Chapter 4
- Mapping the density of features lets you see the patterns of where things are concentrated.
- Density maps are mostly used to look at patterns instead of the usual locations of features.
- Mapping density is especially useful when mapping areas like counties.
- GIS can be used to map the density of points or lines.
- You can map the density of featuresÂ
- There are two ways of mapping density, by defined area and by density surfaceÂ
- Defined area is density mapped graphically by using a dot map or calculating a density value for each areaÂ
- Density surface is created in the GIS as a raster layerÂ
- Density is calculated based on the areal extent of each polygon
- Some GISsoftware allows you to calculate density instantaneously
- When creating the map you specify the value youâre mapping density for and the attribute containing the area
- A dot density map is a method where you map each area based on a total count or amount and specify how much each dot represents
- Dot maps gives readers a quick sense of density in a place
- A dot map simply represents density graphicallyÂ
- GIS can be used to summarize features or feature values for each polygonÂ
- There are many parameters that you specify affect how the gis calculates density surface like cell size, search radius, calculation method, and units.
- Cell size determines how coarse or fine the patterns will appear.Â
Chapter 5Â
- Mapping inside an area allows you to monitor what’s occurring inside it, or to compare several areas based on what’s inside each.
- To find whatâs inside you draw an area boundary on top of the features.
- Finding what’s inside a single area lets you monitor activity or summarize information about the area.
- Finding how much of something is inside each of the areas lets you compare the areas.
- Discrete features are identifiable and unique.
- Listing them or a numeric attribute with them would allow easier comparisons.
- Continuous features represent seamless geographic phenomena.
- Continuous values: are numeric values that can vary continuously across a surface.
- GIS can be used to find out whether an individual feature is inside an area.
- Linear Features and discrete areas might lie partially inside and outside an area.
- Three ways of finding whatâs insideÂ
- Drawing areas: you create a map showing the boundary of the area and the features.
- Selecting the features inside the area: you can 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 attribute of both or compares the two layers to calculate summary statistics for each area.
- Overlaying areas and features: this method lets you find which discrete features are inside which areas and summarize them.
Chapter 6Â
- Finding what’s nearbyÂ
- Lets you see what’s within a set distance or travel range of a feature.
- To find whatâs nearby, you can measure straight line distance, measure distance or cost over a network, or over a surface.
- Distance is one way of defining and measuring how close something is.
- If you are mapping whatâs nearby based on travel, you can use distance or cost.
- You can specify a single range or several ranges
- If specifying more than one range, you can create either inclusive rings or distinct brands
- Inclusive rings are useful for finding out how the total amount increases as the distance increases.Â
- Bands are useful if you want to compare distance to other characteristics.
- Three ways of finding what’s nearby:
- Straight-line distance: you can specify the source feature and the distance, and the GIS finds the area or the surrounding features within the distance.
- Distance or cost over a network: You can specify the source locations and a distance or travel cost along each linear feature.Â
- Cost over a surface: You can specify the location of the source features and a travel cost.Â
- To create a buffer, you specify the source feature and the buffer distance.
- Once youâve created the buffer, you can display it to see whatâs within the distance of the source, or you can use the buffer to select the features that fall within it.