The geographic coordinate system may sound like this really generic term, but it's actually something very specific and it's really important that we understand what it is. When you go into GIS software, one of the very first things that you're going to have to decide is whether you're going to use a geographic coordinate system or a projected coordinate system. Literally, when you create a new map, you're starting from scratch. You want to make some map points, or create some data, or you want to import some data, one of the very first things you should do is set the coordinate system. And in order to do that, you have to understand what they are, and how they work, and which one to choose and why. And so, there's this decision that has to be made to begin with, which is which one are you going to use? A geographic coordinate system or projected, and I'm actually going to spend a lot of time talking about both of these things. They both have their purposes. There's good and bad about both, and you need to make informed decisions about how to use them; and I kind of think of this part as, I'm going to talk a lot about these two things, and there's a lot of theory and conceptual stuff that goes on in order to understand this. But really, what it ends up coming down to is that it's maybe a few seconds of your time in making that decision for a particular mapping project, but you have to make an informed decision, and in order to make that informed decision you have to go through all of this and understand it so that when you make that decision you're making the right one. So, that's really what it comes down to, is this one dialog box, a few clicks of the mouse, something that may take you a few seconds to do but you have to have that knowledge, that understanding and appreciation of what all this is about in order to be able to choose correctly. I have to tell you that part of the reason, part of my motivation for spending a lot of time on this is that when I first started with GIS software, this was not really explained that well to me and I really didn't understand or appreciate the importance of it, and I used to be so mystified by this. All I would be able to do is say ''Well, what was the coordinate system that was used on the paper map? I guess, I'll just use that one or what did somebody else is using? Okay. I'll use that, and I'll kind of cross my fingers and hope that's right,'' and I didn't really know why I was making a choice or what decision to be made. So, I want to spare you that. I want you to understand what this is. It's not that it's super difficult. There's a few kind of things you need to remember. You have to be able to visualize things in 3D a little bit maybe. There's some terminology to remember, but if you get this down now when you're first starting out, then you'll never have to really worry about it again. It'll just be something that's second nature to you and part of your foundational knowledge, that I think is so key to being a very effective, and efficient and useful GIS professional. A geographic coordinate system consists of an angular unit of measure, a prime meridian and a datum, and that datum specifies an ellipsoid, and so there are many different ellipsoids. So, there are many different geographic coordinate systems, and this may sound like one of these things where I'm kind of spending a lot of time in this definition, but there's a key element of this that's super important. So, the angular unit of measure, we understand that, that's latitude and longitude. We know what the prime meridian is, and now we know that the ellipsoid can differ or change depending on who's using the data, where it came from, which part of the world it's in. There's lots of different ellipsoids out there that's also related to the fact that there's lots of different datums out there. And so, remember, this is all part of the definition of the geographic coordinate system. So, if the ellipsoid changes, then you're actually changing the definition of the entire geographic coordinate system. And what that means is, if you change the ellipsoid, you're changing the geographic coordinate system and that will change where things are mapped or located on your map. It may seem strange but bear with me here, I think this will make sense to you. So, this is the overall definition, and if you change part of that definition, you're changing the coordinate system; and if you change the coordinate system, things will be mapped differently and maybe not in the way that you wanted. Maybe some errors will be introduced, maybe things will have shifted and maybe you'll notice that and maybe you won't, but if you don't you may actually have errors in there that you're not even aware of, and we want to avoid that of course. So, if we take this definition and look at it in terms of the software, this is just a dialog box from ArcMap, and you can see we have a geographic coordinate system folder and a projected coordinate system folder. So, you'll notice first by the way that these are folders. It's not like there's just one geographic coordinate system that everybody uses for everything all the time. I wish it was that simple but it's just not. There's actually folders for Africa, and for Antarctica, and Asia and so on. So, within each of these folders is actually multiple geographic coordinate systems. So, for example, with North America here, so this is a folder for North America. These are all different geographic coordinate systems. They're all using essentially the same thing. They're all using an angular unit of measure that's the same. They're all using the same prime meridian, but they're using different ellipsoids and different datums, and that's where the key is for this. If we scroll down, you'll see that there's NAD83. And in fact, there's actually even different flavors of NAD83 that have been specialized for particular purposes, but for now all we're going to worry about is the fact that there is one here called NAD83 and we can use that one. And you'll notice that when I select that, you can see the properties of that geographic coordinate system, and if we look at those properties you'll see, for example, that we have our angular units. So, this is part of our definition of a geographic coordinate system. So, we have our angular unit, which is a degree. Great. That makes sense to us. We have a prime meridian, that's Greenwich. Okay. That makes sense too, and we have a datum which uses a spheroid, which has a semi-minor axis, a semi-major axis here. We have a flattening, which is roughly 1/300. So, here it's 1/298.25722 bla bla bla, et cetera. And so, these are all the components of our geographic coordinate systems. So, this is actually implemented in the software. That's where you'll see it, and I really encourage you when you see dialog boxes like this and you see all this terminology and all these numbers to not let your eyes glaze over and kind of go, "I don't know what all that stuff is, but it probably looks right. Looks kind of technical though." What I want is for you to look at that and say, ''Oh, I know what that stuff is. I know what the definition of a geographic coordinate system is. I know what a primary unit is and all that stuff.'' And so, you can look at that and be comfortable and say, ''Yeah. That makes sense. That's what I want." I want you to be able to make informed decisions about how you're operating with your GIS. So, wait a minute. It says here spheroid, not ellipsoid and I kept talking about an ellipsoid. This is something that I think is a little bit unfortunate, but Esri likes to use the term spheroid. I like to use the term ellipsoid. The only reason really, they mean the same thing. The only reason I like to use ellipsoid is that it's much more obvious that we're talking about a flattened version of the Earth as opposed to a sphere, which is a perfectly round version of the Earth. What I worry or my concern is that if we're talking about spheres and spheroids, that it might be easy to confuse the two. So, I prefer to say sphere and ellipsoid, but in the software if you see spheroid with the oid on the end, that's the same thing as an ellipsoid. So, it's a little bit of terminology. You'll come across both terms depending on what you're reading or what software you're using. So, I do think it's important to at least point that out.
