Thessaloniki urban growth simulation

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WHAT IS IT?

This is a model based on the NetLogo model "Structure from Randomness 2" ( Wilensky, U. (2007). NetLogo Urban Suite - Structure from Randomness 2 model. http://ccl.northwestern.edu/netlogo/models/UrbanSuite-StructurefromRandomness2. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.), redesigned in a way to simulate the urban growth of an existing city, and in this case the city of Thessaloniki, Greece. Except from the random initial data that is introduced in the model, there is also included a map of the area of Thessaloniki with color indication for each different type of landscape condition, such as the sea (Thermaikos Gulf) and the rivers (Galikos river and Axios River) the steep slopes of the mountainous areas, the steep slopes of the coastal areas of the region and a small urbanized area (the urban area of the late 19th century Thessaloniki). In each color is given a different initial level of attraction, and along with the small random variations of the initial data, the model is driven in a seeming coordinated but always different direction for each time that is running.

The basic idea here is to show how simple initial conditions of just one variable (in this case the landscape formations) can lead to a structured image that simulates the urban growth of a well known environment, and a comparison of the today's image of the city growth with the results of this model can reveal the fact that the urban environment is driven by few, simple, but yet unpredictable attractors.

In a way to imitate in a more realistic way the process of urban growth, in the model, along with the positive feedback of the levels of attraction, is also introduced a negative feedback,trying to simulate the concept of the carrying capacity of complex systems, such as urban environments.

HOW IT WORKS

Each grid cell is representing the minimum area unit of a two dimensional simulation of a physical landscape, having a variable value, named as level of "attraction". In this case, attraction is the measure of the amount of how attractive is a place for becoming urbanized.

The level of attraction of each cell varies, depending on the color of the map that is loaded in the "world" window, and in most cases is a random number.

Each tick of the model, the cell takes on a new attraction value, which is the average of the potential values from its Von Neumann neighborhood (that is, itself and its 4 neighbors) from the previous step, plus a random number, depending on the color that already has or becomes, and the total amount of the urbanized cells on the world window.

The attraction values are not displayed directly in the graphical view. Instead, a cell is shown as white in the view when it becomes urbanized, which occurs when its attraction value exceeds the threshold specified by the THRESHOLD slider.

HOW TO USE IT

By pressing the SETUP button the browsing window opens to load the image of the map of a given area (in this case the map of Thessaloniki)

Press the GO button to run the model. Press the GO ONCE button to run the model just a single tick. The model may run a bit slowly due to the calculations of the negative feedback.

The THRESHOLD slider controls the threshold which cell's attraction value must reach in order to become urbanized.

The CELLS ABOVE THRESHOLD plot records (at each tick) the number of cells that are currently above the threshold. Note that this is different than plotting the number of urbanized cells, which is a monotonically increasing function.

The URBAN CELLS plot shows the total amount of the cells that are urbanized in the given time "t".

THINGS TO NOTICE

The plot of urban cells except from the initial tick is increasing rather slowly in the beginning an is becoming faster after a while, but after a specific amount of time the rate is slowing down and is finally stopped when the system exceeds its carrying capacity.

Also notice the fractal geometrical form of the urban masses that are developed in the model and compare them with the actual urban growth of the city of Thessaloniki from the starting point of the model (late 19th century) till today's urban landscape.

THINGS TO TRY

Choose a medium threshold level (such as 4 or 5) and let the model run. The default level is set at 4.5. Then try different threshold levels.

When the model is slowed down by reaching the systems carrying capacity, one can stop the model by deactivating the Go button, an rerun the model with the GO2 button, that runs the model with an extended carrying capacity.

Also try to introduce a different map of another real or hypothetical urban environment by using the same color schemes that are used in this model.

EXTENDING THE MODEL

One can try to extend the colored areas that affect the levels of attraction, or even extend the variables that control the model, to have an even more complex and perhaps more realistic behaviour of the system.

If anyone wants to see the model running fast by removing the "negative feedback" feature, one can erase the part of the calculation of the code that is written right after the (-) (minus) symbol.

There is only one version of this model, created about 7 years ago by Dimitris Giouzepas.

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