anyway just to make sure that we're all thinking about it in the same way and that you really have an appreciation of how it works and how it helps us describe locations on the surface of the Earth. If we want to tell somebody where something is, of course, you can do it in a relative way, you could say it's down the block or it's across town or it's next to the library, but what if you want to tell them exactly where it is? How do we come up with a framework or a system to be able to tell somebody in any part of the world where something is? Like how could you say, "I don't want to tell you exactly where that thing is, a tree or a fire hydrant or whatever it is, and say, I want you to be able to go to that exact location anywhere on the surface of the Earth." Well, we're going to use a coordinate system to do that and we're going to talk about how that relates to finding locations, storing GIS data, and that will lead us into a conversation later on map projections. longitude and latitude is based on an angular unit of measure. So, if we have a spherical Earth which for now we'll just treat it as though it's a perfect sphere, it's not exactly, we'll get to that later, but for now just think of it as being this perfect sphere, and you can't use just a regular grid like you might use a Cartesian coordinate system or something like that, we've got this 3-dimensional round sphere, we have to come up with a system that will allow us to describe locations on the surface of that sphere. So, how are we going to do that? We use this angular unit of measure, and it's based on the idea of having two axes that we're going to measure angles from the equator, and the prime meridian. So, for any given location we're going to measure an angle East or West of the prime meridian, and we'll talk about what that is in a minute, and an angle North or South of the equator. So, if we want to describe the location of something such as the Eiffel Tower in Paris, we can use latitude and longitude angles in order to describe where that is on the surface of the Earth. The coordinates here would be read as 48 degrees 51 minutes 30 seconds North of the equator, and two degrees 17 minutes 40 seconds East of the prime meridian. That's how we read them. Here the notation is latitude and then longitude, and this can get confusing because GIS likes to do it the other way around, we'll get to that in a minute, but for now, just think of it in terms of what you would probably normally expect to hear it as which is latitude and longitude. So the latitudes first, that's North or South of the equator and longitude is East or West of the prime meridian. Not so difficult, you've probably seen this before, the main thing that I really want to emphasize right now is that these aren't just coordinates, I worry that sometimes people think of them just as numbers, like well, "Yeah, they're coordinates." They're angles, and they're angles in relation to these axis on this round globe. The fact that the angles will be really important later on when we talk about datums and how this affects the actual coordinates that are used to store locations based on the datum that they're stored in them, we'll get to that later, but for now, I just want to emphasize this idea that these are angles. Okay, you got it? Angles, good. So, if it was you were trying to come up with a frame of reference for the spherical earth, what would you work with? Well, let's have a look. We have this axis that the Earth spins on, so okay, that might be something we could use and we could draw a line along that plane or actually create a plane along that line let's put it that way, and here we're using it, there's the prime meridian, we'll talk about how that's defined in a minute, but that's going to be one of our axis for our frame of reference, and then the other is going to be the equator which we perpendicular to that and we'll measure angles away from that, where those cross we just referred to that as the origin as you would with any reference system with two axis like a Cartesian Coordinate System where they cross they just call that the origin. So, we can measure angles away from the equator, for example, here, this is an angle of 10 degrees from the equator to that line. It's an angle of 10 degrees. Now, if we extend that line around the Earth, that becomes, well, if I just circle the whole thing, that becomes a line of latitude at 10 degrees, and that's what we mean by that angle, so we draw an angle of 10 degrees, we extend that line around the Earth, that becomes our line of latitude that's at 10 degrees. So, anything that exists anywhere in the world along that line can be said to be at a latitude of 10 degrees. So we can do that for 20 degrees, 30 degrees, really, it would be at one degree increments, so I'm just putting some of them on here so you can get a sense of how it works, all the way up to 90 degrees at each of the poles, we have 90 degrees North latitude at the North Pole, 90 degrees South latitude at the South pole. So, you see that there's an annotation here that says N so that we know that this is North of the equator, S so that we know that that's South of the equator. All right, I'm hoping that's fairly straightforward for you. Longitudes are a little bit different. All we really know is that okay, well, we have a North pole, so we need to draw a line that we can use to measure angles away from there, and well, where's that line going to be? Really, it could be kind of arbitrary and in a way it is, but we've all agreed on the same arbitrary line, and as long as everyone's using the same line then we're okay, and so that's what we refer to as the prime meridian. So, the prime meridian is zero degrees that runs through Greenwich England, we'll talk about that more in a minute. We measure angles away from that, so a 90 degree angle from that prime meridian would be 90 degrees, in this case, East of the prime meridian, and then it goes all the way to 180 degrees away from the prime meridian, and that's known as the International Date Line. We do the same thing on the other side, so just like we did with the North and South, here we have 90 degrees East of the prime meridian, so we're measuring angles this way, here we have 90 degrees West, here we're measuring them that way, and in the same way as we did before, we're actually extending this numbering system so that you can have one degree East of the prime meridian, two degrees and so on, you get the idea. So we have our lines of latitude, we have our lines of longitude, when we put those together we get this grid that we refer to as the Graticule. That's another bit of terminology that you should remember, it's something that we'll refer to from time to time as I might recommend that you had a graticule to your map, and so now you'll know what we're talking about, it just means the grid that is described or defined by the lines of latitude and longitude. So, it turns out that latitude and longitude is good for describing locations on the surface of the Earth, but that coordinate system is not very good for measuring distances. In other words, you wouldn't really want to tell somebody that the distance from here to another town would be so many degrees latitude or longitude, and why is that? Well, if you're describing the distance between these two lines as being 40 degrees. So, this is a meridian, this is what we would call this line, this is a meridian as well, so the prime meridian is the one that goes through Greenwich here, any other line that's on that axis are part of that system is just called the meridian, so we have a meridian here and we have a meridian there. So, the distance in terms of the angle between those two is 40 degrees, and you think, "Okay, what's the problem? Well, the problem is that that's 40 degrees, that's 40 degrees, that's 40 degrees, and that's 40 degrees, and that's 40 degrees. The problem is that that these meridians, it's not necessarily a problem, it's just a fact, these meridians converge at the pole. In other words, they are getting closer and closer together as you go farther North and the same thing happens as you go South. In other words, 40 degrees means different distances at different latitudes, this distance here is very different than this distance up there. So, if we don't have a consistent system or way of measuring distances, then it's useless. If I said to you, it's 20 miles to get from here to the shopping mall and it say, "Oh well, it depends on what part of the Earth Uranus is what 20 miles actually means, it could be really far, it can be really close", that's not going to work very well, is it? So, why do I point this out? Well, as we'll see later, we have to come up with a different coordinate system that is consistent, that we can use to measure things like distances on the ground or on a map that will work much better. So, I'm planting that seed now so that you see that it's good for describing locations, but not so good for describing distances, just remember that.