In Geographic Information Systems (GIS), a link refers to an operation that connects information stored in different data tables, allowing them to be used together. Specifically, a link is an operation that relates two tables using a common field, without altering either table. This is a fundamental concept for managing and analyzing spatial data which often involves attributes stored in separate tables.
Understanding Linking in GIS
GIS data typically consists of geographic features (like points, lines, or polygons) linked to attribute information describing those features. This attribute information is often stored in tables. Sometimes, related information is stored in other tables that are not directly attached to the spatial data layer. Linking provides a way to connect these separate tables based on a shared piece of information, such as an ID number or name.
Think of it like having a map of parcels (features) and a separate spreadsheet listing property owners and their contact information. A "link" allows you to look at a parcel on the map and immediately access the owner's details from the spreadsheet, as long as both the map data table and the spreadsheet have a common field (e.g., "Parcel_ID").
How Linking Works
The core principle of linking is matching rows between two tables based on identical values in a specific field. This field is called the key field.
- Source Table: The table you are starting from (often the attribute table of your spatial data layer).
- Destination Table: The table containing the additional information you want to access.
- Key Field: A field present in both tables with matching values that establish the relationship.
When you link tables, the GIS software uses the key field to find corresponding records in the destination table for each record in the source table.
Link vs. Join vs. Relate in GIS
While the term "link" might be used generally, in GIS software, this operation is most commonly implemented as either a Join or a Relate. Both are types of linking operations, but they handle the relationship between tables differently. The definition provided ("without altering either table") aligns most closely with a Relate.
GIS Operations for Linking
- Relate: Establishes a dynamic connection between two tables. When you select a record in one table, you can view related records in the other table. Crucially, a Relate does not append the fields of one table to the other. The tables remain separate structures, and the link is temporary (for the current project or session). This operation is ideal for one-to-many or many-to-many relationships (e.g., one parcel can have multiple owners listed in a separate table).
- Join: Appends the attributes from one table onto the attribute table of another spatial layer. This does alter the structure of the source table by adding new fields. Joins are typically used for one-to-one or many-to-one relationships (e.g., one parcel has only one owner in the owner table you are joining). While a Join permanently alters the primary table (structurally adding fields), the underlying concept of relating tables based on a common field fits the general idea of "linking" data. However, the reference's emphasis on "without altering either table" points specifically to the behavior of a Relate.
Table: Comparing Relate and Join (Types of Linking)
| Feature | Relate | Join |
|---|---|---|
| Definition Match | Closest match (Doesn't alter tables) | Alters primary table structure |
| Result | Related records viewable | Attributes appended to primary table |
| Table Structure | Tables remain separate | Primary table is modified |
| Relationship | One-to-one, One-to-many, Many-to-many | One-to-one, Many-to-one |
| Access | Bi-directional access to related data | Data from joined table is directly accessible in the primary table |
Practical Examples of Linking
Linking (Relating or Joining) is essential for many GIS tasks:
- Connecting Parcel Data to Ownership Records: Link a parcel layer's attribute table to a table of property owners using a Parcel ID.
- Linking Stores to Sales Data: Relate a store location layer to a table of daily sales figures using a Store ID.
- Connecting Environmental Monitoring Sites to Readings: Link a point layer of monitoring stations to a table of water quality readings using a Station ID and date.
- Relating Census Blocks to Demographic Information: Join a census block group layer to a separate table containing detailed demographic data.
By using links, GIS professionals can integrate disparate datasets to gain richer insights and perform complex spatial analyses without the need to merge all data into a single, potentially cumbersome, table.
