Gravity model
An accessibility method where every destination contributes, but nearer ones count more, with the fall-off set by a distance decay parameter.
A gravity model measures accessibility by letting every destination contribute something, weighted by how far away it is. Near destinations count fully, distant ones count less, and the rate at which that weight falls off is the distance decay parameter. The name comes from the analogy with gravitational attraction, which falls with distance in the same shape.
The advantage over a simple catchment count is that nothing flips at a boundary. If you count everything within fifteen minutes, a park at fourteen minutes counts fully and one at sixteen counts not at all, which is a bad description of how people behave. A gravity model treats the sixteen minute park as slightly less useful than the fourteen minute one, which is closer to the truth.
The cost is a parameter you have to justify. Distance decay is not observable, it is chosen, and different values produce different maps from identical data. Sensitivity is the term for this setting in GOAT. Pick it deliberately, say what you picked, and if the result is close to a decision, show what happens when you change it.