In Chapter 4, the author focuses predominantly on mapping density and how to interpret the maps. There are two methods of displaying density. You can show the density for each area graphically using a dot density map, or you can calculate a density value for each area and shade each based on this value. You can also display a density surface using either graduated colors or contours.
The cell size determines how coarse or fine the patterns will appear. The smaller the cell size, the smoother the surface.
Calculating cell size: Convert density units to cell units, then divide by the number of cells, and then take the square root to get the cell size (one side)
Natural breaks: Class ranges are based on groupings of data values.
Quantile: Each class has the same number of cells in it.
Equal interval: The difference between the high and low values is the same for each class
Standard deviation: The classes are defined by a number of standard deviations from the mean of all values in the layer.
Discrete features: Unique, identifiable features. You can list or count them or summarize a numeric attribute associated with them. They are either locations, such as student addresses, crimes, or eagle nests; linear features, such as streams, pipelines, or roads; or discrete areas, such as parcels.
Continuous features: Represent seamless geographic phenomena and include things like spatially continuous categories or classes, such as vegetation type or elevation range.
Three ways of finding what is inside:
Drawing areas and features -You create a map showing the boundary of the area and the features. You can then see which features are inside and outside the area.
Selecting features inside the area – You specify the area and the layer containing the features, and the GIS selects a subset of the features inside the area.
Overlaying the areas and features: The GIS combines the area and the features to create a new layer with the attributes of both or compares the two layers to calculate summary statistics for each area on the fly.
Using GIS, you can find out what’s occurring within a set distance of a feature. To find what’s nearby, you can either measure a straight-line distance, measure distance or cost over a network, or measure cost over a surface. This will help you decide which method to use.
After identifying which features are near, there are three methods for gathering your information:
List – An example of a list is the parcel-ID and address of each lot within 300 feet of a road repair project.
Count – The count can be a total or a count by category.
Summary statistic – a total amount, such as the number of acres of land within a stream buffer, or an amount by category, such as the number of acres of each land cover type (forest, meadow, and so on) within a stream buffer
Three ways of finding what is nearby:
Straight line distance – specify the source feature and the distance, and the GIS finds the area or the surrounding features within the distance. Use straight-line distance if you’re defining an area of influence or want a quick estimate of travel range.
Distance or cost over a network – specify the source locations and a distance or travel cost along each linear feature. Use cost or distance over a network if you’re measuring travel over a fixed infrastructure to or from a source.
Cost over a surface – specify the location of the source features and a travel cost. Use cost over a surface if you’re measuring overland travel.