Chapter 4
Chapter four discusses how to map density. Density shows the highest concentration of a feature. It is useful for showing patterns on a map and in areas that vary in size. Using GIS, you can either map the density of points, lines, or data from a specific area that has already been summarized. You can create a density map based on features by area or density surface. When mapping by defined area, you can map density graphically, using a dot map, or by calculating a density value for each area. A density surface is usually a raster layer in GIS. Each cell gets a density value. This method gives the most detail, but takes the most effort. Mapping density by area should be used if your data is already summarized by area, while a density surface should be used if you have individual locations, points, or lines. On a dot density map, it is common to display the dots for smaller areas, but provide the boundaries for larger areas. This keeps the data easy to read. There are four parameters that affect how the GIS calculates the density surface. The cell size determines how coarse (large cell) or smooth (small cell) the patterns appear. The search radius determines how generalized the patterns are, with a larger radius providing more generalized patterns. The GIS counts only the features within the search radius, which creates overlapping rings. When using the weighted method, it gives more importance to features closer to the center of the cell, which results in a smoother density surface. The units you choose should be appropriate for the features you are mapping. A density surface is displayed with graduated colors or contours. Graduated colors use a different shade for each value. The most common classifications are natural breaks, quantiles, equal intervals, and standard deviation.
Chapter 5
Chapter five covers mapping what’s inside to see if activity occurs in an area or summarize activity in several areas to compare them. If an activity does occur within a specific area, action needs to be taken. Through summarizing multiple areas, you can document where there is greater activity happening. You can do this by drawing an area boundary on top of the features, selecting the features inside the boundary, or combining the area boundary and features. Single areas can be a service area, a buffer that defines a distance around a feature, an administrative or natural boundary, a manually drawn area, or the result of a model. Multiple areas can be contiguous, disjunct, or nested. Discrete features are unique and identifiable, such as locations, linear features, and discrete areas. Continuous features are seamless geographic phenomena, such as spatially continuous categories and continuous values. Within an area, GIS can provide you with a list of features, the number of features, or a summary based on feature attributes. If a feature is partially outside of an area, you can choose whether to include it or not. If you need a list or count, include the partial features. If you need to know the amount of something within an area, include only features that are entirely in the area. There are three ways to find what’s inside: drawing the area and features, selecting the features inside the area, and overlaying the areas and features by creating a new layer with the GIS. Drawing should be used if you only need to see the features inside a single area, selecting is used if you need a list of features fully or partially inside the area, and overlay is good for multiple areas or if you need a list or summary of values.
Chapter 6
Chapter six discusses mapping what’s nearby so you can see what’s within a set distance or range of a feature. Finding what’s within a set distance shows the features inside an area within a set distance. Traveling range can be used to define the area served by a facility. You can measure straight line distance, measure distance or cost over a network, or measure cost over a surface. You may have the option of calculating distance assuming the earth is flat (planar method) or using a curved earth (geodesic method). Planar is used for small distances, and geodesic is used for large regions, such as a continent. Once you identify the features near a source, you get a list, count, or summary based on their attributes. You can specify a single range or multiple ranges by creating inclusive rings or distinct bands. Inclusive rings are useful for studying how the amount increases as the distance increases. Distinct bands are useful if you are comparing distance to other characteristics. Straight line distances are used to see which features are within a given distance of a feature. Creating a buffer allows you to see what’s within the distance of a source. Using selection is similar to a buffer, but the GIS doesn’t create a boundary. You can have the GIS calculate the distance between each location and the closest source, which is useful for seeing which source is closest and comparing the distance with other factors. When making the map, you can have the locations color-coded by distance or source, a spider diagram, or use graduated point symbols. Streets are common for finding what’s nearby. Each street segment is tagged with an impedance value, the most common being distance, time, and money.
