Cursor Datasets

SIS is able to read and display spatial data from Oracle, SQL Server and PostGIS. There are also a number of other sources that SIS is also able to read. For example ESRI shp files, MapInfo Tab files etc.

Cadcorp have developed the concept of a cursor dataset. The philosophy of a cursor dataset is that only a small part of a large dataset is loaded into the system at a time, based on the screen extents.

As you navigate around the map, the data loaded in the system changes to match the view. This form of dynamic indexing maximises the speed with which SIS is able to read and display data from the source, and allows access to large datasets.

The following datasets are considered to be cursor datasets:

• Oracle Spatial
  
  
• PostGIS
  
  
• Microsoft SQL Server Spatial Dataset
  
  
• SHP files (when a prj  file are present)
  
  
• MapInfo (TAB)

Cursor datasets have the following characteristics:

• By default they extract data from the source that is required for the current map window view extents. This makes SIS memory efficient.
  
  
• Panning in the map window triggers a redraw which forces SIS to extract data from the source. This ensures that the data is current.
  
  
• Some commands and menu options are specific for cursor datasets. For example spatial queries for overlays based on spatial data from Oracle Spatial or Locator can be executed server side rather than client side. These commands will be discussed in detail later.

SIS can use some types of dataset as a cursor dataset.

Hot snapping

'Hot' snapping enables you to select features and snap to vertices easily on an RDBMS cursor dataset overlay by clicking on them with the left mouse button (whilst the cursor is in its default select mode i.e. arrow as the mouse cursor). There is a performance overhead for this option so the feature can be switched off in the Options [Application] dialog:

Note: If the 'Hot' snapping radio button is set to off map tips will not display.

With 'Hot' snapping switched off use the following ‘hot’ keys to select features while the mouse cursor is hovering over the feature:

A	=	to select Polygon items
L	=	to select LineString items
P	=	to select Point items
C	=	to select the Centre of an item
V	=	to select a Vertex
M	=	to select the Middle of a LineString
X	=	to select the cross of two LineStrings

See Snapcodes for further details of ‘hot’ keys and snapcodes.

Transactions: Begin, Commit and Rollback

When you work on a cursor dataset, you can organise the changes into larger units, called transactions.

First, you begin a transaction, by choosing the Begin Transaction command in the Maps Control Bar:



Now, all the edits and additions you make to the cursor dataset are tracked. At any stage, you can choose to do one of two things to this set of changes:

• commit them all to the dataset, by choosing Commit Transaction from the local menu
  
  
• discard them all, by choosing Rollback Transaction from the local menu

Commit Transaction applies all the changes in the transaction to the dataset, making them permanent. Rollback Transaction discards all the changes you made since you selected Begin Transaction, returning the dataset to its original state.

The Transaction commands are available as follows:

• from the dataset’s local menu in the Maps Control Bar as shown above, or
  
  
• in the Cursor Dataset Details dialog, this can be displayed by clicking the Details... button on the Dataset tab of the Overlays dialog.

See RDBMS Cursor Dataset Transactions and Lockouts for details of transactions.

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