Users of Foursquare City Guide and Foursquare Swarm (in addition to users of the thousands of apps built by our API and data partners) routinely interact with our venues on a map. A map is the most natural backdrop against which to present geospatial data. Yet, the problem of determining where exactly to drop a venue's map pin is a surprisingly difficult one. A common, but limiting, solution is to geocode a venue's street address to a pair of latitude/longitude coordinates. At Foursquare, however, we have accumulated a massive location dataset that has allowed us to sidestep the geocoding approach altogether. In this post, we will describe how we harness the patterns inherent in user behavior to continually improve map pin placement over time.
Unbeholden to Geocoders
Geocoding venue addresses is not only expensive to do at scale, but also severely limited by the ability of geocoders to parse and error-correct a wide variety of international addresses. In addition, commercial geocoders' address coverage and resolution vary widely across, and even within, countries. The point a geocoder returns could be the exact rooftop or doorstep coordinates in the best case, or an interpolation against published address ranges along the length of the street in the average case, or simply the centroid of the postal code or city.
A lot of place databases are essentially like “yellow pages" in that they contain basic information of the kind you might find listed in a business directory (or that you could call a place and get over the phone). Geocoding is often the only way that place records can be rendered on a map at scale. But at Foursquare, for every one of over twelve billion check-ins to date, we know where a user was physically located when they checked in at a specific venue.
From Filtering to Clustering
In the past, our approach to using these check-ins to place map pins was rather straightforward — for every venue, we simply computed the centroid of the latitude/longitude coordinates of its check-ins, and dropped the pin at that centroid. One problem with this approach is that GPS data is noisy, so not all check-ins are equal. Mobile phones report a horizontal accuracy along with latitude/longitude coordinates, where a numerically larger value corresponds to a higher error or lower confidence. When we noticed our centroids were thrown off by check-ins with large horizontal accuracy values, we naturally resorted to filtering them out, but that did not solve the entire problem either.
As we visualized check-ins at one venue after another, we noticed the underlying pattern in human behavior that generates check-ins staring right back at us. Most users check in when they're physically inside a venue. But sometimes, they check in from the parking lot, or from across the street as they walk in or out. Less frequently, they might cheat at the Foursquare Swarm game and check in from somewhere else altogether in order to get coins, unlock stickers, or become mayor. We can usually detect fraudulent check-ins, but regardless, the key observation here is that check-ins are inherently clustered, with some outliers.
Check-ins at a Red Robin in Bellevue, WA showing distinct clusters centered around the actual restaurant location and on the other side of the street.
Our new approach, therefore, was to identify clusters in the check-ins, and use the centroid of just the “dominant," or largest, cluster as the map pin location. Ideally, this cluster should consist of the check-ins that originated from inside the venue itself. There will typically be smaller clusters corresponding to legitimate check-ins from the parking lot or across the street. The rest are noise for our purposes.
Combining DBSCAN and Random Forests
Having reduced our situation to a flavor of clustering problem, our goal became to build a system that was robust to outliers and which made no assumptions about the size or shape of individual clusters or how many clusters there were in the data. We identified DBSCAN, a clustering algorithm that is well known and tested in the industry, as being an effective option for our purposes. DBSCAN (Density-Based Spatial Clustering of Applications with Noise) is a highly versatile algorithm, and, as the name suggests, it is particularly well suited to datasets that contain noise in a way that more popular clustering methods, such as k-means, are not. At its core, once again as its name suggests, DBSCAN is density-based, and especially when operating in two-dimensional space, the algorithm reflects the intuition by which our brains visually pick out clusters of points.
The DBSCAN algorithm takes two parameters, canonically referred to as MinPts and Eps. Without going into too much detail, MinPts is a number of neighboring points and Eps is a radius, and together, they intuitively describe the density criteria for points to form a cluster. The fact that there are only two parameters should technically make it easy to evaluate different models, but the catch is that the distribution of check-in densities in the real world varies greatly as a function of venue shape and size, which in turn are a function of category or chain, and location. So, the check-ins at a suburban Target look very different from the check-ins at a bar in Manhattan. There is no single value of MinPts and Eps that would work globally.
Distribution of check-ins at a Target in Tucker, GA (left) and at Sweet and Vicious, a bar in New York (right). Note that the map on the left is zoomed out to show a much larger area than the one on the right.
We overcame this limitation by training a Multi-Output Random Forest to predict the best values of MinPts and Eps for each individual venue given a feature vector including, among several other things, its category, where it is geographically located, and various raw and geo-normalized user engagement statistics. We then fed those predicted parameter values to the DBSCAN algorithm to detect clusters of check-ins at that venue.
Before going any further, it's helpful to establish how we define map pin accuracy. For our purposes, a venue's map pin is accurate if it is within 50 meters of its real-world location and on the same side of the street. The latter requirement is especially important in order to avoid routing users up the wrong side of the street and it is this condition that our map pins most frequently used to fail the test on.
We built up a ground-truth dataset from a sample of venues from across the world and in a variety of categories. For each of them, we had a member of our internal data quality team draw the true shape of the venue as well as the shape of the street block surrounding it. The first shape is used to test if a map pin is within 50 meters of the real world location. The second shape is used to test if it is on the same side of the street. If a point is within the same street block as the venue, it is, by definition, on the same side of every surrounding street as the venue is.
Top: The true shape (left) and street block shape (right) of a hair salon in Greece.
Bottom: The true shape (left) and street block shape (right) of a car wash in Turkey.
Even though the street block shapes in both cases contain other streets, these are pedestrian or private streets, and our definition excludes them.
Next, we used this ground-truth dataset to synthesize labeled training, validation and test datasets for our Random Forest model. The way we did this was by enumerating several combinations of MinPts and Eps for every venue in the ground-truth dataset, and including as labels only those values that generated an accurate dominant cluster centroid, which we determined by programmatically checking for containment within the two hand-drawn shapes.
In the end, an impressive 44% of the venues in the test set had their pins moved to a better location and only 7% to a worse one. We shipped this model to production, where it went on to recompute tens of millions of venue map pin locations.
Where We Will (Probably) Go From Here
At Foursquare, we have always believed in the power of mobile location data, and we are uniquely positioned in the industry to bring this power to bear on such a diverse set of problem domains. We continue to research other innovative uses of our location data to improve the coverage and quality of our venue database. Watch this space to learn more about how we're using machine learning techniques to consume large amounts of data from external sources and use it to intelligently augment our own.