# Transition Spread Distribution

The **Transition Spread Distribution** Property specifies the frequency distribution of spread distances possible for a particular Transition Group. The values entered for this property are used to simulate slow spreading transitions, such as the spread of invasive plants, over several timesteps. State classes that are contagious for a particular transition group are identified in this property. Each timestep the model samples from the Spread Distribution for each neighbor of each cell that is contagious and tracks the accumulated spread distance in that direction. Once the accumulated spread distance exceeds the center to center distance between neighbors, the model will apply a transition from the contagious transition group to the target neighbor. A transition will only be applied if a transition pathway has been defined for the current state class of the target neighbor. The fields for this property are:

### Iteration (optional)

This value represents the Iteration in which the spread distribution is specified. The spread distribution continues through subsequent iterations until another Iteration value is encountered by the model for the same transition group in the same stratum, at which point the new values for spread distribution will be applied. If no iteration is specified, the spread distribution will be applied across all iterations.

### Timestep (optional)

This value represents the Timestep in which spread distribution is specified. The spread distribution continues through time until another Timestep value is encountered by the model for the same transition group in the same stratum, at which point the new values for spread distribution will be applied. If no timestep is specified, the spread distribution will be applied across all timesteps.

### Primary Stratum (optional)

The Stratum for which the spread distribution applies. If no Primary Stratum is specified the spread distribution is applied across all Primary Strata.

### Transition Group

Represents the Transition Group for which spread distributions are specified.

### State Class

State Classes which are contagious for a Transition Group should be identified in this field. For example, for modeling invasive plant spread, each state class that already has the invasive plant present would be identified in this field. Note that for each state class users must create a set of records that define the frequency distribution of spread distances for a particular transition group. Different state classes can have different Spread Distributions.

### Maximum Distance (m)

The upper bound of the transition spread distance bin (in meters) for which the frequency is specified. Note that records which share the same Vegetation Type, Iteration, Timestep, Transition Group and State Class are sorted according to Maximum Distance. The record with the smallest Maximum Distance has a lower bound of zero for it's corresponding bin. All other records have the Maximum Area of the previous record as the lower bound for their corresponding bin. Note that when sampling from the distribution spread distance is interpolated linearly within the selected bin.

### Relative Amount

The relative frequency for the spread distance bin corresponding to this record within the group of records that share the same Vegetation Type, Iteration, Timestep, Transition Group and State Class. Note that the sum or Relative Amounts need not add to one or one hundred. The model will internally normalize these to a proportion of events at runtime.