Hi Andy, This is a know limitation for most spatial optimization problems. Let’s take an example where you requested for 50 zones and got only 49 for 9419 features in the input. This tool builds zones by aggregating features until the zone building criteria are met. So in this case, the zone building criteria is 50 zones which means that each zone should have about 188 features (total number of features being 9419). The tool randomly picks an initial feature (called seed) and starts aggregating the spatial neighbors until the number of features in the zone reaches 188. Then the tool randomly picks another feature and continues the process. Now imagine that there are features left as “islands” between two completed zones which have not being assigned to any zone (highlighted features in the below image). Because there is a “spatial constraint” for the zones to be continuous, the tool aggregates these leftover features to the nearest zone. That’s the reason you might see more than 188 features in a zone and because of this spatial constraint and the leftover island features, the tool wasn’t able to find a perfect 50 zones solution. For spatial optimization problems, there’s a trade off between speed and precision. If you want to explore more combinations of solutions and get better results (closer to perfect solution but a perfect solution isn’t guaranteed in any spatial optimization algorithm), try increasing the population size and number of generations to refine your results, in the Advanced Parameters section of the Build Balanced Zones Tool. To know more about the advanced parameters, see Build Balanced Zones. (Try running it with 1000 population size, for 1000 number of generations! It’ll take about 15 minutes, but you’ll get an idea between precision and speed!) An alternative is to increase the number of zones to say, 51. In that instance, the number of features required per zone will reduce and the chances of getting 50 zones will increase. The important thing to remember is that like any other spatial optimization problem, it is not guaranteed to reach a perfect solution. The tool will find an optimized solution. Also remember that even if the number of zones is the only constraint provided in the tool, there is always a spatial constraint to make the zones continuous. Hope that helped!
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