Unit 2 - Introducting GIS
Introducing GIS - narrative to follow PowerPoint presentation
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People often ask me what I do… and I tell them I work with Geographic Information Systems or GIS. In order to give them a quick, one line answer -- I tell them GIS is a digital map system linked to a database. People still give me funny looks, but they know what a map is and they also know what a database is from personal experience. We use them all of the time, such as indexing phone numbers, email address, etc. But here is a more formal explanation of a GIS.
Slide 1 - Cover sheet slide
Slide 2 - Whatever map you want. This refers to the fact that a GIS can produce as many maps as your data will allow. So if we have information about cities, rivers, lakes, etc., then we can produce these maps showing this information. We can also simply turn off these "layers".
Slide 3 - A GIS map contains layers. A geographic feature is anything we can map, such as a city or river. A "layer" is a collection of these features. This is how a GIS organizes information. But there's more than the eye sees.
Slide 4 - Layers may contain features or surfaces. As mentioned before, each geographic object is called a "feature". Features have a specific shape, size, and geographic location, as seen next.
Slide 5 - Features have shape and size. So you could represent a city with a point, a road or river with a line, and a lake or wetland with an area -- known as a "polygon". Note that if you zoomed in farther, you could represent a city with a polygon to show its boundary, and even a road (also known as right of way) as a polygon. When you zoom in this far, often this is called "cadastral mapping" (as we will discuss later in this class) suggestive of using CAD based drawing systems.
Slide 6 - Layers may contain features or surfaces. Take a second to think about how each line or polygon could be drawn by the GIS software. The most efficient way to depict a "feature" would be to use a series of dots (or nodes) connected by lines. Using this method in a GIS (called vector-based drawing), the computer can keep track of direction and adjacency, for example the flow of a river or which parcels of land are next to a river. We call this "topology".
But GIS software can also use another method, a much simpler way to represent certain aspects of the world, such as the ocean depth. See the PPT slide where you can decipher the ocean depth by the darker color. This is known as a "surface" (rather than a feature) in GIS talk. In this case, the most efficient way to depict a surface is with a series of small dots (or pixels) adjacent to one another (called raster-based drawing). The most basic raster image is a photograph.
Slide 7 - Surfaces have numeric values. "A closer look at this raster of ocean depth shows that it is composed of square cells. Each cell holds a number value." This means the only information known to this map is depth indicated by the darker color. The computer knows the depth and color as a specific number.
Slide 8 - Features have locations. Of course the real core of a GIS is the fact that computer software knows your absolute location, whether that is latitude and longitude, or Universal Transverse Mercator (UTM), a simple x & y-axis metric system to establish your coordinate position.
Slide 9 - Features can be displayed at different sizes. It is easy to take for granted, but the scale of a GIS map is flexible and variable. This means you can zoom into close range or out to see the big picture. You can also bring together different maps and easily integrate their scale (as well as other factors, such as different map projections and coordinate systems).
Slide 10 - Features are linked to information. Remember what I tell people when they ask what is a GIS? I tell them "it's a map linked to a database." Well here is the database part. It is in the form of simple tables. For example, with a country we may want to know: it's name, energy per capita (abbreviated ENERGY PERCAP) on the column heading, net migration, urban percentage, greenhouse emissions, etc. Knowing this "attribute" information, we can ask many questions.
Slide 11 - "The link between features and their attributes makes it possible to ask question...." What are the national capitals? Which cities have populations over five million? All of the answers to these questions come from the database tables. We perform what is known as a "query" (or table search) to find many of these answers. Of course this is very easy for a computer to do.
Slide 12 - Features have spatial relationships. We could also ask the GIS software to perform an analysis of spatial relationships, such as proximity, adjacency, overlap, containment, etc. For example, which cities are within 50 kilometers of a river? Of course this is also very easy for a computer to do. But if we were to use standard paper maps, this would be much more cumbersome to calculate. Next follows an example of solving agricultural problem.
Slide 13 - Analysis from overlap. In this case, we are looking for a place to grow the nutritious grain of amaranth, typically grown in areas of moderate elevation in Mexico. With this analysis, you are trying to find ideal conditions for this crop considering: Elevation, Rainfall, Growing Season, and Soils.
Slide 14 - Result of analysis. By comparing these layers you discover that amaranth could be grown in many other places among the Americas. This is just one example of comparing layers to discover patterns on the landscape.