Prequel

A Prequel is an Grid Overlay which serves as input for another (geographical) calculation model, overriding Keyed Attribute feature values.

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. These values are 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.

Uses

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.

Configuration

A prequel is configured using the following parameters:

• Overlay ID of the Grid Overlay that uses a prequel
• Overlay ID of the Prequel Overlay. Must be different from the Overlay that uses the prequel
• Prequel name. Which name can be used depend on the Grid Overlay the prequel is configured for.
• Optional Timeframe, as an integer representing the timeframe to select the data from.
• Optional Previous, as a Boolean indicating whether the data from the prequel should be selected from the previous iteration (true) or in this iteration (false).

Prequel calculations

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, a Combo Overlay can be used to precompute water levels for a Water Overlay. However, incorrectly precomputing the water depth rather than the water level will cause significantly different water initialization.

Prequel calculations are automatically performed in order. Any Grid Overlay which relies on a Prequel will be computed after their Prequels are computed.

Any warnings which occur in Prequels are also reported as a warning in the Grid Overlay which relies on that Prequel's results.

Examples

The following how-to guides demonstrate using Prequels to compute complex inputs or further process model outputs.

• How to set a prequel to a Grid Overlay
• 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)

Notes

• Circular references in Prequels is only supported if one of the prequels is configured as 'previous'.
• An overlay can never be a direct prequel to itself, an intermediate overlay such as a Combo Overlay or Iteration Overlay should be used instead.

See also

• Prequels (Average Overlay)
• Prequels (Combo Overlay)
• Prequels (Distance Overlay)
• Prequels (Heat Overlay)
• Prequels (Sight Distance Overlay)
• Prequels (Subsidence Overlay)
• Prequels (Traffic NO2 Overlay)
• Prequels (Traffic Noise Overlay)
• Prequels (Travel Distance Overlay)
• Prequels (Water Overlay)
• Prequels (Watershed Overlay)