Attribute Data

Attribute data describes the characteristics of spatial features:

• Each row (record) represents a spatial object and each column (field) describes a characteristic of that object
• Two types of attribute data: textual or numerical

Attribute data is managed by relational database. It is a collection of tables (relations) that are defined for each class of objects describing the same subject

• Allows to organize, update, and interrogate data of different subjects
• Each table can be managed separately from other tables, but can be connected when needed (i.e. for queries)

There are two types of attribute tables in ArcGIS:

• Feature attribute tables: has access to the spatial data and uses a FID to link the spatial data with the attribute data
• Non-spatial attribute table: stores additional information and has one field in common with the feature attribute table

The connection between tables is made though a key (i.e. an ID), which is a common field whose values can uniquely identify a record in a table. They cannot be linked through the FID.

Types of relationship between tabular data

• One-to-One relationship
• One-to-Many relationship
• Many-to-One relationship
• Many-to-Many relationship

There are 4 measurement levels associated with attribute data:

• Ratio - has an absolute 0
• Interval - distance between numbers is meaningful
• Ordinal - can be ordered
• Nominal - only named variables

Attribute analysis is done by spatial queries which search for and select a subset of features and table records. All query expressions in ArcGIS use SQL to formulate these search specifications. The purpose of this is to:

• Explore the data
• Focus on a data subset of interest

Basic structure in SQL (i.e. syntax) is: SELECT FROM

• Logical operators: =, >,
• Arithmetic operators: +, -, *, /, ^
• Boolean operators: AND, OR, NOT, XOR
• Mathematical functions (trigonometric functions (e.g. sin, tan), logarithms, etc.)

EXAMPLE: Select* (=all columns) From ContaminatedSites Where “Toxicity” > 8