In most cases, a Grid Overlay calculation will reference the presence of Attribute values which are related to vector data, such as Buildings or Areas. This is then rasterized to provide input for the calculation model. For a number of calculation models, it is possible to configure a Prequel instead. This is a Grid Overlay, the output of which serves as that same kind of input.
Prequels can be used for a number of use-cases. Some common examples include:
- Increasing the detail of input data by providing it as a nearly-analog grid rather than a discrete set of values through polygonal features.
- Feeding the output of a previous calculation (from an earlier run, or running in parallel) into a following calculation.
- Including the effects of a geographical Action via a Combo Overlay in a controlled way in the input for a following calculation.
Any kind of Grid Overlay can serve as a Prequel. This allows for great flexibility, but also means care must be taken to select a prequel which has the correct type of data. For example, using a Combo Overlay to precompute water levels for a Water Overlay makes perfect sense. However, incorrectly precomputing the water depth rather than the water level will cause significantly different water initialization.
A number of how-to's available are examples of how Prequels can be used to either compute more complex input for calculation models, or further process the output of other calculation models.
- How to create an evacuation routes overlay
- How to use a WMS to estimate foliage (Heat Overlay)
- How to set up pre-initialized water levels (Water Overlay)
- Circular references in Prequels are prohibited. An Overlay cannot be its own Prequel, be a Prequel to its own Prequel, etc.