Tooill – Week 2

Chapter 4- 

• Density maps are used for identifying patterns rather than showing the precise location of something. They are more useful for mapping areas.
• For a density map, densities for a specific area can be represented by different shading or a density surface. You can map the densities of points or lines. You can also map data that is summarized by area. 
• Density of features (number of businesses in an area) or feature values (like how many employees are at each business) can be mapped. 
• Density by defined area can be graphed using dots or density value. Calculating density value- divide the number of features by the area of the polygon.
• Density graphed by a density surface- Created as a raster layer. Every cell in that layer is assigned a density value, which can be found using the number of features within a radius of the cell. This type of graphing gives the most detail, but is the most difficult to make. 
• Which method should you use? 
  • Defined area- when you already have data already summarized by area or when you want to compare administrative or natural areas with defined borders.
  • Density surface- Shows the concentration of points or lines.
• Calculating a density value for defined areas-
  • “Add a new field to the feature data table to hold the density value.” (Mitchell, 2020)
  • “Then, assign the density values by dividing the value you’re mapping by the area of the polygon.” (Mitchell, 2020)
  • “If the density units are different from the area units, you’ll need to use a conversion factor in the calculation to change the area units to the density units.” (Mitchell, 2020)
• Creating a dot density map- 
  • “The GIS divides the value of the polygon by the amount represented by a dot to find out how many dots to draw in each area.” (Mitchell, 2020)
  • GIS places dots randomly within the area. The dots do not actually represent locations. 
• Cell size- determines how coarse patterns on your map will be. 
  • 1. Convert density units to cell units (1 sq. km = 1,000 m * 1,000 m = 1,000,000 sq. m)
  • 2. Divide by the number of cells(1,000,000 sq. meters / 100 cells = 10,000 sq. meters per cell)
  • 3. Take the square root to get the cell size (one side)
• Search Radius- the larger the search radius, the more generalized the patterns in the density surface will be. The smaller it is, the more local variation shown. 
• Contour lines- Contour lines connect points of equal density value on the surface.

Chapter 5-

• Finding what’s inside-
  • Single area- ex. Customers within a proposed sales territory or a service area around a central facility.
  • Several areas- ex. the number of businesses within a group of zip codes.
• Multiple areas- 
  • Contiguous- such as zip codes or water sheds.
  • Disjunct- such as state parks.
  • Nested- such as 50- and 100-year floodplains, or the area within 1, 2, and 3 miles of a store
• In your map, linear features and discrete areas may not all fall inside the set boundary of a map. You can choose to include features only fully in a designated area, only the parts that lie within the boundary, or what partially lies in a designated area. 
• 3 ways of finding what’s inside-
  • One- drawing areas and features. This method is good for seeing whether one or a few features are inside or outside a single area.
  • Two- Selecting the features inside the area. This method is good for getting a list or summary of features inside a single area or areas. It’s also good for finding what is in a certain radius of another feature. 
  • Three- Overlaying the areas and features. This method is good for finding which features are in each of several areas or finding out how much of something is in one or more areas.
• Drawing areas and features:
  • Locations and lines- draw them using a single symbol or symbolize them by category or quantity. After, draw the boundary of the area on top.
  • Discrete areas- (1) Shade the outer area with a light color and draw the boundaries of the area features on top. (2) Fill the outer area with a translucent color or a pattern on top of the discrete area boundaries. (3) Draw the outer area boundary with a thick line, and the discrete area boundaries with a thin line in a lighter shade or different color.
  • Continuous features- Draw the areas symbolized by category or quantity (as a class range), and then draw the boundary of the area or areas on top.
• Selecting features inside an area- 
  • specify the features and the area. 
  • GIS checks the location of each feature to see if it’s inside the area and flags the ones that are.
  • It then highlights the selected features on the map and selects the corresponding rows in the feature set’s data table. 
• The vector method- GIS splits category or class boundaries where they cross areas and creates a new dataset with the areas that result. Each new area has the attributes of both input layers.
• The rastor method- GIS compares each cell on the area layer to the corresponding cell on the layer containing the categories. It counts the number of cells of each category within each area, calculates the areal extent by multiplying the number of cells by the area of a cell, and presents the results in a table.

Chapter 6-

• Finding what’s nearby-
  • An area of influence is measured by a straight line distance.
  • Travel to or from a feature is measured using distance or travel cost.
  • Travel can be measured over a geometric network, such as streets or deer walking to a stream.
  • Can also measure nearness using time it takes to travel there (ie. through heavy traffic would warrant more time).
  • You can measure distance as the Earth being flat (planar method) or you can use the curvature of the Earth (geodesic method).
  • You can get distance information on several features, not just one. 
  • Inclusive rings are useful for finding out how the total amount increases as the distance increases.
  • Distinct bands are useful if you want to compare distance to other characteristics.
• Using straight line distance-
  • (1) Create a buffer to define a boundary and find what’s inside it.
  • (2) Select features to find features within a given distance.
  • (3) Calculate feature-to-feature distance to find and assign distance to locations near a source.
  • (4) Create a distance surface to calculate continuous distance from a source.
• Centers- Source locations in networks. Usually represent centers that people, goods, or services travel to or from.
• Geometric network- composed of edges, junctions (points where edges meet), and turns. To get accurate costs to travel through a junction, make sure that (1) edges are in the right place, (2) edges actually exist, (3) edges connect to other segments accurately, and (4) there are correct attributes for each edge.
• Impedance value- the cost to travel between the center and surrounding locations for a street segment.
• Creating a boundary-
  • List all individual locations.
  • Get a count of locations in the area covered by the selected segments.
  • Have data summarized by area. ie. you want to total the number of households per census block to find out how many households are within a 15-minute drive of a recycling center.
  • Get a list, count, or amount for linear features or areas, ie. the total length of salmon streams within a half-hour drive of the town.
• “You can limit the area for which the GIS calculates cost distance values by specifying a maximum cost. The GIS stops calculating cost distance when all cells within the specified cost have been assigned a value. Any remaining cells are not assigned a value on the output layer. If you don’t specify a maximum cost, the GIS calculates a value for all cells in the study area” (Mitchell, 2020).