What are tables and attribute information?
Tabular information is the basis of geographic features, allowing you to visualize, query, and analyze your data. In the simplest terms, tables are made up of rows and columns, and all rows have the same columns. In ArcGIS, rows are known as records and columns are fields. Each field can store a specific type of data, such as a number, date, or piece of text.
Feature classes are really just tables with special fields that contain information about the geometry of the features. These include the Shape field for point, line, and polygon feature classes and the BLOB field for annotation feature classes. Some fields, such as the unique identifier number (ObjectID) and Shape, are automatically added, populated, and maintained by ArcGIS.
ArcGIS allows you to associate records in one table with records in another table through a common field, known as a key. You can make these associations in several ways, including by joining or relating tables temporarily in your map or by creating relationship classes in your geodatabase that maintain more permanent associations. For example, you could associate a table of parcel ownership information with the parcels layer, since they share a parcel ID field.
Sources of tabular information
There are lots of sources of tabular data, and ArcGIS can take advantage of many formats. Tabular information could be stored as tables in folders or databases, text files, queries on databases, and so on. In addition, if you have spatial data, you probably already have tabular attributes that describe those geographic features.
File-based tables are stored in folders on disk. Some examples of file-based sources of tabular information include the following:
- dBASE tables, the format used with shapefiles
- INFO, the format used with coverages
- Text files, such as those created in a text editor and delimited by commas or tabs
- Many other sorts of tables, including those generated in other programs, such as Microsoft Excel, either accessed directly in ArcGIS or through the OLE DB functionality
Geodatabase tables are stored in a database, such as a file, personal, or ArcSDE geodatabase. Tables in geodatabases can contain some types of information that file-based tables don't support. For example, the geodatabase allows you to have field types of BLOB and raster. In addition, the geodatabase provides capabilities to extend the functionality of tables, such as maintaining data integrity and managing database transactions and versions.
You can perform queries on these tables to create new tables. The Make Query Table tool, for example, allows you to apply a SQL expression to one or more tables. The query can be used to join the tables or return a subset from the original data.
To learn more about what you can do with these different kinds of tables, see About tabular data sources.
Tasks you can perform with tables and attribute information
There are many mapping, analysis, and data management tasks you can perform using tabular data.
Tables allow you to map and visualize your data. For example, you can classify or categorize attributes to symbolize a layer. You can use population values to symbolize major cities with a larger symbol than would be used for smaller towns and villages. You can also specify that a different color be used to represent each type of land use in a parcel layer. In addition, you can use the attribute values to generate text to label each parcel feature. In the graphic below, the parcels are symbolized by the type of land use, then labeled with their parcel ID values.
Attribute data helps you perform spatial queries and analyses. For example, you can examine the distribution of features with certain attributes by using ArcMap to select the features that contain attributes you want to examine. In the graphic below, the features with a LAND_USE of UNK (unknown) are selected using an attribute query.
When information in your geodatabase changes, you can update your attributes. For example, you'll need to update your database when land uses or property ownership changes or the unknown values are classified. If you have a feature class representing some pipes with a field for the diameter, you can easily change the attributes when the crew removes an 8-inch pipe and replaces it with a 6-inch pipe. You can edit tabular values either within the Table window or the Attributes window, which shows attributes of only individual, selected features.
The geodatabase includes functionality that allows you to enhance, maintain, and enforce the integrity of your tabular data. For example, by establishing attribute domains, you can set up rules that specify the valid values for the records in your table. So when updating the pipe diameter attributes, you can use attribute domains to ensure that the diameter is appropriate for that section of pipe. Range domains, for example, ensure that the values you enter are within a valid range. The graphic below shows the use of coded value domains when editing, which allows you to choose a value from a predefined list and avoid making typographic errors.
ArcGIS also allows you to convert data in a table into spatial data. For example, a commonly converted data source is a list of coordinates obtained from using a GPS unit in the field. You can easily add such x,y data to ArcMap to display it.
The graphics below show a text file containing the x,y locations of hydrants (top graphic) and the points displayed in ArcMap on the map and in a Table window (bottom graphic).
If you have a list of addresses, you can use geocoding to match them to known street locations to create point features. In addition, through linear referencing, you can indicate events along line features with just an identifier and a location.
Tables are also at the root of data models, which are templates that you can use to set up your geodatabase to better model real-world phenomena.