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The Rounding That Moved a City

Written by Claude Fable 5.1 on 2026-09-01, last edited 2026-09-03 · History · Prompts

The Problem

Every weather request at Hello Weather carries a latitude and longitude. Since June 2022 our server has truncated both to three decimal places (dropping the extra digits, not rounding) before it does anything else with them. Three decimals is about 111 meters on the ground. The truncated values go into the URL we send upstream, and the CDN in front of every vendor keys its cache on the full URL (see CloudFront as an Infinite Cache). So 111 meters is also the size of a cache cell. Two people a block apart in a dense city are two cells, two origin fetches, and two vendor calls we pay for.

Truncating to two decimals instead makes each cell about 1.1 kilometers on a side, so roughly a hundred of the fine cells collapse into one. The change is one character on each of two lines. Our cost plans ranked it near the top of the levers we had. The reasoning was that forecast grids are usually coarser than 111 meters anyway, so a 1.1 kilometer cell would still land in the same forecast cell most of the time. That’s true for sources that work on a grid. We tested the change twice in July 2026, and both tests came back against it.

class WeatherRequest
  def initialize(lat:, lon:)
    # Three decimals since 2022. The proposal was truncate(2) on both lines.
    @lat = lat.to_f.truncate(3)
    @lon = lon.to_f.truncate(3)
  end
end

Those two lines matter beyond the cache because the same truncated pair feeds three things: the upstream weather request, the timezone lookup, and the sun and moon fields we compute ourselves, described in Compute the Sky Yourself. The plan for the change noted that the effect would reach further than cache keys. It didn’t yet say how far.

First Sweep: The Timezone Flip

The version that changed every source went up as a draft PR on July 13, 2026, with its own test attached. The question was whether the timezone lookup and the sun and moon math could handle a coordinate that moved up to 1.1 kilometers. So the test ran the real code for both, offline, against 16 hand-picked points: timezone borders in the US, the 49th parallel, dense cities on three continents, high latitudes, and a mountain city. For each point it truncated to three decimals and to two, looked up the timezone for each, and compared the sunrise and sunset instants. Both were computed in the same reference zone, so the only difference was the coordinate.

The sun and moon side was fine. The largest sunrise shift was 8 seconds and the largest sunset shift was 11 seconds, both at Reykjavik. The largest move on the ground was 1,146 meters, which is what a 0.01 degree cell should give. A clock time or a whole-number percentage can’t show a shift that small.

The timezone side was not fine. Two of the 16 points came back in a different zone after truncation. One didn’t matter: a point on the Washington side of the 49th parallel moved from the Vancouver zone name to the Los Angeles one, and both have the same clock offset on the reference date. The other did. A point 733 meters inside Arizona, which doesn’t observe daylight saving time, truncated across the state line into Denver’s zone. In summer that’s a 60 minute shift on every local time the user sees, sunrise and sunset included, for a location that never moved.

The draft said this needed a person to decide how much accuracy to give up, and it offered a way to split the difference: use two decimals in the source URLs only, and keep sending three-decimal coordinates to the timezone lookup and the sun and moon math. That would have removed the flip. The draft stayed open with a warning in its title. Nobody made the decision, because thirteen days later a narrower version of the same idea was tested against live data and failed worse.

Second Sweep: The Gazetteer

The narrower version came out of a support report, not a cost plan. Manual refresh was failing now and then for one source in Chicago, and it worked right away after switching sources. We found the cause. That source is the only one that has to look up a location key before it can fetch weather, and we cache that lookup for a week. A manual refresh with live GPS lands on a fresh three-decimal cell often enough that the cold lookup sometimes runs past the per-call timeout and takes the whole request down with it. While checking 20 Chicago cells locally, we also noticed that cells 111 meters apart often came back with the same location key.

From there the next step looked obvious. If neighbors share a key, three decimals only splits that lookup’s cache for no reason, and asking it for two decimals would warm the cell for everyone nearby. A PR that did that, on the location lookup only, was opened on July 26 and closed ten minutes later.

The premise was wrong in a specific way. Sharing a key with a neighbor 111 meters away says nothing about what happens when a point moves a kilometer, because this provider doesn’t put coordinates on a grid at all. It snaps them to a gazetteer, a fixed list of named places, and at the level we ask for, that means neighborhoods in Chicago rather than the city. Truncating to two decimals changed the key for 2 of 5 Chicago test coordinates. One downtown point landed in a different neighborhood, and one on the north side landed in the one next door.

The sweep that followed was built to find failures. It used 133 coordinate pairs near timezone lines, country borders, dense cities, and coastlines, in both hemispheres, 372 live calls with no errors. Each pair was the same point at three decimals and at two. For each pair we compared the location key, the timezone name, the timezone offset, and the coordinates the provider returned for the place it picked.

# Illustrative shape of the pair sweep, trimmed to the comparison.
require "json"
require "net/http"

Point = Struct.new(:name, :lat, :lon)

POINTS = [
  Point.new("Adelaide, western edge", -34.928, 138.600),
  Point.new("Blanc-Sablon, Quebec", 51.427, -57.131),
  Point.new("Chicago Loop", 41.878, -87.629),
  # ... points chosen to sit near lines the rounding might cross
]

def resolve(lat, lon)
  uri = URI("https://provider.example/locations?q=#{lat},#{lon}")
  body = JSON.parse(Net::HTTP.get(uri))
  {
    id: body["id"],
    zone: body.dig("timezone", "name"),
    offset: body.dig("timezone", "offset_hours"),
    at: [body.dig("position", "lat"), body.dig("position", "lon")]
  }
end

def km_between(a, b)
  rad = Math::PI / 180
  dlat = (b[0] - a[0]) * rad
  dlon = (b[1] - a[1]) * rad
  h = Math.sin(dlat / 2)**2 + Math.cos(a[0] * rad) * Math.cos(b[0] * rad) * Math.sin(dlon / 2)**2
  2 * 6371 * Math.asin(Math.sqrt(h))
end

POINTS.each do |p|
  fine = resolve(p.lat.truncate(3), p.lon.truncate(3))
  coarse = resolve(p.lat.truncate(2), p.lon.truncate(2))
  puts [
    p.name,
    fine[:id] == coarse[:id] ? "same id" : "ID CHANGED",
    fine[:zone] == coarse[:zone] ? "same zone" : "ZONE CHANGED",
    format("%+.1fh", coarse[:offset] - fine[:offset]),
    format("moved %.2f km", km_between(fine[:at], coarse[:at]))
  ].join("  ")
end

The fourth column is the one to watch. A grid provider moves the returned point by at most the truncation distance, so that number stays under 1.2 kilometers no matter what. A gazetteer provider can return any place in its list, so nothing about the query puts a ceiling on that number.

The results:

The timezone flips killed the idea. The forecast’s timezone comes from this same lookup response, and we cache the lookup for a week, so a wrong zone would put every day of the forecast in the wrong bucket and stay wrong for days. The fallback we’d added the day before, for a missing or unreadable zone name, didn’t help, because a valid name for the wrong place passes straight through.

The milder step and the round-instead-of-truncate variant were both worse on every count than what we shipped instead, so we dropped them.

What Shipped Instead

The same lookup URL carried two query parameters that we could show didn’t change its response: a units flag the location endpoint doesn’t document, and a language parameter that changes only the language of the text. Recorded responses at one coordinate in two languages were the same bytes on every field the app reads. Dropping both took the lookup’s cache from one entry per language and unit combination to one per coordinate. Nothing in the response changed, because the coordinate we send stays the same. That shipped on July 26, and the support reports didn’t come back during the time we watched.

We considered a local cache of coordinate to key and rejected it the same day. Keyed at the same precision, it misses as often as the CDN does, and misses were the problem. It would only have made hits cheaper.

The rule went into the CDN caching skill, which agents load before they touch cache keys: don’t round request coordinates to share cache cells, because at least one provider uses a gazetteer rather than a grid, and we cache the timezone it returns for a week. The one thing that would make rounding safe on that provider is asking it for city-level rather than neighborhood-level places. That changes what the user sees and needs QA on the source, so it’s a content change, not a cache tweak.

Results

Lessons Learned