Methodology and sources

The short version

• Open, official data We use public sources that allow reuse, including on a site with ads, as long as they’re credited: the U.S. Census Bureau, the U.S. Geological Survey (USGS), Spain’s Instituto Geográfico Nacional (IGN) and statistics office (INE), OpenStreetMap, Natural Earth and the World Bank, among others. The full list with licenses is at the end.
• The math runs in your browser Distances, areas, coordinates, datum shifts, the county or municipality of a point and the population inside a radius are calculated on your own device, with the boundaries and figures the page downloads. Mapsfera has no server that receives what you look up: when outside data is needed (an elevation, an address, a route), your browser asks the public service that provides it directly, and each section below names it.
• How accurate to expect It depends on the tool. The table below sums it up, and each section explains it with numbers.
• Found a mistake? Tell us through the contact page, with the link to the tool and what you expected to see. We review every report and log each fix, with its date, on the Corrections page.

How accurate each number is

Base map

Almost every tool shares the same map. MapLibre GL JS, an open-source library, draws it in your browser from OpenFreeMap tiles, which are built from OpenStreetMap. We use its “Positron” style recolored to Mapsfera’s palette, with place names in the page’s language. When a tool needs it, hillshading goes underneath, computed by MapLibre from the Terrain Tiles elevation data (the same as in elevation).

Sources

• Tiles: OpenFreeMap, free, with no API key, and commercial use allowed with credit. The map shows “OpenFreeMap © OpenMapTiles Data from OpenStreetMap.”
• Map data: © OpenStreetMap contributors, under the ODbL 1.0 license.
• Aerial imagery in the area calculator: USGS Imagery Only from The National Map (public domain) in the lower 48 states, and IGN’s PNOA imagery (CC BY 4.0) in Spain.

Known limitations

• The base map is there to help you find your way. Each tool’s numbers come from its own sources, listed below: if the base map and a tool disagree (a boundary, a town name), go with the tool’s source.
• The map uses web Mercator, which stretches areas far from the equator, so a circle looks slightly elongated toward the poles. Measurements are never taken on the flat map: they’re calculated on the Earth.
• Free aerial imagery is only available for the US and Spain.

Elevation and profiles

Sources

• Terrain Tiles, everywhere except single points in Spain: a worldwide elevation model published in the Amazon Web Services open data registry. For each area it uses the best public source available: the USGS 3D Elevation Program (3DEP) in the US (10 m), Copernicus EU-DEM across most of Europe (30 m), NASA’s SRTM in most other places (30 m), national models in a few countries and NOAA’s ETOPO1 for the ocean floor. Almost all of it is public domain or openly licensed; the attribution list is below.
• In Spain, the IGN’s MDT05: the national terrain model, with a value every 5 meters, made from airborne laser (lidar) surveys. Licensed CC BY 4.0 ign.es and queried live from the IGN’s WCS service (servicios.idee.es, coverage Elevacion4258_5).

How it’s calculated

A single point. We download the zoom-14 Terrain Tiles tile that contains it. Each pixel stores the height in its three color channels (the Terrarium format) and is decoded with the formula below. Then we interpolate between the four nearest pixels (bilinear interpolation, the standard method for terrain models).

In Spain (the mainland, the Balearic and Canary Islands, Ceuta and Melilla), we first ask the IGN for a 2 × 2 cell window around the point and apply the same interpolation. If the IGN has no value there (it returns zeros in Portugal, France or at sea) or doesn’t answer within 8 seconds, we fall back to Terrain Tiles.

A profile. The line is split into evenly spaced samples, at least one every 5 m and at most 1,000, and each sample takes its elevation from Terrain Tiles. The zoom level depends on the length: 14 up to 5 km, 13 up to 30 km, 12 up to 120 km, 11 up to 400 km, 10 up to 1,500 km and 9 beyond, and it drops if more than 48 tiles would be needed. Distance along the profile is measured on a sphere with a radius of 6,371,008.8 m (the haversine formula).

Climb and grade. Total climb adds up every ascent and total descent every descent. To avoid counting noise, a change of direction only counts once the route has climbed or dropped more than 3 m on the terrain model, or more than 5 m with a GPX file’s own elevation, which is noisier. Maximum grade is the steepest over a 100 m stretch (or a fifth of the route if it’s under 500 m), so a single glitch in the model can’t inflate it.

Time. The English version uses Naismith’s rule, with no breaks. The Spanish version uses the MIDE method common on Spanish trail signs.

Accuracy

With Terrain Tiles, the real detail is that of each area’s source, not the pixel: 10 m in most of the US and about 30 m in Europe and most of the world. At that scale the terrain gets smoothed: peaks come out a little lower and canyon floors a little higher.

For single points in Spain, the MDT05 gives a value every 5 m. As a check, at 39 of the 43 Spanish provincial capitals whose reference point matches the IGN gazetteer, the MDT05 and the official elevation differ by 1 m or less.

In What is my elevation?, where your device places you matters most: a few meters with GPS, but tens or hundreds of meters with Wi-Fi or a cell network, which in the mountains changes the elevation a lot. That’s why we show the accuracy your browser reports.

Known limitations

• Profiles use Terrain Tiles only, even in Spain, and on sharp summits they fall a few dozen meters short: Mount Teide comes out at 3,679 m on a profile, while the MDT05 gives 3,712 m at its summit and the official figure is 3,715 m. So a point’s elevation may not match exactly between the elevation finder and a profile.
• It’s the height of the ground above mean sea level. Buildings, trees and the floor you’re on don’t count.
• It isn’t your GPS altitude: GPS measures height above an ellipsoid, a mathematical shape that doesn’t follow sea level, and the difference can be tens of meters.
• At sea, the global source gives the depth of the ocean floor, as a negative number.

Where am I and addresses

Sources

• Your location: provided by your browser (GPS, Wi-Fi or cell network), with an accuracy radius we show next to the result.
• United States: the Census Bureau’s TIGERweb, queried live for the state, county, city or census-designated place (CDP) and ZCTA. Public domain.
• Spain: municipalities, provinces and regions from the IGN’s administrative boundaries database (BDDAE), data as of August 10, 2026 (CC BY 4.0 ign.es, simplified); addresses, building numbers and postal codes from CartoCiudad, the IGN’s official street directory (CC BY 4.0).
• Rest of the world: Photon, Komoot’s search engine on OpenStreetMap data, and countries from Natural Earth 1:10m, version 5.1.2 (public domain).

How it works

The country. If the point falls in a Spanish municipality, it’s Spain; if TIGERweb returns a state, it’s the US. Otherwise it’s the country of the nearest OpenStreetMap address, if it’s within 300 m; failing that, the Natural Earth country that contains the point (or one within 1 km, on the coast). If there’s none, we tell you you’re at sea.

A municipality in Spain. A point-in-polygon test. The province file picks the candidates (the province containing the point and any whose boundary is within 210 m), only their municipalities are loaded, and we find the one that contains the point. If the point falls in none but is within 30 m of one (on the shore, where simplification trims a few meters), we return that one. Within 100 m of another municipality, we warn you.

Address to coordinates. Each address is looked up in Photon (the Spanish version of the site also checks CartoCiudad), and we keep the result that best matches what you typed, house number and town included, not just the first one. For the reverse, the address of a point, Spain uses CartoCiudad and everywhere else uses Photon. Each result says how precise it is: building, street, town, municipality, postal code or named place. Batches go one address at a time, about one per second, up to 100 per batch.

Accuracy

The Spanish boundaries are simplified to 10 m so they load fast on a phone. Measured over the 4.37 million vertices of every unit 150 m or larger, the error is 2.4 m on average, 1.3 m at the median and 10.7 m at most. That’s far less than the source’s own uncertainty: the IGN says most boundary lines are accurate to about 40 m, and some stretches are provisional. That’s why the “near the boundary” warning kicks in at 100 m.

Known limitations

• ZIP codes are delivery routes, not areas, and the Postal Service doesn’t publish boundaries. We use the Census Bureau’s ZIP Code Tabulation Areas (ZCTAs), which match them almost everywhere; P.O. box and single-building ZIP codes have no ZCTA.
• Natural Earth borders can be off by hundreds of meters: the bridge at Irún, on the Spain–France border, shows up in France. In Spain, the IGN municipality takes precedence.
• Natural Earth shows who controls each territory in practice: Kosovo, Taiwan and Western Sahara appear separately. That’s not a position on who they should belong to.
• At sea there’s no county or country: boundaries stop at the coast and we don’t draw territorial waters.
• Spanish postal codes aren’t open data, so we give the postal code of the nearest building but can’t draw its area.

Coordinates and national grids

No outside data here: it’s all formulas running in your browser. We wrote them ourselves, without libraries, and test them against 501 reference cases computed with PROJ 9.8.1, the coordinate transformation software behind QGIS and most GIS programs.

How it’s calculated

Formats. Decimal degrees, degrees-minutes-seconds (DMS), degrees and decimal minutes, UTM, MGRS, geohash and Plus Codes. The reader detects the format on its own, with N, S, E and W letters, and tells you if it swapped latitude and longitude or assumed the northern hemisphere.

UTM. Transverse Mercator with the Krüger series up to the sixth power (n⁶), in Karney’s 2011 form, accurate to the millimeter thousands of kilometers from the central meridian. We use the GRS80 ellipsoid (less than 0.1 mm from WGS84), a scale factor k₀ = 0.9996 and a false easting of 500,000 m, with the Norway and Svalbard zone exceptions.

National grids. Each has its own projection (transverse Mercator, Lambert conformal conic, Swiss oblique Mercator or the Dutch oblique stereographic), using the formulas in EPSG Guidance Note 7-2 and from swisstopo, plus its datum shift:

• None when the system is already on ETRS89: UTM ETRS89, Lambert-93 (France), Irish Transverse Mercator and PT-TM06 (Portugal).
• Official grid file (NTv2, interpolated bilinearly): Spain’s IGN SPED2ETV2 grid from 2009 for ED50 to ETRS89 (EPSG:15932) and Germany’s BeTA2007 for DHDN to ETRS89 (EPSG:15948). We take them from the PROJ network (cdn.proj.org) and convert them to a lightweight format for the browser.
• 3- or 7-parameter Helmert from the EPSG registry: CH1903+ in Switzerland (EPSG:1647), Amersfoort in the Netherlands (EPSG:4833) and OSGB36 in Great Britain (EPSG:1314).

WGS84 (what GPS uses) and ETRS89 are treated as the same: in Europe they’re a little under 1 m apart today.

Accuracy

The projections match PROJ within 1 mm for UTM and 1 cm for national grids, and converting back lands within a ten-millionth of a degree (about 1 cm) of the start. The datum shift is what limits accuracy, as rated by the EPSG registry:

Datum shift for each system and its accuracy

Decimal places matter too: 5 decimal places of a degree get you to about 1 m, and 6 to about 11 cm. More adds nothing, since not even a phone’s GPS is that precise.

Known limitations

• State Plane coordinates for the US aren’t available yet.
• For Great Britain and the Netherlands there are official centimeter-level methods (OSTN15 and RDNAPTRANS2018) that we don’t use yet: with Helmert the error is about 2 m and 1 m.
• There’s no NAD27 conversion: older USGS topo maps in NAD27 can be off by tens of meters, up to about 100 m. US data in NAD83 is read as WGS84; today the two are within about 1 to 2 m, which matters only for survey work.
• UTM only exists between 80° S and 84° N; near the poles another projection (UPS) is used, which we don’t provide.

Distances, routes, drive-time zones and areas

Sources

• Geodesy: GeographicLib, Charles F. F. Karney’s library (MIT license), on the WGS84 ellipsoid.
• Routes, distance matrices and stop ordering: OSRM, on the public FOSSGIS server (routing.openstreetmap.de). Drive-time zones: Valhalla, also run by FOSSGIS. Both use the OpenStreetMap road network (ODbL).
• FOSSGIS offers them for free on three conditions: one request per second at most, no heavy use, and credit with a link to fix the map. They keep route requests in their server logs, as their site says.

How it’s calculated

As the crow flies. The geodesic, the shortest path on the WGS84 ellipsoid (semi-major axis a = 6,378,137 m, flattening f = 1/298.257223563), solved with Karney’s 2013 method, accurate to nanometers. The haversine formula does the same on a sphere and can be off by up to 0.5%; we show it to explain the idea, but we don’t measure with it.

The halfway point is halfway along the geodesic (not the average of the coordinates), and the bearing is the compass direction you leave in. Between cities, each city is a single point: in the US, its interior point from the Census Gazetteer, except San Francisco, Anchorage and Juneau, where we use the OpenStreetMap city center; in Spain, the town seat in the IGN gazetteer. ZIP codes use their ZCTA interior point.

Radius map. The circle is drawn on the Earth: 128 points at the same true distance from the center. Its area and perimeter come from the spherical formula, because that’s the circle you see. To find which states, counties or ZIP areas it touches, the circle and the boundaries are projected onto a Lambert azimuthal equal-area projection centered on the circle (it preserves areas) and clipped; a unit counts if more than 1,000 m² of it is inside.

Area. The polygon is measured on the WGS84 ellipsoid with GeographicLib, as it lies on the Earth, without flattening it onto a map first. The perimeter is the sum of its sides, each along the shortest path. It’s the horizontal (plan) area: slope isn’t counted.

Routes. OSRM finds the fastest route by car, bike or on foot along the roads you can use, at each road’s normal speed, with no traffic. The distance matrix takes up to 50 places (1,225 pairs, each in both directions, because the trip there and back can differ). The multi-stop route takes up to 25 stops: with fewer than 10 it tries every possible order; with more, it uses a fast method that gives a very good order, though not always the best one.

Drive-time zones. Valhalla walks the roads and paths you can use from the starting point and draws the outline of what’s within each time or distance: up to 4 zones, 60 minutes or 100 km (62 miles) at most. By car it uses each road’s normal speed; by bike, about 18 km/h (11 mph); on foot, about 5 km/h (3 mph). Each zone’s area is measured with GeographicLib, and its reach is the longest straight line from the start to its edge.

Accuracy

For straight-line distances and areas the math adds no error: what counts is where you put the points (and that a city is a single point). On a hillside the ground has more surface than its plan area: 2% more on a 20% slope and 12% more on a 50% slope.

Routes are only as good as the OpenStreetMap road network in the area. There’s no traffic: at rush hour you’ll take longer and get less far.

Known limitations

• OSRM doesn’t say when a point has no road nearby: it snaps it to the closest road, even on another continent (New York to London by car came back as a 2,150 km route, because it moved New York to Portugal). So if a point is more than 30 km from its road and more than a quarter of the route, we show “no route”; in the distance matrix, beyond 30 km. For drive-time zones, if the start is more than 2 km from any road or path, we show “no route” too.
• By car, routes use ferries where there’s no other way, and the crossing counts toward the distance and time. If a route has a straight stretch of more than 80 km (50 miles), we warn you it’s almost certainly a ferry.
• The routing services are free and public: if they’re busy, wait a moment and try again.

Spain maps, blank maps, worksheets and quizzes

Sources

• US states: the us-atlas 3.0.1 package, built from Census Bureau cartographic boundary files (public domain).
• Countries and capitals of the world: Natural Earth 1:10m, version 5.1.2 (public domain).
• Spanish provinces and regions: the IGN’s BDDAE (CC BY 4.0), the same used in Where am I.
• Country areas: the World Bank (indicator AG.SRF.TOTL.K2, latest year 2024, CC BY 4.0), with the ±10% rule below.

How they’re made

These maps don’t use the base map: they’re drawings (SVG) that are projected and simplified in advance. The US uses the Albers projection with Alaska and Hawaii in insets; the world, Equal Earth; Europe, an azimuthal equal-area projection centered on 12° E, 52° N; and Spain, a conformal conic projection with the Canary Islands in an inset. Each border between two regions is simplified only once (Douglas-Peucker, about half a pixel), so neighbors fit with no gaps or overlaps. Each region’s area is measured on the unsimplified shape.

In the quizzes, the 193 UN member states and Vatican City count as countries. Kosovo, Taiwan, Palestine, Western Sahara and other countries’ territories appear in gray but aren’t asked. Worksheet answers (which state is smallest, which country is largest) come from this same data, not from memory.

The data maps split values into six shades by rounded quantiles (each shade holds about the same number of counties, with breaks at easy-to-read numbers). Population change uses fixed breaks: gray between −1% and +1%, and darker green or brown beyond 5% and 10%. Every scale runs light to dark, so it still reads in grayscale or printed in black and white.

Accuracy and known limitations

• These maps are for viewing, playing and printing, not measuring: simplification is a fraction of a pixel at their size.
• Country areas: we use the World Bank’s figure only if it’s within ±10% of what we measure on the map. Natural Earth alone would give Algeria 2.31 million km² and Libya 1.63 million (officially 2.38 and 1.76), changing the order. But the World Bank has errors too: it has listed Canada at 15.6 million km² since 2018, sometimes includes territorial waters (Croatia, Dominican Republic, Sweden) and for Greenland counts only ice-free land. Where we don’t use it, we give the map measurement as approximate (“about … km²”).
• Some capitals have more than one answer, and all of them are accepted when typing (Pretoria, Cape Town or Bloemfontein for South Africa; Sucre or La Paz for Bolivia).
• There are no river or mountain maps yet.

Population and other data (Census, INE, World Bank)

Sources

• U.S. Census Bureau (public domain): Vintage 2025 population estimates (July 1, 2025, with the April 1, 2020 estimates base) for counties, states, cities and towns; the 2020–2024 American Community Survey 5-year estimates for census-designated places, ZCTAs and county median household income (in 2024 dollars); and the 2025 Gazetteer and cartographic boundary files.
• Spain’s INE: official population figures from the municipal register (table 29005) as of January 1 of 2025, 2020 and 2015, published December 11, 2025. INE allows reuse, including commercial reuse, with credit.
• Spain’s IGN: the area of each municipality and the location of its seat, from the national gazetteer (CC BY 4.0).
• World Bank: country areas (CC BY 4.0), with the ±10% rule described under maps.

How it’s calculated

Population in a radius. Population is published per ZIP area (ZCTA) in the US and per municipality in Spain, with no detail on where people live inside each one. So we work out what share of each unit’s area falls inside the circle and count that same share of its population. This is areal interpolation, the standard method when data only exists by area.

We also show a range: the low figure counts only the units entirely inside, and the high one counts every unit the circle touches, in full. The true population is always between the two. In the US we don’t have ZCTA shapes, so each ZCTA is treated as a circle with the same land area around its interior point. In Spain we use the real municipal boundaries.

Counties and states. Population change compares the July 1, 2025 estimate with the April 1, 2020 estimates base. Density is population divided by land area. Connecticut shows its nine planning regions, which replaced its counties in Census data in 2022.

Accuracy

Population figures are the official ones, untouched. For Spain we publish the IGN’s municipal areas; as a check we also calculate them from the boundaries, and for municipalities over 1 km² the difference is 0.01% at the median and 1.4% at the 99th percentile.

Known limitations

• Population in a radius assumes people are spread evenly within each unit. With large circles the range is narrow, because most units end up entirely inside or outside; with small circles it widens, and in rural areas, where ZIP areas are large, the estimate is rougher. The tool warns you when that happens.
• Outside the US and Spain we don’t estimate population in a radius yet: there’s no open population grid in the site so far.
• Census-designated places have no annual estimate, so they use the ACS figure; 33 places have neither. ZIP codes without a ZCTA (P.O. boxes, single buildings) don’t appear. There’s no data for Puerto Rico or the territories.
• One county, De Baca County, New Mexico, has no published median household income.

Place pages

For now, place pages exist only in Spanish. There are 55: Spain’s 52 provincial capitals and autonomous cities, and its 3 highest peaks (Teide, Mulhacén, and Aneto), each with its elevation, coordinates and UTM.

Sources and method

• Capital elevations come from the IGN’s population centers (IGR Poblaciones), at each town’s reference point, rounded to the meter. Peak elevations come from the IGN’s summit table, and peak positions from the summit node in OpenStreetMap, 3 to 10 m from the IGN’s point.
• Every capital is checked against the MDT05 terrain model at the same point. We don’t use the IGR figure if its point is more than 200 m from the town seat in the national gazetteer or its elevation is more than 30 m off the MDT05; we publish the gazetteer point and the MDT05 elevation instead, and the page says so. That happens for 11 capitals: Ávila, Burgos, Albacete, Vitoria-Gasteiz, Huesca, Badajoz, Almería, Palma, Barcelona, San Sebastián, and Alicante.
• UTM coordinates use the Krüger series to n⁶, the GRS80 ellipsoid and k₀ = 0.9996, in the official datum: REGCAN95 in the Canary Islands and ETRS89 elsewhere.

Make maps: coloring, drawing, pins, data maps and viewers

Sources

• Outlines: US states and counties and ZIP Code Tabulation Areas from the U.S. Census Bureau (public domain), countries from Natural Earth 1:10m and Spain’s provinces and regions from its national mapping agency (IGN, CC BY 4.0), all simplified. Sample figures from Census estimates.
• Addresses in a list or table: Photon (with OpenStreetMap data), and CartoCiudad (IGN) for Spain.
• Areas in the true size map: the World Bank (with the ±10% rule described under population), the Census 2025 Gazetteer and the IGN.

How it’s calculated

Everything happens in your browser: what you color, draw or pin, and the files you open in the viewers, are never sent anywhere. Only the addresses you ask us to look up leave your device. The viewers read KML, KMZ, GeoJSON and GPX; if a GeoJSON file is in meters in another system (UTM, for example), they convert it to latitude and longitude with the same formulas as the coordinate converter. Lengths and areas of what you draw or open are measured on the WGS84 ellipsoid with GeographicLib.

In data maps, numeric colors use five classes with roughly the same number of items each (quantiles) and rounded breaks; categories get one color for each of the nine most common. The color-blind-safe palette is Okabe and Ito’s.

True size. The Mercator map enlarges everything 1/cos(latitude) times in both directions: twice as big at 60°. When you move a country, we redraw it point by point at the scale of its new latitude, so its real area never changes.

Known limits

• Simplified outlines lose islets and inlets: for official work, use the original files.
• A misspelled or ambiguous address can land in the wrong place: check the points on the map.
• In the viewers, custom icons in a KML or KMZ file show up as colored dots.

Sun and moon

Sources

• Sun and moon positions, phases and eclipses: Astronomy Engine 2.1.19 by Don Cross (MIT license), with the VSOP87 models for the sun and an abridged ELP model for the moon. It’s accurate to one arcminute.
• Local time at each place: time zone boundaries and the IANA time zone database (see Time zones).
• To check it: tables from the U.S. Naval Observatory (USNO), Spain’s national astronomical observatory (OAN) and NASA’s eclipse catalogs (F. Espenak).

How it’s calculated

Sunrise and sunset. The moment the sun’s upper edge touches a sea-level horizon, allowing for 34′ of refraction and its apparent radius: the same definition the USNO uses. Civil, nautical and astronomical twilight start and end with the sun’s center at −6°, −12° and −18°. Golden hour runs from +6° to −4° and blue hour from −4° to −6°. Everything is worked out for the day from midnight to midnight in local time.

Sun and moon position. Azimuth and apparent elevation, with average refraction; for the moon, also from your exact spot (its parallax). An object’s shadow is its height divided by the tangent of the sun’s elevation, on flat ground. The moon’s illumination comes from Sun–Earth–Moon geometry. In the calendar, a supermoon is a full moon less than 360,000 km away, and a blue moon is the second full moon in a calendar month.

Day and night map. Night is drawn from the point where the sun is straight overhead: caps around the opposite point for sun elevations of −0.83° (sunrise and sunset), −6°, −12° and −18°. In live mode it updates once a minute and pauses when the tab is hidden.

Accuracy

Against 4,878 USNO sunrise and sunset times for 2026 in seven places, 1,460 for 1900 and 2100, 1,460 OAN times, 3,816 twilight times and 1,409 moonrise and moonset times, we’re never more than one minute off. All 50 moon phases of 2026 match the USNO to within a minute, and the four eclipses of 2026 match NASA: peak time within 1–4 seconds and each stage’s duration within 1.1 minutes. The sun’s position is within 0.16′ of USNO celestial navigation data, and the edge of night on the map within 0.3 km.

Known limits

• The horizon is a sea-level one: mountains and buildings aren’t included. Behind a ridge, the sun rises later and sets earlier.
• Refraction changes with temperature and pressure, and can shift the real sunrise and sunset by a minute or two.
• Pages for each place (“sunrise today in…”) live on our tide site, TidePlanet.

Time zones

Sources

• Time zone boundaries: timezone-boundary-builder 2026e by Evan Siroky, the variant with oceans and full IANA names (America/Denver, America/Phoenix, America/Indiana/Indianapolis). They come from OpenStreetMap and carry its license, the ODbL (© OpenStreetMap contributors). The simplified boundaries the site downloads, in the /datos/husos/ folder, are offered under the same license.
• Times, UTC offsets and daylight saving changes: the IANA time zone database built into your own browser.

How it’s calculated

The boundaries are simplified to 50 m and split into 430 tiles: to find a point’s zone, your browser downloads only its tile (a few KB) and looks it up in under a millisecond. The time, the UTC offset and the dates of clock changes aren’t stored in any table of ours: your browser computes them with the IANA database. The world map uses the same boundaries simplified to 3 km, and the US and Europe maps, to 400 m.

States split between two zones are found by checking a point every 0.015° inside each Census county, leaving out a 1.5 km strip along the edge. The meeting planner counts working hours Monday to Friday, and handles the 23- and 25-hour days when clocks change.

Accuracy

Against the original boundaries, with 20,000 test points near borders, the ones that land in the wrong zone are at most 46 m from the line. Times are only as good as your browser’s database: if a country changes its clocks at short notice, an out-of-date browser may lag behind.

Known limits

• In Xinjiang and some disputed territories (Abkhazia, the West Bank, Aksai Chin…) two times are used side by side. We give the official one and the page mentions the other.
• The US map leaves out the Aleutian Islands west of the 180th meridian.

Map lines

Sources

• Countries, lakes, seas, populated places and the date line: Natural Earth 1:10m, version 5.1.2 (public domain).
• Towns near the prime meridian and places on each side of the date line: GeoNames cities1000 (CC BY 4.0).

How it’s calculated

The latitude of the tropics and polar circles depends on the tilt of Earth’s axis. We compute it for January 1 of the current year (when the page is built and, on the map, in your browser) with the IAU 2006 formula for the mean obliquity of the ecliptic (Capitaine et al. 2003; IERS Conventions 2010), leaving out nutation, which moves it by ±9.2″ (about 280 m). The tilt shrinks by about 0.47″ a year, so the lines shift 14–15 m every year. Crossings, distances and the area of the polar caps were calculated with the latitude on January 1, 2026: the gap grows by about 15 m a year, far below the accuracy of the crossings.

Crossings come from cutting each line against the unsimplified Natural Earth polygons, and lengths and distances are measured on the WGS84 ellipsoid with GeographicLib. A line counts as passing through a country’s waters when it comes within 12 nautical miles of its coast (territorial waters) plus a 3 km margin, because those miles are measured from the baseline, not the shoreline.

Accuracy

Natural Earth borders can be a few hundred meters off and some islets are missing, so entry and exit points are good to about a kilometer. Each line’s list of countries matches the reference sources.

Known limits

• The International Date Line has no legal definition: its course at sea is approximate.
• With Natural Earth outlines, the Arctic Circle barely touches Grímsey, and Mirny Station in Antarctica falls 1.3 km south of the Antarctic Circle (other sources put it north).
• Natural Earth lacks Rolas islet, where the equator crosses São Tomé and Príncipe, so the country shows up “through its waters”.

Bearings, horizon and other whole-Earth math

Sources

• Magnetic declination: the World Magnetic Model WMM2025 from NOAA (NCEI) and the British Geological Survey (public domain), valid 2025–2030, with the coefficients from its official file.
• Earth’s layers and density: the Preliminary Reference Earth Model (PREM; Dziewonski and Anderson, 1981).
• Land or sea and countries: Natural Earth 1:10m; Census states and Spain’s IGN provinces for random points. Ground elevation: AWS Terrain Tiles, and the IGN MDT05 model in Spain.

How it’s calculated

Bearings. Start and arrival bearings of the shortest route are computed on the WGS84 ellipsoid with GeographicLib; the rhumb line (constant bearing) uses the isometric latitude and the exact meridian arc. Declination comes from WMM2025: a degree-12 spherical harmonic sum with the yearly change up to the chosen date. Magnetic bearing = true bearing − declination (east declination is positive).

Horizon. A sphere of 6,371 km. The distance to the horizon over the ground is R′·acos(R′/(R′+h)), and the part of a distant object hidden by the curve is R′(1/cos((D−d)/R′)−1). Refraction uses an effective radius R′ = R/(1−k) with the standard coefficient k = 0.13 used in surveying; near the ground it can range from −4 to +16 (Hirt et al., 2010).

Antipode and tunnel. The antipode of (φ, λ) is (−φ, λ±180°), which is exact. Land or sea is tested against Natural Earth countries, with a 1 km coastal margin. The figure on the page (13.1% of land has land at its antipode, ±0.03%) comes from a grid of 20 million evenly spaced points. The tunnel is the chord between the two points: its length, depth and dip are plain geometry; the layers it crosses and the frictionless fall time come from PREM. Through the center it takes 38 min 11 s, matching Klotz (2015); with uniform density, 42 min 10 s for any tunnel.

Random location. Uniform on the sphere: longitude 360u − 180 and latitude asin(2v − 1), so every square mile is equally likely (picking latitude evenly would crowd the poles). Inside a country, state or province, points are drawn in its bounding box until one falls inside.

Geographic center. Three centers on the sphere: the center of gravity (the mean of 3D vectors, weighted if you add weights), the point of least total distance (the geometric median, by Weiszfeld’s method) and the simple average of latitudes and longitudes, shown so you can see how it fails. An area’s center is its spherical centroid, from the exact first-moment formula of a spherical polygon.

Accuracy

Our WMM2025 code matches the official test values: the 12 points of the technical report’s table to the published rounding, and the 100 points of the long table to within 0.001 nT. The model itself has a global average declination error of 0.36° in 2025 (0.40° in 2030). The tunnel and horizon use a sphere: against the ellipsoid, the tunnel’s length changes by 0.6% at most.

Known limits

• Near the magnetic poles a compass can’t be trusted: we warn when the horizontal field drops below 6,000 nT (caution) and 2,000 nT (blackout). Nearby iron, cars or magnetic rocks also pull the needle.
• The horizon ignores any terrain in between, and real refraction changes day to day.
• There’s no single accepted definition of a geographic center (as the USGS points out): the result depends on the method and the outlines used.

National parks and map embeds

Sources

• US national parks: boundaries from the National Park Service Land Resources Division (public domain), official areas from the NPS Listing of Acreage of September 30, 2026 (gross acres), and the date each park was established from the NPS’s official index (The National Parks: Index 2012–2016) and its Recent Changes to the National Park System page (updated October 26, 2026), checked against the laws.
• Driving distance and time: OSRM on the FOSSGIS server, with OpenStreetMap data.

How it’s calculated

We measure the shortest geodesic on the WGS84 ellipsoid (GeographicLib) from your point to the nearest point of each park boundary (zero if you’re inside) and to a point inside the park. On request, OSRM routes to the road closest to that boundary point, assuming clear roads, for the 15 nearest parks you can drive to. Parks with no road access (from each park’s NPS directions page) and parks on other islands are flagged instead of routed.

Google Maps embed code. The code is built in your browser in one of three forms: the maps.google.com/maps?…&output=embed address, widely used though not in Google’s documentation; Google’s Maps Embed API with your own key, which never leaves your browser; or the OpenStreetMap embed, with its box worked out from the zoom and size. Nothing loads from Google or OpenStreetMap until you ask for the preview.

Accuracy

Park boundaries are simplified to 200 m, so the distance to a boundary can be off by about 200 m.

Meet in the middle, thematic maps, widgets and posters

Sources

• Travel times for meeting points: OSRM on the FOSSGIS public server, with OpenStreetMap data (ODbL). Places to meet (cafés, restaurants, parks, towns…): OpenStreetMap, read from the same OpenFreeMap tiles that draw the map.
• US thematic maps: median household income and median age from the American Community Survey 2020–2024 5-year estimates (tables B19013 and B01002), with their 90% margins of error, and population from the Census Vintage 2025 estimates (public domain).
• Spain thematic maps: mean net income per person and per household (2024) and mean age (people in private households on January 1, 2025) from INE’s Household Income Distribution Atlas (ADRH 2024, released October 19, 2026), and population from the municipal register as of January 1, 2025.
• Posters: OpenStreetMap data (ODbL) through OpenFreeMap vector tiles, with our own styles.

How it’s calculated

Meet in the middle. We try about 80 spots in two rounds: first a grid around the geodesic midpoint and along the line between people, then towns or places near the best one. For each spot OSRM gives the travel time from each person, and we pick the one with the smallest gap between the longest and shortest trip, as long as the longest trip is no more than 5% (or 2 minutes) over the shortest possible longest trip; if several have a similar gap, the one with the least total time wins. Times assume clear roads, with no traffic or public transit; bikes at about 9 mph and walking at about 3 mph.

Thematic maps. Each map sorts values into six classes by rounded quantiles on a light-to-dark green ramp. Counties and municipalities use the same simplified Census and IGN boundaries as the rest of the site’s maps.

Widgets. They use the same math as the full tools: the circle’s area on a sphere and distances on the WGS84 ellipsoid with GeographicLib. The distance widget gives the straight-line distance only. Blank-map widgets are pre-drawn SVGs from the printable blank maps.

Posters. To download one, a second, hidden map redraws the same framing at 300 DPI in pieces that line up to the pixel, and the tool itself writes a lossless PNG and PDF. On phones, A2 and 50 × 70 cm come out at a lower DPI to avoid running out of memory. Every poster carries the OpenStreetMap credit.

Known limits

• Meeting points take a few seconds: the public routing server handles one table of travel times about every 10 seconds.
• ACS figures are estimates with a margin of error, which is wide in small counties: the map shows it.

Terrain Tiles attribution

Terrain Tiles provide elevation outside Spain, every profile and the map’s hillshading. They were produced by Mapzen with its open-source Joerd software and are now published in the Amazon Web Services open data registry. They combine elevation models from many countries, and several of those licenses require showing this attribution list. Here it is, word for word:

Terrain Tiles sources, the area they cover and their license

Sources and licenses

Every data source and third-party program Mapsfera uses, with its license and the date or version of the data. Each name links to the source.

Data sources and software Mapsfera uses, with their licenses

How to cite us

If you use a number from Mapsfera in a paper, a class or an article, cite the tool and the date, for example: “Mapsfera, Distance between two points, mapsfera.com, accessed October 31, 2026.” Please also credit the original source named in each section, such as “U.S. Census Bureau,” “Data: IGN (CC BY 4.0)” or “© OpenStreetMap contributors.”

Something doesn’t add up? Let us know and we’ll look into it. Changes are logged on the Corrections page.

This page was last reviewed on October 31, 2026.