Key concepts
- A hex is one hexagon on the map, covering roughly 15,000 m²
- A visit is a record of presense, within a cell, for a given day.
Map structure
The map cell structure are based on the H3 spatial indexing standard, from Nick Rabinowitz via Uber. We use resolution 10 of the H3 grid, so there are 33,897,029,882 cells. (12 of these are pentagons.) Higher resolutions are not calculated from base data; lower resolutions are used for visualization when "zoomed out" to large areas. This is to prevent leaking greater precision, because resampling is dangerous and the partial intersection of H3 cells compounds that problem. (See the last image in this article for details on this risk.)
The public addresses for cells use the encoding regime from Placekey.
Visit levels
Behind the scenes, visits are designated with one of 5 levels of increasing weight of evidence, listed below. Visits at the trace and path levels represent movement and are drawn with thinner lines.
| trace | ambient tracking | phone logging |
|---|---|---|
| path | explicitly recorded movement | outdoor workout logs |
| dwell | detected linguring in a location | a six-minute pause during a run |
| breadcrumb | location metadata from an explicit action | photos |
| checkin | explicitly recorded presence | Swarm checkins |
What day is it?
It can be surprisingly difficult to determine what day a visit occured on. Many data sources give only a UTC ("universal") time, which is fine if you're in Greenland or Ghana, but usually it's a bit more complicated. Some sources will offer a timezone, but even that doesn't alway help. If you ride your bike across a timezone border, for example, Strava will only tell us the timezone you began in.
So we record everything in UTC, and use the location and time to determine what the local time was for each visit, before aggragating to days. We use the Who's On First (WOF) gazetteer to determine which timezone a cell is in. This could be an incorrect or controversial choice! Between the imprecision of maps and projection of cells onto the Earth spheroid, there may be some hexes that are incorrectly designated. If you have visits near midnight that appear a day too early or late on your map, let us know and we can manually assign the hex a more sensible timezone.