Mitchell Ch. 2,3,4

Ch. 4

Why map Density?

• emphasizes patterns rather than individual locations
• mapping areas of different sizes

When the location of each individual feature is mapped it may be difficult to distinguish how many are in a highly populated area.

Deciding What to map

Areas can be shaded based on density value

OR by creating a density surface

What kind of Data do you have?

1. density surface- uses GIS  to map the density of points or lines
2. density value- maps data already in defined areas like counties or forest districts

Do you want to map the density of  features, or feature values?

• features- are things like the location of businesses
• feature values- values like the number of employees at each business

Two ways of mapping Density

Mapping Density for Defined Areas

1. map graphically with a dot map
2. map density value for each area

Dot map

• each dot refers to specified number of features

each dot = five business

• dots are distributed randomly within defined area
• do not represent actual locations of features
• closer together dots mean higher density of features in an area

• This method is good for showing clustered features.

Density value= total# or value of features/area of polygon

• each area shaded based on density value

Downside: can see which areas have higher density but not specific distribution of density within area

Creating a Density Surface

created with GIS raster layer

each cell gets a density value based on # of features within a specified radius of the cell number of businesses per square mile

Data may be locations of features

OR data collected from sample points

Choosing a Method

Map density by area

• for data already summarized by area or to compare data within defined borders

– Trade-off : easy BUT doesn’t show exact centers of density

Create a density Surface

• for looking at concentration of point or line features

–Trade-off: more precise and shows centers of density BUT not as easy and requires more data processing

Mapping Density for Defined Areas

Calculate density values for areas

• Density value= total#  features/area of polygon
• pay attention to units, make sure that the density units are the same as the area units.

Note: density value fro each polygon area applies to the entire area and the actual density for any given point may be very different from the overall value.

Creating a Density Surface

GIS defines  a neighborhood around each cell

• based on an input search radius

GIS totals the number of features within neighborhood and divides this number by the area of the neighborhood = value assigned to each cell

This can also be done with  data values instead of features. GIS totals the value for all features within the neighborhood.

Important features that influence appearance of patterns:

• Cell size- how coarse or fine details of patterns appear

• Search radius

larger radius = more generalized patterns

smaller radius = more local variation ( but a really small radius may not show broader patterns)

• Calculation method

–simple method- value based on features within radius of each cell

• cells with no features within radius are not given a value

–weighted method- cell value derived from function that gives more importance to features near the center of each cell. Every cell is assigned a value even if it is very distant from any features. This results in smoother density surface.

• Units- choose units of appropriate magnitude for the size of the features being mapped

In order to create a density surface from data summarized by defined areas use: centroids- center points of the defined areas to create density surface.

Displaying a Density Surface

Graduated Colors

• custom class ranges- setting a range high and low value for each color
• choose a standard classification scheme

Common schemes include:

• Natural breaks- class ranges based on
• Quantile- each class has the same number of cells in it.
• Equal Interval- difference between high and low value is the same for each class
• Standard Deviation- classes defined by the number of standard deviations from the mean

More classes create a smoother looking map

BUT using too many classes may result in blending of colors making classes difficult to distinguish

Density surfaces are usually done in shades of a single color.

For classes defined by standard deviation shades of two different colors are used for values above and below the mean

In general,   Darker = “more” (higher density)  and  Lighter = “less” (lower density)

Contour lines

–connect points of equal density.

–show rate of change.

Contour intervals- specify how far apart in density value the line will appear.              Closer lines = more rapid change

Shaded area and contour lines can be used together on the same map to communicate both areas of highest density and rate of change

Looking at Results

• patterns on a map can vary depending on how a density surface was created
• in the process of making a density surface data is smoothed and generalized so that patterns stand out. Maps created this way are not meant to give detailed information about a specific location.

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