# Dispersal less than cell size, spread distribution from center or edge?

Syncrosim Forums ST-Sim & State-and-Transition Simulation Models Dispersal less than cell size, spread distribution from center or edge?

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• #21197
Tony Ciocco
Participant

We have a transition spread distribution where many of the distance for many of the bins is less than the cell size (e.g., 10m dispersal distance for a 30m resolution grid). How does SyncroSim handle these situations? In other words, how does it calculate dispersal distance from an origin cell? From the center? From the edge?

#21199
Sarah Chisholm
Moderator

Hi there,

Leonardo, who is our ST-Sim expert, is out of office until Tuesday next week.
According to our ST-Sim documentation, the spread distance is accumulated over timesteps with the origin point at the center of a cell. Here’s the corresponding excerpt from the documentation:

“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.”

Here’s a link to the Transition Spread Distribution documentation: https://docs.stsim.net/reference/prop_transition_spread_dist.html

I will confirm with Leonardo when he is back in office next week if I missed any details,
Sarah

#21200
Leonardo Frid
Keymaster

Hi Anthony,

Sarah is correct, spread distance is accumulated over time for each possible source and destination cell combination. ST-Sim uses the age of the source cell as a proxy for how much time has elapsed and hence how many draws to make from the spread distribution for each source/destination vector. For this reason, the initial age of the source cells is important. In your simulations you will see spread happening soonest to neighbors of the oldest source cells. Spread distance is calculated from center to center. When this distance threshold is exceeded, the destination cell will transition.

Regards,
Leonardo

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