Isobath: five cities a day, and a pin instead of an answer

01  What Isobath is

Isobath is a free daily geography game built on one rule: you never type an answer. Each day gives you five real cities, one at a time. A name appears over the water — Lisbon — and your job is to put a pin where you think that is. Tap to drop it, drag to adjust, confirm; the chart draws an arc to the true location and hands you a number: off by 412 km.

Five rounds, five numbers, one total, in about two minutes — the same slot a word game occupies in the morning. No account, nothing to install, no paywall: the game is ad-supported and runs entirely in your browser.

That is the whole thing — no levels, no currencies, no daily quests. Isobath asks only that you know, roughly, where things are, and it is unusually honest about how roughly.

02  Place it, don’t name it

Most free daily geography games are name-entry games. Globle asks you to type a country and answers with heat; Worldle shows a silhouette and asks which country it is. Both test recall: you either know the name or you do not.

Isobath inverts that. There is no text field anywhere in the product: the only input it accepts is a coordinate. You are not asked whether you know that Reykjavík is in Iceland — you are asked to point at Iceland, from memory, on a chart with no labels, with your thumb. Those are different skills, and the second is much harder to bluff: knowing roughly where somewhere is and finding it on a blank map are separated by hundreds of kilometres.

It also works where the polished alternative cannot. GeoGuessr, the best-known one, is built on licensed Street View panoramas — a dependency this one does not take, and one many school and office networks block outright. Isobath ships its own chart and calls no imagery service, so it loads on a throttled connection and plays where tile-based games are firewalled. With no keyboard involved, it fits a phone: one thumb.

03  How a day is built

Everyone in the world gets the same five cities. They are derived, not served: the day’s puzzle is a pure function of the UTC date and a fixed pool that ships with the game — no server call, no stored state, and yesterday’s puzzle still computable tomorrow. UTC rather than your local midnight matters more than it sounds, or Auckland and Los Angeles would be on different days at once.

The pool is curated, not scraped at runtime: 354 cities across 153 countries, in five difficulty tiers of 71, 71, 71, 71 and 70. Tier one is a household name; tier five is a real city you have to think about. The rounds walk up the tiers, so round one is usually placeable and round five usually is not.

Two rules stop a day feeling like one corner of the world five times over. No country appears twice in a day, and no two cities in a day are closer than 1 200 km.

Because the puzzle is a function and not a record, every past day is still there: /day/YYYY-MM-DD publishes a day’s five cities with their locations revealed, thirty days back. Today’s answers are never on those pages, and your score is recorded for the live day only.

04  What the chart is, and what it deliberately is not

The map is drawn by the game itself from Natural Earth 1:110m vector coastline data, which is in the public domain: shorelines in a survey-print style — two values of ocean and paper, stencil type, hairline rules. There is exactly one colour in the whole product, a magenta flare, and it is spent on your pin and the arc of your error.

It shows coastlines and nothing else. No political borders, no country outlines, no labels, no flags — not hidden, not toggled off, simply absent from the geometry. That is a game decision first, because a labelled map answers the question for you. It is also why the chart stays out of arguments it has no business in: places whose sovereignty is disputed are left out of the city pool.

Two limits, stated plainly: a game map that pretends to be a map is worse than one that admits it is a board. At 1:110m the coastline is generalised and small islands are dropped, and the projection flattens away from the equator. Distances are great-circle estimates on that simplified geometry — close, but not surveyed, and not for navigating anything.

05  The verdict line is a diagnosis

After each round the game stamps a verdict, not just a number. Under 25 km is a pinpoint, under 150 km is close, under 600 km is the right region, under 2 000 km is the wrong region, and beyond that a continent off.

Where the geometry can say something sharper, and provably true, it says that instead. Every one of those lines measures your own two coordinates rather than guessing what you meant. Cross the equator by more than eight degrees and the stamp reads WRONG HEMISPHERE. Land within two degrees of latitude but more than eight of longitude and it reads RIGHT PARALLEL, WRONG MERIDIAN; the mirror case reads RIGHT MERIDIAN, WRONG PARALLEL. Past roughly 16 000 km you are NEARLY ANTIPODAL, which is at least a distinction.

That is more useful than it sounds. RIGHT PARALLEL, WRONG MERIDIAN is not an insult but a diagnosis: your sense of how far north or south a place sits was fine, and your sense of how far around the globe it sits was not — two separate habits you can fix separately.

06  The total, the benchmark and the streak

The five distances add up to a total in kilometres, and that total is the whole score — no time bonus, no multiplier, no streak arithmetic folded in. You can check the arithmetic on paper, which is rather the point.

The result places your total against a benchmark curve and prints a line like BETTER THAN 68% OF OUR BENCHMARK. Read that carefully, because the game is careful about it: the curve is a fixed reference that ships with the build and is computed in your browser, not a live comparison with other people. Isobath has no server and counts no players, so it cannot know what anyone else scored — hence the wording: benchmark, not scores.

The streak is the part that brings you back. Playing the day’s puzzle extends it; missing a UTC day resets it, and the reset is visible before you play, because it is a fact about the calendar rather than your next tap. It lives in your browser alone, beside your last thirty days of results.

At the end you get a share card rendered in your browser: the day’s chart with its five error arcs, the total, the percentile, the streak. It carries no city names, so posting your result cannot spoil the day for anyone who has not played.

07  Questions people ask

Is Isobath free, and do I need an account?

Free, and no account. There is nothing to install and nothing to sign up for, no paywall on any part of the game, and the pins you place are never uploaded. It is supported by ads, with consent handled through Google’s consent tool.

How is Isobath different from Globle or Worldle?

Those are name-entry games: you type or recognise a country name. Isobath accepts no text at all — the only input is a coordinate. You drop a pin on a chart with no labels, and the score is the great-circle distance between your pin and the city, in kilometres.

Does Isobath use Street View, Google Maps or satellite imagery?

No. The chart is drawn by the game from public-domain Natural Earth 1:110m coastline data. There is no Street View, no satellite imagery and no third-party map tiles, which is also why it plays on networks that block imagery hosts.

Can I play previous days?

Yes. Every past day is at /day/YYYY-MM-DD with that day’s five cities revealed, and the archive at /day/ lists thirty days back. Today’s answers are never published, and your score is only recorded for the current day.

What does “better than 68% of our benchmark” mean?

Your five-round total compared against a fixed reference curve that ships with the game and is computed in your browser. It is not a live comparison with other players — Isobath has no server and receives no scores, so it cannot know what anyone else scored.

Does it work on a phone, and does it work offline?

It is built for a phone: one thumb, a 44-pixel pin target at every zoom, pinch to zoom with the pin anchored to its coordinate, and three zoom levels you can also step through by double-tap or with + and −. Offline, no — the page needs the network once, after which each round is computed locally.

08  More from the shelf

• Dewkeep — Keep a creature. Grow a little world.
• Tilewright — Draw seven tiles. Outword the AI.
• Yearmark — The year you were born, as a poster.
• Chess — Beat the engine. Post the win.

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