Linked Bitmaps

Raster data can often exceed the amount of physical memory in the computer (e.g. Ordnance Survey 1:10000 Published Mapping series black-and-white raster files are approximately 8MB in size when in Windows BMP format.)

Pasting files of this size into SIS Desktop 9 will quickly use up the memory available.

However using Linked Bitmaps can allow computers with relatively small amounts of memory to display large quantities of raster data.

The contents of a Linked Bitmap are only required when the bitmap is being drawn on screen.

If sufficient memory exists, the contents will remain in memory and will not be re-read. However, if SIS Desktop 9 detects it needs more memory (say to draw another Linked Bitmap when the view has been panned) the contents of the an existing Linked Bitmap will be removed from memory to make more space.

The original Linked Bitmap is then only re-read if it has to be drawn again. This swapping of files is all carried out automatically without the need for intervention from the user.

Linked bitmaps should not be deleted from disk as this will cause the image to be removed from your map view. Simple bitmaps may be deleted from disk as they are stored on the overlay itself.

Note: If you are considering using raster data extensively, there is no substitute for large amounts of computer memory.

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