Souvenir Survey: Skipping Stones

It’s been approximately 10 months since the Buddy Adventure feature was released, and with it new ways to interact with your favorite Pokémon. One of these new interactions is the ability to receive a variety of unique souvenirs from buddies who have reached Ultra buddy level or higher. While these souvenirs don’t currently offer any in-game benefits besides the joy of collecting them, our researchers were interested in studying them because of their apparent association with different environments and real-world features. Perhaps they might even hide a clue about Pokémon biomes. Since the launch of our investigation, our researchers and their buddy Pokémon have been hard at work collecting nearly 6,000 souvenirs.¹

As Niantic has indicated, some souvenirs are more difficult to find than others. This article will focus on one of the rarest souvenirs: the Skipping Stone, found by only a handful of researchers. Let’s see what our team has uncovered!

Key Findings

  1. Information from OpenStreetMap (OSM) can influence the availability of certain souvenirs.
  2. Skipping Stones can be found in Level 14 S2 Cells that contain select OSM “water” related tags.
  3. There are at least 7 OSM tags that can trigger Skipping Stones.
  4. OSM data is likely pulled from the same date as nests and restricted areas (February 25, 2019).
  5. When Skipping Stones are available in an area, your buddy has around a 3.9% chance of finding one.

OSM Tags & Water Features

Initial observations from travelers on the Road hinted that souvenirs were tied to locations. A souvenir that is commonly found in one location might be nonexistent in another. To explain this further, we need to take a closer look at mapping in Pokémon GO.

Pokémon GO uses publicly accessible OpenStreetMap (OSM) data to generate much of the visible (and invisible) game world. OSM stores metadata about map features using “tags,” which use key-value pairs to describe objects; for example, the tag building=house has “building” as the key and “house” as the value. OSM tags have been shown to influence many of the game’s features, including biomes, EX Raid eligibility, nests, and areas with restricted gameplay. Knowing this, we hypothesized that OSM tags might be underlying the observed differences in souvenir distributions.

When it comes to the Skipping Stone souvenir item, we found a lead in the descriptive text: “A smooth stone that is the perfect shape for skipping on water.” This hints at a connection to water-related features. Indeed, of the 81 Skipping Stones found by our researchers, all of them were in the proximity of an OSM-tagged water feature. We were able to collect souvenirs in numerous locations with a single variation of water-related tagging in the vicinity, confirming a handful of OSM water tags that we are confident can act as “triggers” of Skipping Stones. As OSM uses a plethora of water-related tags of varying obscurity, this is likely not an exhaustive list.

Confirmed Skipping Stone Tags
waterway=stream
waterway=canal
natural=water
waterway=dock
natural=wetland
waterway=drain
waterway=river

No other tags were consistent across locations, leading us to believe that the presence of just one of these OSM-tagged water features is the primary factor when it comes to determining Skipping Stone availability.

However, it is unknown whether the presence of a single water tag guarantees that Skipping Stones will be available. Researchers found two locations where water features were present (along with other factors discussed below), but were unable to find any Skipping Stones (n=133 and 68). One location was near a ‘waterway=river’ and ‘waterway=stream’ tag, while the other was in the proximity of a very small ‘natural=water’ and ‘waterway=stream’ tag. More investigation will be required into these anomalies.

Image: This lake in Austin, TX, USA which features the “natural=water” tag is able to produce Skipping Stones. Map © OpenStreetMap contributors.

Other Possible OSM Tags

Not all OSM tags could be tested in isolation; some OSM tagging schemes are hierarchical and are often found in combination with more common tags such as “natural=water.” While Skipping Stones were found near these water-related tags, our researchers were unable to confirm that they produce Skipping Stones without another viable OSM water tag.

Possible Skipping Stone Tags
water=lake
water=pond
water=river
natural=bay
harbour=yes
seamark::type=harbour

Likewise, our researchers looked at other locations which did not generate Skipping Stones. Some of these areas have OSM-tagged water features which appear visually in-game, such as swimming pools. In absence of a viable water tag, we were unable to find Skipping Stones near the following OSM Tags.

Tags that likely do NOT trigger Skipping Stones
basin=retention
landuse=basin
leisure=swimming pool
natural=coastline

Given the uncertainty discussed above as to whether water features guarantee Skipping Stone availability, further locations will need to be tested to confirm these findings.

Spatial Considerations

In the previous section, we stated that researchers could only find Skipping Stones in the “proximity” of water features. But how close do you need to be?

After some investigation, we discovered that Skipping Stone availability is not directly determined by a trainer’s distance to a water feature. Instead, it is linked to S2 cells. S2 cells are an efficient method for indexing spatial data such as large-scale maps. The globe is divided into smaller and smaller grid cells, creating “levels” that give spatial hierarchy. Levels with low numbers are associated with the largest grid sizes, and progressively higher level numbers refer to smaller and smaller grids, with Level 30 cells being just a few centimeters across. S2 geometry is also a hidden cornerstone of key game mechanics including PokéStops, Gyms, EX Raids, biomes, and much more.

We discovered that your buddy’s ability to find a Skipping Stone is determined by whether or not you are standing within a Level 14 S2 cell that overlaps or contains relevant OSM water features. Level 14 cells are about the size of a city block. If you are standing anywhere inside an eligible Level 14 cell, your buddy might find a Skipping Stone. Note that the location where your buddy first finds the souvenir determines its contents, rather than the location where you choose to open it.

In the map below, two locations where a researcher gathered souvenir data are shown, indicated by a green checkmark (✓) and red X mark (✗). Both locations are nearly equidistant to the nearby lake border. However, the location denoted with a checkmark is inside a Level 14 cell that overlaps the lake, while the X mark is not. Skipping Stones were found at the checkmark, but never at the X mark.²

Image: A lake in Princeton, NJ, USA with “natural=water” tags. Skipping Stones found in the s2 cell labeled which contains the tag. Skipping stones were not found in the s2 cell labeled which does not have the tag. Map © OpenStreetMap contributors.

Similar tests were conducted at two other locations with identical results. It’s possible that researchers were simply unlucky to not find a Skipping Stone, or that they weren’t available for a different reason, but the repeated trials give us some confidence that this hypothesis is correct.

OSM Date

Now that we’ve explored the connections between Skipping Stones, OSM tags, and the spatial scale, one final question remains: which version of OSM is being used?

Pokémon GO does not draw from OSM in real-time; instead, the game uses various static versions pulled from the OSM database at different times. Data is rarely updated, and several features use different datasets. The current map data used for nests and restricted areas is suspected to be from on or around February 25, 2019. We hypothesized that this data might also be linked to souvenirs. To test this hypothesis, we looked at two locations that were added before and after the aforementioned map update was sourced.

Image: Two ponds containing the “natural=water’ tag, tagged before (left) and after (right) the map update. Skipping stones were found in the left pond, but not the right. Map © OpenStreetMap contributors.

In the above example, the left pond produced 3 Skipping Stones out of 84 souvenirs sampled. However, we failed to find any Skipping Stones in the right pond after 93 souvenirs (estimated 2.4% likelihood). It’s possible that the pond on the right does not trigger Skipping Stones for another reason, but the observations strengthen our hypothesis that the OSM database from February 25, 2019 is being used to determine souvenir availability.

Probability

In the nearly 6,000 total souvenirs gathered so far for this study, Skipping Stones had an overall rate of 1.41%. However, using the bulk estimate is misleading given all of the information we’ve determined about which locations are eligible for Skipping Stones at all. If we filter for several locations that meet the above criteria (and were chosen after criteria were determined), researchers found 22 Skipping Stones in a sample of 562 souvenirs, an overall rate of 3.91%.

Location Souvenir Count Observed Rate
1 181 6 3.31%
2 169 9 5.33%
3 55 2 3.64%
4 73 2 2.74%
5 84 3 3.57%
Combined 1-5 527 18 3.91%

The combined rate for the above locations is significantly higher than that of all locations (p-value < 0.01). We have not been able to collect enough data to determine conclusively whether the probability of receiving a Skipping Stone is identical at all locations where they can be found. However, it’s clear that even with the criteria met, Skipping Stones have proven to be rare!

Parting Words

We hope you enjoyed taking a deep dive into OSM water tags and their apparent links to the Skipping Stone! If you’re on the hunt for these elusive souvenirs, use OpenStreetMap to find areas that are tagged with one of the water features we’ve highlighted in this article. While all Skipping Stones have been found near OSM water tags, there are exceptional locations yet to yield Skipping Stones. Our researchers will be on the lookout for other factors at play. Interestingly, most of the confirmed OSM water tags had water Pokémon spawning immediately nearby, leaving open the possibility that all tags associated with the “freshwater biome” also generate Skipping Stones. Remember, buddies will be able to find Skipping Stones so long as your avatar is within the same level 14 S2 cell as the water feature, no need to take your phone for a swim!

We suspect that the other fourteen souvenirs may also use a combination of OSM tags and S2 cell geometry, and our researchers are on the case! Keep an eye out for our next article which will delve into different types of souvenirs!

Until then, travelers, see you on the Road!


Acknowledgements

Article authors: Scientist Titleist and Senior Researchers CaroKann, RDC-DCIfan68, and Sinkalingsveis
Header graphic concept: Hectordraw
Project Leaders: Lead Researchers HotBlondey and Belle
Data collection: 238 Silph Researchers

Footnotes

¹This total, or any of the analysis in the article, does not include the GO Fest exclusive souvenirs “Pikachu Visor” and “Confetti.”

²In the pictured example, a total of 63 souvenirs were gathered at the ✗. 12 souvenirs were collected at the ✓ before a Skipping Stone was found. Researchers also collected souvenirs from three other locations immediately adjacent to a Level 14 Cell that had Skipping Stones, for a total of 178 souvenirs without a Skipping Stone.