Massachusetts Institute of Technology
Department of Urban Studies and Planning

11.220 Quantitative Reasoning and Statistical Methods for Planning

ArcGIS

HOME   |   TEST-OUT   |   BRUSH-UP   |   MATERIALS

ArcGIS > Four Issues

1. Data Screening.

Pay careful attention to mapped data. For example, the block groups with no housing units needed to be excluded from our display of population in order to show the true distribution.

2. Multiple Polygons for an Entity

A shape file of town boundaries for Massachusetts indicates there are 631 polygons, but we know there are only 351 cities/towns in the state. Why the difference? In GIS any polygon becomes a record in the database even if a town is made up of islands. For example, the city of Boston has many islands which are separate polygons. When you join other data for the 351 city/towns in a GIS map, many cities/towns will be duplicated. That's usually O.K . for some thematic mapping. However, when you classify the map data into quantiles, the GIS will count each island as a polygon. This problem leads to mis-representation of the data because quantile classification groups the data points in the map into categories of equal number. In this case, you need to use MS-Access to calculate the correct quantile.

3. The pattern on the map for the same data may vary based upon the areal unit of analysis. The difference between Census Tract vs. Census Block Group is following.

4. Classification of the data into a different number of categories can show completely different distributions. The following illustrate these distributions.

	By Blockgroup	By Tract
5 Groups
11 Groups

Created by Myounggu Kang on January 25, 2004. Edited by Rhonda Ryznar on January 19, 2005.