Chapter 1: Introducing GIS Analysis
GIS analysis is a process for looking at geographic patterns in data and the relationships between features. A framing question helps you figure out what information you need. The next step is understanding your data and what you need it to do to help answer your questions. The features, and attributes, and potentially needing to make new data by calculating new values in the data table. You then choose a process to get the information you need process the data and analyze the results. There are a lot of different types of features: discrete, continuous phenomena, or summarized by area. Discrete lines and locations can be pinpointed, continuous phenomena like temperature can be measured anywhere (there’s no place that has no temperature). Continuous data is usually interpolated from a series of discrete points. Summarized data is the count/density of individual features within defined boundaries (number of households in a county). Geographic features can be represented with vectors or rasters. Vector features are a row in a table, and features are defined by x,y locations in space, good for discrete data. Rasters represent features as a matrix of cells in continuous space, best for continuous numeric values. Continuous categories can be represented by either. Raster can be used for discrete features when combining them with other layers. Categories are groups of similar things that help you organize and make sense of your data. Ranks put features into an relative order of higher to lower. Counts and amounts show you total features, counts are the actual number of features on a map and amounts are any measurable quantity associated with a feature. Ratios show the relationship between two quantities and are created by dividing one quantity by another for each feature. Proportions are a ratio that shows you what part of a total each value is. Densities show the distribution of features or values per unit area.
Chapter 2: Mapping where things are
People use maps to see where or what an individual feature is, but by looking at the distribution of features on a map patterns emerge that help you better understand the area you’re mapping. Mapping where things are can show where you need to take action or what areas meet your criteria. You can explore causes for patterns you see. What information do you need from the analysis? Just whether features are present or not, or whether certain features occur in proximity to one another or repeatedly in the same areas. When preparing the map its important to cater the level of detail to your audience, whether technically skilled or general public level of knowledge that will require two different levels of information in order to be effective. In preparing your data each feature needs geographic coordinates, they need category values to identify its type and potentially subtypes. To make a map you need to tell the GIS what features to display. You can map them as a single type with a single symbol, or you can map by different categories and symbolize them distinctly. The number of categories is important to keep condensed in order to be easily understandable, at smaller scales you can get more detailed. There’s details on grouping categories effectively, choosing symbols, mapping recognizable reference features, all of which help make your map understandable. All of this comes together to assist in your analysis of geographic patterns. Features clustering together or appearing in patterns may infer meaningful relationships between those features or a common underlying cause.
Chapter 3: Mapping the most and the least
People map the most and the least to find places that meet their criteria and take action or to see the relationships between places. Mapping features based on quantities adds a sense of feature density that just the location of features doesn’t provide. Like before you can map discrete features, continuous phenomena, or summarized by area. Locations and linear areas usually represented by graduated features and areas are often shaded to represent quantities. Continuous phenomena can be defined areas or a surface of continuous values. Areas displayed as graduated colors and surfaces can be graduated colors, contours, or a 3D perspective view. Data summarized by area usually displayed by shading each area based on its value or using charts to show the amount of each category in each area. You want to keep in mind whether you’re exploring the data or presenting a map to your audience when you’re constructing your map. Quantities can be counts, amounts, ratios, or ranks. Counts and amounts let you see the value of each feature as well as its magnitude with other features. Counts and amounts can be used for discrete features or continuous phenomena. If summarizing by area using ratios will make it clearer, because using counts or amounts can skew the patterns if the areas vary in size. Using ratios evens out differences between large and small areas, or areas with many features and those with few. Averages are good for comparing places that have few features with those that have many. Proportions show you what part of a whole each quantity represents. Densities show you where features are concentrated. Ranks put features in order from high to low and show relative relationships rather than measured values.