When learning about GIS overlay analysis, I think it's really useful to go back to the very beginning, and that's Ian McHarg. He was a pioneer in the spatial analysis and thinking that went into overlay analysis. The work that he did was directly borrowed by the people that developed the first GIS software in the first computerized version of this, and they were heavily influenced by the work that he's done. So, because he had such a big influence on GIS and the way it developed, I think it's worth spending a couple of minutes of learning about who he was and what he did. Ian McHarg started the program in Landscape Architecture at the University of Pennsylvania in 1994. In 1969, he wrote a very influential book called "Design with Nature." He was a partner in a consulting company, and was asked to find a route for new highway on Staten Island that would have what was defined as the "least social cost." Before this, the standard way to route a highway was to leave its engineers to just pick the shortest or most cost-effective route between two points. He created a method that was based on the new field of environmental science as well as on social values. So, what he did was he mapped out 16 factors that he thought should be considered including natural factors such as slope and soil drainage, but also things like scenic value and recreation value, and this was new at the time. So, each map represented one factor where the darker the tone the greater the cost and defined both in practical terms like slope and in social terms like historic value. What he did was he traced these out on plastic overlays that we're transparent, that you can see through, and then overlaid them together. So, when you overlay these on top of one another with each map representing one factor where you have these darker tones, that's where more of these dark areas would overlap with one another, and it would visually indicate where the most number of factors were overlapping, and that would indicate the highest amount of social cost or whatever way you wanted to find these costs. When you shine a light through it, then it would become really clear. So, you create these transparent layers, you draw on them, you overlay them, you shine a light through it, and then you can see the combination of these factors. So, this created a composite. As I said, the darkest areas have the highest cost, the lightest areas had the lowest costs. So, when this was done, Ian McHarg was able to draw what he considered to be the best route that he recommended for the Staten Island highway. So, this method was directly borrowed by the early developers of GIS software, and we still see it in use today. We map each geographic theme separately, and then combine them to look for spatial relationships. So, each map is a geographic theme. So, you might have elevation, or satellite imagery, or census data, or locations of restaurants, and we can look for relationships between these geographic themes and see how they interact with each other in space, and combine them in various ways in order to be able to model them and look for relationships. So, that's really what it comes down to is that, this may seem at this point in time like, "Oh, yeah. Okay. I guess that sounds kind of interesting," but back then in the 60s, late 60s, early 70s, that was a fundamental shift in thinking and how to analyze things. So, this idea of taking these layers and combining them together, and seeing where things overlap in space, and combining them to look at things like cost is really fundamental to the whole concept of overlay analysis in GIS.