Land of a Thousand Pools

This page shows the outdoor swimming pools in the municipality of Mostar. I found the pools in satellite images with a Python script. I did not do a survey on the ground. The map shows each pool that the program found. The table shows the quantity of pools near each settlement.

Where the pools are

Method

1. I took the boundary of the municipality from OpenStreetMap. The area is 1,143 km².
2. I made the search area smaller. I kept only the parts near a building. This removed 88 percent of the municipality. A pool is almost always near a building.
3. I downloaded 20,619 satellite image tiles. One pixel shows 0.217 m of the ground. A pool of 8 m is thus 37 pixels wide.
4. A colour filter found the blue shapes in each tile. The filter kept a shape only if the size and the form were correct. This step gave 4,295 possible pools.
5. I looked at 600 of these shapes and marked each one. A shape was a pool, or it was not a pool.
6. A neural network learned from these 600 examples. The network then looked at all 4,295 shapes.
7. The program found 1,998 pools.

Accuracy

A count alone is not enough. You must also know the quantity of errors. I thus did two tests.

1. For the first test, I looked at 200 pools from the result. 165 of them were correct pools. The precision is thus 0.83.
2. For the second test, I counted all the pools in four areas by hand. The program found 93.5 percent of them. The recall is thus 0.935.
3. I also counted the pools in two areas outside the search area. I found no pools there. The smaller search area is thus safe.

I corrected the count of 1,998 with these two values. The corrected count is 1,763 pools. The true quantity is between 1,550 and 2,481.

Limits

1. The satellite images are not all from the same date. The dates are between 2019 and 2024.
2. The program cannot find an indoor pool.
3. The program can miss an empty pool. The program can also miss a pool with green water.
4. The program does not count a pool with a surface area of less than 8 m².
5. The name in the table is the name of the nearest settlement. It is not an administrative area. Many pools in the city of Mostar thus show the name of a village near the city.

Pools by settlement