Overlay Data Schema

A schema defines a sub-set of data, and is used to control the display of the data-oriented parts of the user interface, notably the table window, themes and MapTips™. A schema consists of a list of the data columns for the overlay.

Note: See What is a Schema?

Schemas are used to define the data columns displayed in the Table Window. Each column in a Schema has an expression and a description. You can also use Schema as a filter to display only specific data columns from the original dataset.

With Editable Cursor Based Datasets such as Oracle, PostGIS, SQLServer and FDB, you can access the data columns directly through the Dataset/Properties dialog.

Exercise caution when defining data columns in the Dataset/Properties dialog as the base dataset is being edited directly. This could be adding, renaming or altering columns in an Oracle, PostGIS or SQLServer database. However a Schema can be edited without losing any data as it is simply a view of the data columns.

The Schema column’s expression is often just an item property name (e.g. Item ID for the item’s ID number). However the expression can be more complicated involving mathematical operators, spatial searching functions and database-joins.

The Schema column’s description is related to the expression. But Cadcorp SIS Desktop does not try to interpret it which means you can make it more readable (e.g. "Item Identification Number").

When you create a Schema column, the data does not have to exist. Instead the Schema just tells Cadcorp SIS Desktop what data you want to view. If the data does not exist, then Cadcorp SIS Desktop will show empty values in the Table Window. If the Schema column expression is a simple item property (e.g. "MyName$") and the items are editable then SIS Desktop 9 will let you add new data values as attribute data.

To edit the overlay schema use Overlays or the Schema local commands in the Maps Control Bar:

TIP: See also Creating a Schema.

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