Berlin's private pools

The results have now been published at Tagesspiegel.

We found a total of around 23,000 pools. Contrary to expectations, however, pools in Berlin are less a sign of wealth than of space. There is also an uneven distribution of pools across the city.

In the graphic, each blue dot represents a pool. The dots cluster together and form a pattern on the map of Berlin. However, the dots are not necessarily located where there is particularly high wealth. In Steglitz-Zehlendorf, for example, Berlin's wealthiest district, our model identifies 1,100 pools. This makes Steglitz-Zehlendorf one of the three districts with the fewest pools per 1,000 inhabitants. Only the inner-city districts of Mitte and Friedrichshain-Kreuzberg and Charlottenburg-Wilmersdorf have fewer pools.

Contrary to the assumption that pools are a sign of wealth, our analysis shows that most pools are located in Marzahn-Hellersdorf, the district with the lowest median income.

Space matters

More than money, space plays a role: the pattern shows that pools are mainly found where there are single-family homes with gardens. For example, in Pankow.

One of the areas with the highest pool density is Blankenburg, where there are 94 pools per 1,000 residents.

In addition to the gardens of single-family homes, allotment gardens are a popular spot for pools. The pools located in allotment gardens are marked in yellow on the following map. Almost a quarter of Berlin's pools are located in allotment gardens.

The pools form a ring around Berlin. The problem is that the pools are mainly located in cooler areas. In the heated center, where there is a lot of concrete, there are very few pools.

Tagesspiegel also examined how much water Berlin's swimming pools can hold and what water consumption is generally like on hot and cooler days in Berlin.

Method

The pools appear tiny on the satellite images, often measuring just 15 pixels in the raw data. They come in different colors and shapes. Shadows and trees also often make them difficult to detect.

Initial tests did not yield reliable results for Berlin.

We therefore preprocessed the aerial images to make detection easier – improving contrasts and cutting the material into smaller images. We then used Google's object detection model, adjusted it after the first few attempts, and thus “trained” the model for our application.

To filter out as many misidentifications as possible, we subsequently checked a quarter of the data manually and filtered out implausibly large pool areas of over 100 square meters.

The model was rarely wrong when it marked a pool: around 93 percent of the recognized pools were actually pools.

However, the model overlooked some pools: around 82 percent of pools were correctly detected. Thirdly, we only consider pools located on non-public areas of the city, i.e., in residential areas and the like. Pools on roofs were also often not detected—the model incorrectly identified many roofs as pools. We therefore filtered out all detected pools located on built-up areas.

We manually checked more than 3,000 pools to improve the analysis model. Finally, we manually checked a quarter of the results.

Misidentifications are still possible. For example, it is difficult for the model to distinguish between blue round trampolines and round pools – they look very similar from the air.

Nevertheless, the results allow us to classify the number and area of pools in Berlin for the first time.

The images were taken by the Berlin administration in spring 2023. One of the many difficulties is that some pools are covered in winter, most are not filled with water, and depending on their color, they are difficult to see in the shadows. Others may not set up their above-ground pools until summer.

Link to the entire Tagesspiegel article.