What is remote sensing? Well, it's a way of being able to sense things remotely. It's a way of gathering information about an object without actually having to be there to touch it in person. And that could be remotely in terms of being a few feet away or could be remotely in terms of being many kilometers away. So, notice how I mix my units there. So, let's have a look at how this works. Alright. So, if you have an object, something on the surface of the Earth such as a tree, you, in this case are the remote sensor, your eyes are the sensor, and we need a source of energy. In this case, we'll just think of it as light, that's all it really needs to be, and the light comes from the source such as the Sun. It hits an object such as a tree, some of that light bounces off of that, to reflect it off of it and goes in all kinds of directions and one of those directions is towards your eyes and your eyes are sensing that light and your brain is then interpreting what it's getting from that light. So, it may be thinking or seeing that in terms of color and brightness and shape and texture things like that and so what happens then is that your brain has kind of like a library of information about things that it's seen before. So, this is probably not the first tree you've ever seen. So, your brain looks at the pattern of information that it's getting in terms of the light, the amount, the shape, the texture, color, things like that. And it compares it to what it's seen before and says, "what is this thing that I'm looking at?". And of course, this all happens practically instantaneously, you don't have to think about it but then your brain says, "Oh that's a tree". Now you didn't actually have to go over and touch that tree or smell it or feel the bark on it. You're able to just look at it, it might have been a block away or whatever. And you are able to identify it. Not only that, but you can probably identify things like, are there leaves on it? How tall is it? Maybe even what species is it? So, that's information that you're gathering remotely based on nothing more than light being reflected off of that tree. That really is to me the essence of what remote sensing is, is that what you'll see through this section, is that we're doing exactly the same thing really is that you have light coming from the sun and for the purposes of this discussion we're only going to be looking at Sunlight. There are other sources of energy that can be used for this, you know things like radar but for for this section we're just going to look at the sun. So, you've got light coming from the sun and some of that's visible, some of it may not be visible as we'll see but that light comes from the sun, reflects off of the surface of the earth and instead of going into our eyes directly it goes into a sensor of some kind. You can think of it like a camera sensor really, it's a very similar kind of idea and that sensor is registering the same kinds of things, it's how much brightness is there of light or the color of the light if you want to think of it that way and it's recording that as a number. And so, we do that over and over again for different patches on the ground and that gives us a way of being able to collect information remotely from a sensor which could be on a plane or it could be on a satellite, could be on a drone. Wherever it is, those are the vehicles that would carry the sensor, if the sensor is able to collect that data, send it back to us, we can then assemble it and then use it to interpret what we're getting from that sensor in terms of that data, so we can interpret that visually, we could then assemble that into an image and say well what can we see or we can interpret it or analyze it quantitatively, we can analyze it and say are there patterns there that help us recognize certain types of features to see what they are. So that's basically what remote sensing is. So, a more formal definition would be to say that it's acquiring information about a surface without being in contact with it and that we're recording reflected or emitted energy and analyzing it. So, that's just a way of summarizing what I just said. So, the reflected or emitted energy, so you can't actually have things like thermal remote sensing, so if you've probably seen these before, where there's a certain type of sensor that you can point towards let's say a house and see where the heat is escaping from the house, so that's energy that's being emitted from the house that you're able to sense. Reflected energy which is what we'll talk about here really is more, usually energy that's being reflected from say the sunlight. I think it's useful when we're talking about remote sensing to think about color. So, why is it that if we have an object such as the shirt here, that this appears red instead of green or blue? Well, it has to do with what wavelengths of light are reflecting off of that object. Different amounts of color if you want to think of them that way reflect off of objects differently. If you divided up the the spectrum, if we take the light that comes from the Sun, which we just kind of think of as white light, if you split that up, even if you just see a rainbow. You know you're using a garden hose here at a waterfall wherever you see a rainbow. And you can see the different bands of light so there's red, orange, yellow, green, blue, violet, those are the ways we normally describe those. So, those are different sections of the wavelengths of color and we can look at those and think about well, what does that help us do in terms of understanding the information that's coming from an object based on light. So, we look at this shirt, the fact that it looks red means that it's reflecting more red light than other parts of the spectrum. So, it's not reflecting as much blue light or green light or yellow light or orange light. It's mostly reflecting red and that's why we perceive it as red. So, for now I just want you to kind of think about these things, the fact that different objects reflect, different amounts of different light. And what that means is, that we can tell them apart. If all objects reflected like the same way then, they will all look the same to us but the fact that they don't, the fact that they reflect things differently that there's different combinations of light. Something's reflect more red, some reflect more blue, they reflect them in different ways, there's different textures to those, even just looking at this picture of a shirt, you can tell that it's probably like a smooth material like a cotton, it's not a rough material like wool. So, even things like that you're gathering information about that object purely through the light that's being reflected from it. And then, what happens if the shirt got wet for example? What if the conditions of that material change? Then that will change the way that light is reflected, now that that spot there on the shirt is a little bit darker. And the reason it's darker is that the water is absorbing some of that red light now so there's less of it being reflected and so even the conditions can can affect how much light is reflected. So imagine if you think of a beach and the sand the dry sand, will look lighter the dark sand will look or the wet sand will look darker. And so, you have exactly the same sand but the conditions upon which you find it, which could be whether it's wet or dry or other things, will affect how much light is reflected. So, this is all just to say that I want you to think about color, wavelengths, the fact that we're not just looking at light in general as it comes off of an object but different types of light. One way we can think about light and how it hits objects is in a way that we refer to as Radiation/Target interactions. And the radiation for us for this conversation really just means sunlight but it could be, energy that's coming from some kind, it could be a light bulb, it could be from radar device, whatever. For us we'll just think of it as light coming from the sun, that's our form of radiation. And the target here is just a leaf, that just means the object that you're trying to gather information about based on the light that's coming off of it. So, it's Radiation/Target interactions. Okay, so if we have, our source of radiation, in this case the sun, the light that's coming from that towards our object is referred to as the Incident Radiation. And so, what happens, what can happen, when it hits an object? Well, some of that light may be transmitted through the object. When you think about a leaf, if a leaf blocked the light completely, if none of it was transmitted through then if you looked at a leaf, if you held it up to the sun or if you are looking at a tree from underneath, it would look completely black, it would be, imagine if you did that with a brick or something, there's no light coming through it, you get that but with the leaf, you can see that some of the light is coming through, so there is some light that's being transmitted through that leaf. Some of the light will also be absorbed, especially in the case of the leaves, because leaves use sunlight for photosynthesis. So, they actually have to absorb that light in order to be able to use it for food. So, some of that light can be absorbed by an object. Thirdly, some of that light can be reflected, and that goes off back into wherever, okay, away from the object. So, things can be transmitted, absorbed, or reflected. When we're talking about remote sensing, all that we're able to work with is the part that's reflected. We can't sense the part that's absorbed or transmitted. So, what that means though is that when we look at an object, we're looking at combinations of those three things absorption, transmission, and reflection. So, the key when you're thinking about different objects and if you're trying to identify one object versus another, is that it may absorb, transmit, and reflect in different ways. Some objects will reflect more light, some will reflect less light. When we take that a step further, not only is this happening in terms of light in general, but it's also happening in terms of different colors of light or what we call different wavelengths. So, it may be that a leaf absorbs red light more than green light. The fact that a leaf looks green is because more green light is being reflected than other wavelengths. So, red or blue or something else is probably being absorbed by the leaf. Some of it may be transmitted. So, again, I'm just trying to get you to think about in a more systematic way. What happens to light when it hits an object? How can we think about that? How can we describe it? So, let's take this idea and apply it to remote sensing of the earth's surface. So, we have our source of radiation, the sun. We have the incident energy coming in. Some of it is reflected off of the surface of the earth and that is sensed by a sensor. In this case, and mostly for this conversation, I'm going to talk about it in terms of satellite imagery. So, we have a satellite that's passing overhead. It has a sensor that's pointed down at the ground, and it's very sensitive to light that's being reflected from the surface of the earth. So, it comes in from the sun, reflects off the surface, hits the sensor, the sensor records the brightness of that. Then, that's what we use to build an image. So, that's essentially the idea of remote sensing. So, what exactly is being recorded though? It's the amount of light reflected that's stored as a number for one cell. So, what we have here is one square on the ground that we would refer to as a cell. When you think about it, what's happening is the sensor is pointed at the ground and it's just sensing that one square at a time, then the sensor moves through the sky to the next cell over and senses that one, moves over to the next one and senses that one. Obviously, this is happening very quickly, but that's how it's able to sense all of these different cells, and what we do is assemble those into a grid that we can then interpret as an image. So, each one of those cells has one value, one number that represents the amount of light that's being reflected off of the surface to the sensor. Now, we can get more complicated than this. We can actually sense more than one color of light at the same time, we can have multiple sensors; one for blue, one for green, one for red, we'll get into that in a minute. But this is the general idea. So, light's coming in, reflected to the sensor, and it's been quantified as the amount of light that's coming to that sensor as a number, from a scale of could be typically often say zero to 255 with an eight bit sensor. So, there's a range where if there's no light coming from then it's a zero. If it's the maximum amount of light, let's say off of ice or snow, then it'll be 255 and everything else will be somewhere in between. So, I mentioned the sensor can be on a satellite, it can also be mounted on the bottom of a plane, it can be on a drone, there's lots of different ways to be able to do this. But essentially, you have to think of them as two parts. There's the vehicle that the sensor is on, so that's like the satellite, plane, drone. Then there's the sensor itself. Think of it like a camera for now, and there may actually be one or there may be several cameras that are used for different purposes. Some of them may be taking pictures of larger areas, some of them in smaller areas, some of one wavelength of light, or another a bunch of different wavelengths, but that's the general way that this works. So, if you imagine yourself floating above the surface of the earth. Let's say that you're on that satellite looking down. This is the ground, this is what you're seeing. If we zoom in a little bit, we can get a little more detail here. This is actually part of the campus here at the University of Toronto. I'm just outlining this the outside of the field as a reference for now. So, we have our sunlight that's our source of energy, it's reflecting off of this field, some of that's hitting the satellite, the sensor, and it's going to sense it for that one square, that one cell, at one point in time. So then, it actually does this for every cell in the area, which forms a grid, and then this is what we would actually see. So, what's happening is that, what I've done and the software has done, is its assigned a level of gray or grayscale as we'd call it from black to white. So, black would mean that there's a value of zero for that cell, white would be if it had a value at the maximum, like I said it may be 255, that's a common range from zero to 255. So, these different cells each one of these has a value. So, this one might be 25, this one's a little darker, it might be 10. This one's really bright, it might be 200. You get the idea. So, these are actual cells from an actual satellite image. I've just zoomed in really closely so you can see them and get a sense of how this actually works and how an image is built. If we zoom out a little bit, you can get a better sense that this actually starting to look like something. If we zoom out some more, suddenly you can see that this is an image. Now, instantaneously, I imagine what's happening is that your brain is starting to interpret that, and that's good that's of course normal for you to do. But remember, all that this really is, is an assembly of cells in a grid with numbers, and then I've assigned those gray scale values from black to white. So now, our brains can start to visually interpret this information to say what do we have here? What do we see? I might see a really dark area maybe that's water? I might see really bright area maybe that's something else, like vegetation? Maybe there's patterns that we can see, like different sequences that are repeated that might be buildings? That kind of idea. So, that's all I wanted to do for this part is to just give you an overview of what Remote Sensing is, the basics of what we think about in terms of lights, and color, and reflection, absorption, transmission, and the combinations of those, and how that's used to collect information or data and then actually, it's data because what happens is is that you then assemble that into an image, and then your brain in this case is converting them into information, it's taking that data and assigning it to add more meaning which is what turns data into information.
