Preliminary Tasks
Note: See the section on Spatial Databases for a full description of the use and functions of an RDBMS Cursor Dataset.
For the purposes of this section reference to RDBMS Cursor Datasets refers to data coming from the following where the data has been stored in the databases native spatial data type:
(Note that spatial data can be stored in these databases using the databases non spatial data type, for example View Points data, editable geometry data and OGC SQL92 data.
Before creating an overlay in SIS from spatial data held in a spatial database it is very important that the following preliminary tasks are carried out:
Ensure that the appropriate backdrop has been opened for your geographical areas. UK users should be using the British National Grid backdrop.
Ensure that the Map Window view extents in SIS are set to a scale of approximately 1:2500 (or less) and are focused on an area which is coincidental with the spatial data that will be read from the RDBMS cursor dataset. This can be achieved by displaying boundary data or aerial photography for your area and then using the zoom command.
If the view extents are not coincidental with the data no data will display in the RDBMS cursor dataset overlay. Users will have to pan or zoom to an appropriate location to see data. If the view extents are set too large (above 1:5000) large volumes of data may have to be read from the database to SIS. This may have an adverse affect on the spatial database server, client and network resources.
'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 [File] 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.
Expresssion result cache is a preference that can be switched on for RDBMS Cursor-based Datasets created in dynamic mode. In situations where the result of an expression is static, SIS will remember the value rather than calculate it every time the user pans the map window or takes any action that causes SIS to redraw the overlay. If Expression result cache is switched off SIS will recalculate the values every time the map has to redraw.
Expression result cache can be switched on and off from the Options dialog:
Choose the Miscellaneous tab from the left hand side of the dialog.
Check the Cursor-based Datasets tickbox under the Expression result cache folder.
If you require symbols to be scalar i.e. adjusting in size according to view scale, you should check the Scale:Override tickbox on the Styles tab of the overlays dialog box for the overlay in question.
Symbols will appear on the screen at the size that they were created as it is the dataset scale that matches the view scale. If the dataset scale is doubled then the symbols will double in size for a given view scale.
A symbol can be set to display at a fixed size, i.e. the size it was drawn or a multple of that size. See Symbol - Fixing the Symbol size for a description of how to achieve this.
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