Mapping Quantities: Choropleth Maps Gary Christopherson 4/4/2019 Mapping Quantities: Choropleth Maps Gary Christopherson

Review – Preview Review Preview Foundational issues Symbolization Cartographic processes, geodesy, data models, etc. Symbolization Symbol types Visual Variables Color Data Classification Mapping Quantities Choropleth Maps Dot Density Maps Graduated Symbols Etc. Design Issues Layout Text Web Mapping 4/4/2019

Mapping Quantities Many, if not most maps are about quantities of things tied to points lines or polygons A variety of ways to illustrate quantities This is the start of a series of lectures looking at different ways to map quantities Most of these start with data classification 4/4/2019

Flow Map 4/4/2019

Flow Map 4/4/2019

Graduated/Proportional Symbol Map 4/4/2019

Graduated/Proportional Symbol Map 4/4/2019

Cartograms 4/4/2019

Continuous Surface Map 4/4/2019

Isarithmic Map 4/4/2019

Isarithmic Map 4/4/2019

Dot Density Maps 4/4/2019

Choropleth Map 4/4/2019

Choropleth Map 4/4/2019

Choropleth Map 4/4/2019

Choropleth Map 4/4/2019

Choropleth Map http://www.nytimes.com/interactive/2009/03/03/ us/20090303_LEONHARDT.html?scp=1&sq=recessi on%20map&st=cse 4/4/2019

Definition 1: Choropleth From Greek: Choros = place Plethein = to fill A method of cartographic representation which employs distinctive color or shading applied to areas … usually statistical or administrative areas. (International Cartographic Association) 4/4/2019

Choropleth Mapping What you do Determine symbolization Mapping area features based on quantity – provides another layer of information beyond location What you need: Geometry – polygons Quantifiable attribute/s (Almost always )normalized data (for example, data expressed as a rate or density) Classified data What the GIS does: Stores and displays geometry and attributes What you do Determine symbolization Normalize data Classify Data Graduated color Identify patterns in the data 4/4/2019

When to Use Choropleth Maps Theme in well defined enumeration units Data must work as ratios or proportions Not necessary to portray actual values in each enumeration unit Simplified data assumptions must be acceptable to map maker and map reader 4/4/2019

3D Histogram or Stepped Statistical Surface Choropleth Maps 3D Histogram or Stepped Statistical Surface Choropleth 4/4/2019

Definition 2: Enumeration Units The spatial extent, or area in which data were collected (enumerated). For example, US census data is collected in a variety of spatial units, including blocks, block groups, and tracts. 4/4/2019

Geometry = Enumeration Units 4/4/2019

Geometry = Enumeration Units 4/4/2019

Definition 3: Data Normalization The process of dividing one numeric attribute value by another to minimize differences in values based on the size of areas or the number of features in each area. For example, normalizing (dividing) total population by total area yields population per unit area, or density 4/4/2019

Data Normalization Helpful to view data in tables or graphs – this helps the cartographer understand his data It is often necessary to calculate rates or density measures 4/4/2019

Definition 4: Data Classification The process of sorting or arranging entities into groups or categories; on a map, the process of representing members of a group by the same symbol, usually defined in a legend. 4/4/2019

Classification Choropleth maps use data that have been classified Number of classes usually between 5 and 10, more likely 5 than 10 Classification methods vary depending on data and on the story you are telling ArcGIS includes a number of different classification schemes 4/4/2019

Same Data, Different Classification Schemes 4/4/2019

Preliminary Considerations in Choropleth Mapping Geographic Phenomena Number and Kind of Enumeration Units Map Scale Data Processing 4/4/2019

Geographic Phenomena What is it that is being mapped? What are the appropriate geographic data for the questions being asked and answered? Are there data that can serve as proxy for the question? How does the phenomena behave? Agriculture in the US States? Counties? Etc e.g. crop acres for agriculture Spatial, aspatial, in relation with other geographic phenomenon 4/4/2019

Number and Kind of Enumeration Units Number of units are related to scale More enumeration units = higher resolution, or greater detail Type and Size are determined by scale and questions to be answered Often determined by data available It would be nice to have these data at a higher resolution, but they don’t exist. 4/4/2019

Number and Kind of Enumeration Units Number of units are related to scale More enumeration units = higher resolution, or greater detail Type and Size are determined by scale and questions to be answered Often determined by data available 4/4/2019

Map Scale Appropriate scale is determined by: Necessity – scale should be sufficient to accommodate symbol recognition Available space – the size of the sheet. Too small and scale problems will occur; too large and you have extra space but no extra data 4/4/2019

Appropriate Data Enumeration units must be areal Attribute data must be quantitative Attribute data should be normalized e.g. rates or density rates e.g. percent Hispanic density e.g. population per square mile Assumes data is spread uniformly throughout enumeration unit 4/4/2019

Data Classification and Legends We covered data classification schemes previously List enumeration units Modify legend values if necessary Best to have multiples of 25, or 50, or 100 Try to change class values as little as possible 4/4/2019