Horizontal datum is really important when you're trying to describe locations more accurately and taking into consideration the flattening of the Earth and the ellipsoid that you're using. So let's talk a little bit about the horizontal datum. Okay, with an ellipsoid, they're designed to approximate the Earth's shape for part of the planet, and that's important. Most ellipsoids do not really work that well for the entire planet all at once. They're really kind of if you think of it as being kind of tailored or customized for one part of the earth. So how do we specify which part of the earth a particular ellipsoid was designed for for it to be used? Well, to do that, we use a datum. So let's talk about what a datum is. A datum specifies the ellipsoid used and it's location. So a point on the ellipsoid is linked to a point on the earth. If this is the ellipsoid and this is the earth, Then you'll notice that this part of the earth fits really well with that ellipsoid. But this part does not work as well. Now, this is a kind of a simplification, but I think it works pretty well for us at this point in what we're trying to figure out. And, you'll notice that I've got this blue dot here and what that refers to is the origin and that's the way of specifying where that ellipsoid is connected to the earth, okay? So the datum specifies the ellipsoid, and it specifies how and where it's connected to the earth itself. So if this is the Earth's surface We can have a local datum. And that means that it's specified or connected to the Earth at the surface of the Earth. And these tend to be older datums, before there were satellites and ways of going into space. Whereas now the more modern ones use a more Earth-centered datum which is specifying the center of the Earth's mass as the way of connecting the ellipsoid to the Earth itself. So we have a local datum and we have an Earth-centered datum. So hundreds of ellipsoids have been developed over time, literally dating back to some of the first estimates of the Earth's size by Aristotle. He lived from 384 BC to 322 BC, so we're talking about over 2,300 years ago. So yeah, there's been a long history of people trying to figure out how to accurately measure the earth and the amount of flattening for the earth. So here's an example of two different datums that are very common and popular in North America. These are ones that I am just familiar with and that if you work in North America you will probably come across. These are what are referred to as NAD 27 and NAD 83 and that stands for North American Datum 1927 and North American Datum 1983. And so yes, this was literally adopted in 1927, this was adopted in 1983, NAD 83 and that's the short form or the techie way of talking about it, is still really kind of the standard for a lot of things, even though you think, well 1983 seems like a long time ago. Because it's a standard and because it's something that people adopt that are using it, tons of people over lots of different locations, they don't want to change that standard very often. So even though it's kind of old, it's still the one that's being used. So I wanted to compare these a little bit, in terms of things like the Ellipsoid, Semi-major axis and so on. So the NAD 27 uses the Ellipsoid from Clarke 1866, so that's how old [LAUGH] the Ellipsoid is, that's being used for the datum from 1927. The semi-major axis is here, the origin for this is a local origin. In other words, it's tied to the surface of the earth in this place called Meades Ranch, Kansas. And it had 25,000 control points. So in other words, this is back in the days of literally covered wagons and horseback. They went across the United States and measured locations away from the origin, from this Meades Ranch. So they started there, they measured all these control points from there. And then that was a way for them to create this kind of customized or tailored coordinate system for that part of the Earth based on the ellipsoid and flattening that they use from that. And so I think a useful way of kind of thinking about this is that, think of it this way. You're trying to figure out the flattening for the entire planet, but you're doing that by only taking measurements in the United States, for one part of the earth, and then trying to extrapolate that for the rest of it. And that's a good way, I think, of thinking about why these local ellipsoids especially only really fit one part of the Earth well, as it was really only ever measured or designed around that one part of the earth. So it works well for there, but if you took that same NAD 27 and tried to use it for, say, Africa or Australia or somewhere else, it's not going to fit as well and it's not going to give you as accurate numbers. Anyway so here you see that it says best fitting so it's best for North America as opposed to NAD 83 that uses a much more current ellipsoid which is the GRS 80. So our semi major axis is a little bit different. The datum origin here is the center of the Earth's mass, and there was way more control points used here. These include ones that were measured from space, so we have a lot more control points that are measured more accurately. And so even though it says NAD 83, it actually fits the Earth well enough that you could use it anywhere in the world. Now having said that, there are still ellipsoids that are customized or tailored for different parts of the world even now in the sort of modern age that we're in. So you may still want to find an ellipsoid that's customized for Australia, or Africa, or South America, or Europe, whatever it is that you're working, that fits that part better. So yes, NAD 83 is kind of general purpose and it fits everywhere well. But there are still other ellipsoids that have been created and datums related to that that are more specific or customized for different locations. Now why am I telling you all this? I'm just trying to remind you that there is a practical application to all this, is to kind of jump ahead a little bit. When you're in the software and you're importing coordinates or using data, or you're creating data, you will have to specify the datum, and that datum will specify the ellipsoid. And that will actually have a real effect on where that point that you're creating is located on the surface and how accurate it is. That's really the connection here that we have to kind of a practical application. When I was talking about NAD 27 and I said that the origin of that is Meades Ranch, Kansas, you might be thinking, where is Meades Ranch, why did they use that, and what's that about? Well, at the time I thought this was kind of clever. We're talking about the contiguous United States, the part that's not Alaska and Hawaii, [LAUGH] let's just start with that. And so if you're looking at this kind of object, this shape. If you're trying to come up with a framework as to well, how are you going to measure things away from that, or in that? Why not start with the very center of it? And so it was determined that the very center of the United States was this location called Meades Ranch Kansas. And what do you know, if you go into Google Maps, you'll see that it actually says geographic center of the conterminous United States. And here's a photo from Google of that location. I have not been there yet, but I would love to go. [LAUGH] It looks like it's kind of in the middle of nowhere. Although I'm sure the people that live in Kansas wouldn't think of it that way. But it looks fairly isolated, but I would love to be able to go there and just kind of appreciate the fact that so much of the measurements that are taken were based from this location. And I should mention to you that even thought NAD 27 is an older datum, I've used many datasets that were based on that. I used to work a lot in North East territories in Canada. And a lot of the data that was collected out there was based on topographic paper maps that were still based on the NAD 27 datum. And so those paper maps were based on that, things were traced or digitized off of those maps that were still in NAD 27. And so even though they were digital and they were in our GIS, they were still based on that older datum, and so I had to be aware of that. And then I could, if I needed to, which I normally did, would converted to NAT 83. So just because it's an older datum doesn't mean that you won't ever see it or need to deal with it.
