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If you’ve ever opened a map file and seen your data in the middle of the ocean, this page is for you.

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The basic idea

Every point on a map is a pair of numbers: longitude and latitude (or x and y, or easting and northing, depending on who wrote the file). But those numbers only mean something once you know the rules the file is using to interpret them. A point at “5234000, 6321000” could be:
  • A spot in Berlin (in the German official system)
  • A spot in California (in a US state plane system)
  • Nonsense (if the file thinks those are GPS coordinates)
The “rules” are what we call a coordinate system. A file should declare which one it uses. If it doesn’t, software has to guess, and the guess is usually wrong.

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WGS 84: the one most files use

If you’ve ever looked at GPS coordinates (“the cafe is at 52.5170, 13.3889”) you’ve seen the most common system. It’s called WGS 84 (or “EPSG:4326” if you read technical docs). It uses degrees of longitude and latitude. Range: longitude -180 to 180, latitude -90 to 90. Google Maps, GPS phones, and most online tools use it by default. Most files Georaf works with should be in WGS 84. If yours is, you’re set, everything just works.

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Projected and local systems

Local mapping authorities, surveyors, and engineers often use projected systems instead. Those use meters or feet, measured from a regional reference point. Examples:
  • EPSG:25832 (UTM zone 32N), common across Western Europe
  • EPSG:31370 (Lambert), Belgium
  • EPSG:2154 (Lambert-93), France
  • EPSG:3857 (Web Mercator), what tile-based web maps use internally
They exist because doing math on degrees is awkward. A degree of longitude in Norway covers fewer kilometers than a degree in Kenya. For surveying and construction, meter-based local systems are simpler to work with.

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What can go wrong

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”My data is in the ocean”

Your file is in a projected (meter-based) system, but it doesn’t say so. Software treats the meter values as if they were degrees, and 5,234,000 “degrees” puts you somewhere in the middle of nowhere. Fix: find out which system the file is supposed to be in, set it explicitly, and convert to WGS 84 before uploading.

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”Latitude and longitude are swapped”

Some formats list latitude first, others list longitude first. If they get swapped, your London data ends up near the equator off the coast of Africa. Fix: open a few rows in a text editor and check which number looks more like latitude (between -90 and 90). Then re-import accordingly.

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”It worked yesterday, it’s broken today”

Some tools guess the coordinate system from the file extension or contents. A file that opened correctly in one tool can display wrong in another. Fix: don’t rely on automatic detection for anything that matters. Always set the coordinate system explicitly.

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What Georaf does

When you upload a file, we check whether a coordinate system is declared. If not, you’ll see a failing health check, because we know that’s a recipe for trouble. If a system is declared but it isn’t WGS 84, we quietly convert the data to WGS 84 inside Georaf so it shows up correctly on the map. Your original file is never touched.

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When to worry, when not to

  • You got the file from a familiar source (Google Maps export, GPS app, OSM extract): probably WGS 84, probably fine.
  • You got the file from a surveyor, government agency, or engineering firm: ask what coordinate system they used. If the file doesn’t say, ask before uploading.
  • The map looks empty or your entries are in the wrong country: 99% of the time, this is a coordinate system mismatch.