Why six decimals is harder than it looks

Two rules meet at the sixth decimal place, and they point in opposite directions.

The regulation defines geolocation as coordinates "using at least six decimal digits" (Regulation (EU) 2023/1115, Art. 2(28)): a floor. The Information System's validation rules say that if there are more than six, "the system truncate the value to 6 decimals" (Information System validation rules, Geolocation invalid coordinates): a ceiling. So six is the only precision that survives the trip unchanged.

What six decimals means on the ground

A degree of latitude is about 111 km everywhere; a degree of longitude shrinks towards the poles, so these are the worst case.

Too many: truncation makes duplicates

GPS apps and drawing tools often write ten decimals or more. Cut to six, two points closer than about 11 cm can become the same point. The specification gives an example (GeoJSON File Description, Common errors 7):

-5.8227391234, 144.2567071234  →  -5.822739, 144.256707
-5.8227394567, 144.2567074567  →  -5.822739, 144.256707

Two consecutive identical points in a polygon can break it: a side of zero length, or too few distinct corners. That is Two points in a row become identical at six decimals (COORD-005).

Rounding or truncating? Version 1.5 of the specification called this "rounding" while showing truncation in its example (GeoJSON File Description v1.5, §6.7); the current text says truncation. The difference matters: -5.8227396 rounds to -5.822740 but truncates to -5.822739. The checker flags a pair if either would collapse it, and its fix truncates, so the file you download already says what the system will store.

Too few: JSON hides trailing zeros

Here is the trap in the other direction. A coordinate measured as -22.738700 is written by almost every program as -22.7387, because JSON numbers do not keep trailing zeros. It looks like four decimals. It is six.

Roughly one coordinate in ten ends in a zero at the sixth place. A check that counts decimals coordinate by coordinate therefore complains about nearly every correct file, even the Commission's own example (see the teardown).

The checker judges a plot by its most precise coordinate instead. If any coordinate in the plot has six decimals, the plot was recorded at six. If none does across a whole boundary, the data really was rounded somewhere, and that is Coordinates carry fewer than six decimal places (COORD-004), a warning, because we cannot put the digits back.

In practice

• Export with at least six decimals. More is harmless but wasted.
• Do not pad short coordinates with zeros; it hides lost precision rather than restoring it.
• Run the precision checker on files from new sources.