Chapter 4:
Chapter 4 focuses on mapping density through various methods, including defining why you should map density, deciding what kind of density to map, and how to actually map density. In terms of GIS mapping, density refers to the concentration of features or values within a given unit of area/distance. A density map lets you measure designated features by concentration using a uniform aerial unit like hectares or square miles, so the concentration distribution can clearly be observed. A hectare is a metric unit of area equal to 10,000 square meters.
When discussing why it’s important to map density, Michell gives a multitude of reasons. One being that density maps can help show you where the highest concentration of features/data is located. This makes density maps useful for observing patterns as opposed to observing individual features/locations. These maps are also useful for mapping areas that have a wide variety of sizes. For example, when mapping the number of trees within separate forests, the larger forests may appear to have more trees due to the larger area they take up. However, when mapping for the density of the trees within the forests, you may see more concentrated areas of trees within the smaller forests that are represented through density mapping.
As stated in previous chapters, it’s important to consider what information you want to get from the map before creating it. This will help you decide what methodology to use when creating the map. When mapping density, consider whether you want to map direct features or feature values, as the resulting maps can look very different from each other. There are two ways you can map density: mapping the density by area, or creating a density surface. You should map by defined area if you already have data that is summarized by area, or for comparing certain areas with defined borders. On the contrary, you should create a density surface if you want to see the specific analytical concentration of point/line features. Question: What do you do when a data point falls exactly on an existing defined boundary line?
Chapter 5:
Chapter 5 discusses map/data analysis through the lens of mapping what’s inside. But what does the book mean when it says “finding what’s inside”? It is trying to say that we can identify which geographic features fall within the boundaries of other features using spatial relationships for the purpose of analysis. When in practice using real-life data, this means determining whether points, lines, and polygons are contained within a specific area. This map analysis involves monitoring what’s occurring inside a designated area, or even comparing different areas based on what they contain. In simple words, we are summarizing what’s inside an area using GIS.
According to the book, this can be achieved in a multitude of ways. For one, you can draw an area boundary on top of the existing features. Or you can use an existing area boundary to select and analyze the features inside it. You can even combine the area boundary and its features to create a summary of the area. Just like in previous chapters, the method you choose to use for creating your map depends on the data that is available to you and the information you are looking to gain from creating the map. When taking this into account, the type of data you have matters. For example, finding what’s inside a single area vs what’s inside multiple areas can result in different analytical findings, as you can compare multiple areas to each other for deeper data analysis.
There are three ways of “finding what’s inside” that the book highlights, as I listed previously. In order to draw an area boundary on top of the existing features, all you really need is datasets that show the boundary of the area and the features it contains. To select for and summarize features within an area, you also need datasets that show the boundary and the features it contains, but you will also need the attributes of the features you wish to summarize available. You will need this same set of things in order to perform an overlay of areas and features. Question: Why does it matter whether you’re working with points, lines, or polygons when figuring out what’s inside an area?
Chapter 6:
Chapter 6 discusses how GIS mapping can be used as a tool to analyze what is nearby to an existing feature, allowing you to find out what may be occurring within a specific distance from a feature as well as monitor events within that particular feature’s range. This can be useful for various purposes. For example, finding out what’s within the traveling range of a feature can help the observer define the area that can be served by a facility. An ambulance station can do this to find out how far away possible incoming neighborhood calls are to the specific station. Being aware of what is in the traveling range of a feature can also help with designating areas for a specific use relating to the feature being observed. For example, mapping the traveling range around a lake could allow scientists to identify surrounding wetland areas suitable for conducting environmental protection.
According to the book, you can conduct what was previously described using various methods. One being straight-line distance, in which you specify the feature being analyzed by GIS to measure the direct distance outward from that feature, thus creating an area of surrounding features within that distance. This approach is recommended when you need to create a boundary/select for features around a specific source. To perform this method, you need a layer for the source feature and a separate layer for the surrounding features. Another method you can use is measuring distance/cost over a network. This means to determine the distance or travel cost from a specific source location along a linear feature. This approach is recommended for finding what’s possible within a travel distance/cost of an area located on a fixed network like a road. To perform this method, you need the source feature location, a layer containing surrounding features, and a network layer. Please consider that each part of the network needs an attribute providing its length or cost value. These can either be created manually or chosen from a provided network. Question: Could barriers like rivers or highways affect the analysis of what is nearby to an existing feature?