Chapter 4: Chapter 4 talks about mapping density. Mapping density is important because it allows us to see the highest and lowest concentrations of what we are looking at in a given area. This chapter outlines 2 main methods for mapping density, the first one being mapping by a defined area. We can use a dot map to represent the density of individual locations summarized by defined areas. The dots are distributed randomly within each area; they don’t represent actual feature locations. The closer together the dots are, the higher the density of features in that area. Dot density maps show density graphically, rather than showing the density value. The second method is mapping by a density surface. A density surface is usually created in the GIS as a raster layer. Each cell in the layer gets a density value, such as a number based on the number of features within a radius of the cell. This approach provides the most detailed information but requires more effort. A dot map simply represents density graphically. The dots in a dot density map represent total numbers or values in each area rather than a calculated density value. When creating a dot density map, you specify how many features each dot represents and how big the dots are. You may need to try several combinations of amount and size to see which one best shows the patterns. The larger the amount represented by each dot, the more spread out they will be. Select a value that ensures the dots are not so close as to form solid areas that obscure the patterns, or so far apart as to make the variations in density hard to see. It’s very important when mapping density, in any form, to make sure your map is still easy to understand what you’re trying to map, and picking the type of density map to create is a large part of that.Â
Chapter 5: This chapter mainly focuses on the statistical analysis methods for understanding geographical relationships and patterns, including correlation and regression analysis, to better understand geographical processes. The chapter emphasizes grasping the concept, capabilities, and limitations of these tools. There are 3 ways of finding what is inside. We can draw areas and features, select the features inside that area, or overlay the areas and features. Drawing can be used when we need to find out whether something is inside or outside an area, selecting is used to get a list or summary of what’s inside the area, and overlaying the areas and features is used to find out which features are inside which areas, and summarizing how many or how much by area. When we get the results we need, GIS can create a report of the features we’ve selected. Typically, our summaries can come in either counts, just a count of the selected area, or the frequency of data within an area. The GIS uses either a vector or a raster method to overlay areas with continuous categories or classes. For the vector method, the GIS splits category or class boundaries where they cross areas and creates a new dataset with the areas that result. For the raster method, the GIS compares each cell on the area layer to the corresponding cell on the layer containing the categories. When deciding which one to use, we need to remember that the vector method provides a more precise measure of areal extent but requires more processing and postprocessing to remove slivers and to calculate the amount of each category in each area. And the raster method is more efficient because it automatically calculates the areal extent for you, but it can be less accurate, depending on the cell size you use. A small cell size will give more accurate results but requires more storage space, processing power, and time. Raster overlay also prevents the problem of slivers. It is often faster because the computation that the GIS must do is simpler.
Chapter 6: This chapter is about finding what is nearby. Using GIS, we can find out what’s occurring within a set distance of a feature. Finding what’s within a set distance identifies the area, the features inside that area, and the area affected by an event or activity. Distance is one way of defining and measuring how close something is, but we don’t have to measure nearness using distance; we can also measure what’s nearby using cost. When mapping travel, we can use either distance or cost. Mapping travel costs gives you a more precise measure of what’s nearby than mapping distance and may require more data preparation and processing. When trying to find distance when mapping, we need to decide whether or not to take into account Earth’s natural curvature. The planar method is used when we don’t need to take into account Earth’s curves, and the geodesic method is used when we do need to take Earth’s curve into account. The planar method is appropriate when your area of interest is relatively small, such as a city, county, or state. The results of your analysis will appear as the correct shape when displayed on a flat map. Use the geodesic method when your area of interest encompasses a large region, continent, or even the entire Earth. Output layers created using this method will be displayed correctly on the curved surface of a globe. Inclusive rings are useful for finding out how the total amount increases as the distance increases. Bands are useful if you want to compare distances to other characteristics. To measure what’s nearby, we can use straight-line distance, distance or cost over a network, or cost over a surface. Once the GIS has selected the features, you can get a list, count, or summary statistic based on an attribute.