1. Lecture 15 Spatial Interaction
15-1 Intro to Spatial Interaction Modeling
Spatial diffusion ( Medical geography )
Huff Model (Market Delineation)
Gravity Model ( Migration, transportation, etc.)
Interaction (such as traffic flows between cities) can be modeled with gravity formula.
2018/11/19
Jun Liang, UNC

2. 15-2 The Basic Gravity Model
Two most obvious factors which affect the amount of flow or interaction between any two points:
Population / GDP (Weight)
Distance / Driving Time (Distance)
And gravity model can take them as input to evaluate the interaction between two points.
2018/11/19
Jun Liang, UNC

3. 15-2 The Basic Gravity Model (Cont.)
1,2,4 cities’ population are City 3’s population is
Which link has more traffic than the rest?
2018/11/19
Jun Liang, UNC

4. 15-2 The Basic Gravity Model (Cont.)
Generalizations:
Expected interactions between an origin city i and a destination city j (Iij) will vary directly with the population of the destination city j (Pj) and inversely with the distance between i and j (dij).
2018/11/19
Jun Liang, UNC

5. 15-3 Evaluating Interactions
2018/11/19
Jun Liang, UNC

6. 15-3 Evaluating Interactions (Cont.)
Further step to make the formula more general – add the impact from origin:
From the improved formula, we will have more traffic, if the population of origin were to be increased to
2018/11/19
Jun Liang, UNC

7. 15-3 Evaluating Interactions (Cont.)
2018/11/19
Jun Liang, UNC

8. 15-4 K – The Constant for Gravity Model
2018/11/19
Jun Liang, UNC