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Note: This model is unverified. It has not yet been tested and polished as thoroughly as our other models.

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Surface Walking 2D

[screen shot]

If you download the NetLogo application, this model is included. You can also Try running it in NetLogo Web


This model is a 2D version of a surface-walking algorithm used in "Surface Walking 3D". Turtles approximate a user-defined surface using a simple algorithm that considers the turtle's current position relative to neighboring surface patches.


Turtles exist in a world that consists of two different kinds of patches: surface and non-surface. Turtles follow two basic rules in order to walk along the patch-defined surface:

  1. Look for opposite neighboring patches. If the turtle is currently on a surface patch, it will look for a non-surface one, and vice versa.
  2. Of those neighboring patches of opposite type, identify the one that requires the smallest change in turtle heading for the turtle to face.
  3. Turn halfway to that patch that requires the smallest change in heading to face and take a step.

Why does this algorithm work? Consider any perturbation along a flat surface (in the case of 2D, a curve or angle along an otherwise straight line). In order to traverse the perturbation and remain on the surface, a turtle needs to find and remain on the 'edge' of the surface, and maintain its direction while travelling along this edge. Since turtles in NetLogo always report being on only one patch (the one that the center of their body is over), we maintain the turtle's position along the edge by having it search for a patch that is of opposite type to the patch it is positioned over (Rule 1).

Rule 2 enables the turtles to travel along that edge while maintaining their current direction. Consider a surface in 2d: turtles are able to travel along that surface in one of two opposite directions (for example, they can walk along a circle in either a clockwise or counterclockwise direction). If a surface is flat, a turtle needs not change its heading at all to continue to travel in the same direction along that surface. But, if the surface is curved or has an angle, then in order to continue to stay along the edge of the surface the turtle must change its heading. If it changes its heading exactly 180 degrees, it will reverse its direction of travel. But whether the angle is concave or convex, it will be of some measure between 0 and 360 degrees (non-inclusive). Then the heading change required to continue along the surface without reversing the direction of travel will be less than that required to reverse direction.

Finally, Rule 3 reduces the 'weaving' effects produced by turtles moving toward surface and non-surface patches in order to remain close to the edge by having turtles actually point only halfway to the patch of interest.


NUMBER-OF-TURTLES: Allows the user to adjust the number of turtles that will appear along the shape surface when SETUP is pressed. STEP-SIZE: Allows the user to adjust how far each turtle moves forward during each step. SURFACE-SHAPE: Allows the user to select the surface shape to appear when SETUP is pressed. COLORED-SURFACE?: Allows user to toggle whether the surface on which turtles will walk is colored red, or is invisible. DRAW-SURFACE: Allows the user to add mouse-drawn components to an existing surface by pressing the button and then clicking and dragging anywhere in the view. The area surrounding patches identified by the mouse will be set to behave as a surface, and if the COLORED-SURFACE? switch is on, the area will also turn red. TRACE: Asks one of the turtles to draw a trail as it moves. SETUP: Sets up the environment by creating a surface and placing turtles along the surface. GO: Runs the model by asking turtles to walk along the surface.


Try adjusting STEP-SIZE while the model is running. What happens to the motion of the turtles? What happens to their speed?

Do turtles behave differently on different types of surfaces? Try using the SURFACE-SHAPE chooser to test different shapes. Then, try drawing your own by clicking on DRAW-SURFACE and clicking and dragging your mouse in the view.


What might happen if the STEP-SIZE is set to 1 or larger? Currently, turtles turn half of the way to the edge-patch they identify. What happens if they turn the whole way?

Select "Mickey Mouse" from the SURFACE-SHAPE chooser. SETUP and tell the model to GO. What happens to turtles when they pass over the acute angles where Mickey's ears meet his head? Try adjusting the STEP-SIZE slider while the model is running to investigate.

When does the surface-walking algorithm fail? Why? Use the DRAW-SURFACE button to test different shapes and angles. Does changing the resolution of the world's grid of patches affect turtle motion?


Currently, turtles seek the surface edge by seeking patches that are classified in certain ways. Find and implement another way that patches can be identified or that turtles can identify edge patches.

Try using stamp to trace the trajectory of a turtle over different kinds of surfaces. How might one describe surface-walking accuracy?

The surface-walking algorithm used in this model fails for surfaces (or gaps in surfaces) that are only one patch wide. Why? How might this be fixed?


Note the use of towards to compute headings and min-one-of to make a choice between competing patches.


See Surface Walking 3D (NetLogo 3D) and Virus on a Surface 3D (NetLogo 3D).

Wall Following Example is a simpler version of this example. It is entirely grid-based; the turtles move from patch center to center. The code for this is much less complicated and the turtles are always able to follow the wall perfectly correctly.


Thanks to Michelle Wilkerson for her work on this model.


If you mention this model or the NetLogo software in a publication, we ask that you include the citations below.

For the model itself:

Please cite the NetLogo software as:


Copyright 2007 Uri Wilensky.


This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License. To view a copy of this license, visit or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.

Commercial licenses are also available. To inquire about commercial licenses, please contact Uri Wilensky at

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