About Map Grids
In this section:
Creating a subtype of MapGrid
By reviewing the ArcMap object model diagram, you can see that all the standard grid coclasses available in ArcGISCustomOverlayGrid, Graticule, IndexGrid, and MeasuredGridare subtypes of the MapGrid abstract class.
Looking at the MapGrid abstract class, you can see a custom map grid must implement IMapGrid. You also need to implement IClone and either IPersist and IPersistStream or IPersistVariant, (depending on your development environment), as grids must be clonable and persistable.
The existing map grids also implement IGraphicsComposite. This interface can be used programmatically to access the graphic elements that compose the displayed map grid. You will also implement IGraphicsComposite.
The MapGrid abstract class represents a grid of reference points or lines over
a Map.
Refer to this abstract class as the starting point for any custom map grid class.
Design issues for a custom grid coclass
Before you actually start coding your custom grid coclass, ask yourself if the functionality you require is similar to that available in any of the standard ArcGIS grid coclasses.
- If you want to use one of the standard grids, but
change the way one or two methods or properties work, simply contain an
instance of the required coclass within your custom coclass. Delegate all
property and method calls to the contained object, and adapt as required to
change the way the method works. For example, if you require an IndexGrid that
has extra labels, you can contain an instance of IndexGrid in your class and
delegate member calls to this class. On a call to Draw, after calling the
contained IndexGrid's Draw, you can add your extra labelling as
required.
You may want to base your custom map grid on an existing map grid class by using containment.
- If you want to add functionality to an existing grid, again, you can use containment to hold an instance of an existing grid coclass within your custom grid coclass. Add one or more additional interfaces that provide access to the new functionality. This design is most similar to the approach taken for this example.
- If your grid requirements are significantly different from standard grids, you may want to create an entirely new kind of map grid. In this case you must write most of the behavior yourself but can save time by only implementing the interfaces required for a basic grid to function—that is, those listed above for the MapGrid abstract class.
IIndexGrid and IGraphicsComposite cannot be implemented in VB
IIndexGrid inherits from IMapGrid; therefore, you cannot implement this interface in VB. The VC++ implementation is described. VB programmers could create a basic map grid by implementing IMapGrid and adding their own custom implementation of index-type functionality including user interface.
The IIndexGrid and IGraphicsComposite interfaces cannot be implemented in VB.
Map grid factories
Having reviewed the ArcMap object model and the map grid classes, you may have noticed that the existing map grids are all associated with a factory object.
For more information about MapGridFactories, see the section 'Plugging your custom grid into ArcMap'.
See Also Clippable Index Grid Example and Creating Cartography.