Mitchell
Chapter 4 Mapping Density
Map density shows you the concentration of features, rather than as individual points. This is helpful because sometimes with the naked eye it is hard to tell which areas of a map are most dense when it is shown as individual dots. When deciding how to create your density map you must know the features you are mapping, and the information you need from the map. Density maps are helpful when mapping density points or lines, census tracts, countries, forest districts and more. When creating your map you must decide whether you need to map the density of features or the density of feature values. There are two ways to create a density map 1. based on features summarized by a defined area or 2. by creating a density surface. By Defined Area: This can be done graphically, using a dot map, or by calculating a density value for each area. To calculate density value for each area you divide the total number of features, or the total value of features, by the area of a polygon. Use this method if you have data already summarized by area or you want to compare administrative or natural areas with defined boarders. By Density Surface: Usually created as a raster layer. Each cell in the layer gets a density value based on the number of features within a radius of the cell. This produces more detailed information but requires more effort. Use this method if you want to see the concentration of point or line features. If you have data summarized by areas but want to create a density map you can use the centroids of defined areas to create the density surface, based on the values assigned to each area. When creating a density map you can display the density using graduated colors or contours.
Chapter 5 Finding What’s Inside
Finding what’s inside is important to be able to compare different areas. To find what is inside create an area boundary, and list or summarize the features inside, or combine the area boundary and features to create summary data. To determine which method is best for you, you must look to the data you have and the information you need from the analysis for example if you need a list, count, or summary. You can find what’s inside a single area or several areas. You must decide how to handle features that do not fall precisely inside or outside of your created boundary. If you need a list or count of features, you should include features that are partially in the boundary, whereas if you need to know the amount of something inside the area you should only include the portion inside the area. There are three ways of finding what’s inside. 1.Drawing Areas and Features: Good for finding out whether features are inside or outside an area 2.Selecting the features inside the area: Good for getting a list or summary of features inside an area. 3.Overlaying the areas and features: good for finding out which features are inside which areas, and summarizing how many or how much by area. GIS can create a report for you of your results of the selected features. It can also create statistical results. For overlaying you can use the same kind of analysis you would do with geographic selection if you are overlaying a single area. If you are overlaying several areas on a set of features you can summarize the features by area. To compare areas based on a particular statistic you can create a chart from the table.
Chapter 6 Finding What’s Nearby
You can use GIS to discover what’s occurring within a set distance of a feature, and what is within travel range. Travel range is measured by time, distance, or cost. To decide which measure would be best to use for you, consider what information you. need from the analysis and decide how you would like to measure “nearness”. You can define nearness by setting a distance you specify, or if travel is involved based on distance or travel cost. If you are measuring a small distance it is suitable to use the planar method which assumes the distance you are measuring is a flat plain. If you are measuring a large distance it is suitable to use the geodesic method, which will account for the curvature of the earth. To help you determine the best method for your analysis you should also consider what end result you need, whether that be a list, a summary, or a count. You must also decide how many distance or cost ranges you need. If you are going to do more than one range you can create inclusive rings or distinct bands. If you are trying to find how the total amount increases as the distance increases you should use inclusive bands. If you are looking to compare distance to other characteristics you should use distinct bands. There are three methods of finding what’s nearby 1. Straight-line distance: to define an area of influence around a feature and create a boundary or select features within the distance 2. Distance or cost over a network: to measure travel over a fixed infrastructure 3. Cost over a surface: to measure overland travel and calculate how much area is within travel range.