How to Write Coordinates

Producing a coordinate is more constrained than reading one. When you read, you get whatever the source produced and have to make sense of it. When you write, you pick the format, the precision, and the convention — and downstream consumers depend on all three being internally consistent. This article walks through each choice and the conventions of the major domains. The matching read-side guide is at /learn/how-to-read-coordinates; the format-by-format definitions are at /learn/coordinate-formats-explained.

Choose the format

The choice of format depends on the downstream consumer. Three rules of thumb:

Default to decimal degrees for software, databases, APIs, and any context where downstream consumers are programs rather than humans reading print. Every modern stack supports DD natively; signed numbers sort and compute cleanly; no parsing quirks around degree symbols or hemisphere letters. The NGS NCAT tool treats DD as canonical.

Use DMS for surveying records, paper topographic maps, or contexts where the existing convention is base-60. Surveying datasets typically expect arcseconds; many paper-derived records are in DMS even when the modern digital reproduction is in DD.

Use DDM for marine and aviation work. Both domains inherited celestial-navigation conventions where arcminutes are the natural unit (one arcminute of latitude is one nautical mile by definition). FAA flight plans, marine GPS displays, and most maritime publications use DDM.

Use UTM when you need direct metric measurements within a region — topographic mapping, GIS analysis within a single zone, surveying within a single zone. UTM gives easting and northing in metres, so distance computation within a zone is direct.

Use MGRS when you need compact, radio-friendly grid references — military operations, search-and-rescue, anything spoken aloud. MGRS encodes precision in digit count, so the user can shorten the reference to the operationally useful resolution.

Use Plus Codes when integrating with Google's ecosystem or for places that lack street addresses. Plus Codes are compact, the format is documented openly, and Google Maps supports them as a first-class input.

There is no universally right choice. The wrong choice is one that the downstream consumer can't parse, or one that loses precision the source recorded.

Choose the precision

Precision is how many digits the recorded coordinate carries. The honest rule: match precision to the source's accuracy. A coordinate with more decimal places than the source supports records false precision, which downstream consumers may interpret as more accuracy than the source genuinely has.

For coordinates sourced from a smartphone GPS — accurate to about 4.9 m under open sky per GPS.gov:

• DD: 5 or 6 decimal places. Beyond 6 is decoration.
• DMS: arcsecond decimals to 0.1 (~3 m at the equator) or 0.01 (~30 cm). The latter is borderline false precision for smartphone GPS but matches DD-6-decimal.
• DDM: three decimal places of arcminutes (~1.85 m at the equator).
• UTM: whole metres.
• MGRS: 5+5 digits (1 m precision). For radio communication, 4+4 (10 m) or 3+3 (100 m) is often more useful.
• Plus Code: length 10 (~14 m). Length 11 (~3 m) is the maximum honest precision from a smartphone GPS.

For coordinates from address geocoding — typically accurate to 5–30 m for well-known addresses — DD with 5 decimal places is enough. Six decimal places is borderline; seven is decoration.

For surveyed monument coordinates — sub-metre accuracy — DD with 7 decimal places is honest, and UTM at centimetre precision is acceptable.

For continental-scale references — a country, a region — DD with 1 to 3 decimal places is plenty. An MGRS 2-digit reference (10 km square) is appropriate at this scale.

The general principle is the same across formats: precision in the written value should not exceed the precision the source genuinely resolves. A reader who sees a 7-decimal coordinate will treat it as if the source supports 7-decimal accuracy. Don't encourage that assumption when it's false.

Format the components correctly

Each format has typographic conventions worth getting right.

Decimal degrees. Latitude first, longitude second (ISO 6709). A comma between them, with optional space: 40.7484, -73.9857. Sign in front of the digit, no whitespace between sign and number (write -73.9857, not - 73.9857). Decimal point (.) for English-language data; continental European data uses decimal comma (,) and a non-comma pair separator (semicolon, pipe, or slash). Always be explicit about locale when sharing data across borders.

Degrees, minutes, seconds (DMS). Use the degree symbol ° (not the letter o), the prime ′ (or ASCII apostrophe ') for arcminutes, and the double-prime ″ (or ASCII double-quote ") for arcseconds. Avoid curly typographic quotes (the auto-corrected ' ' " " that word processors produce) — many coordinate parsers reject them silently. Hemisphere letter immediately after the number with no space: 40°44'54.24"N, not 40°44'54.24" N. Zero-pad arcminutes and arcseconds to at least two digits before the decimal: 40°04'05.10"N, not 40°4'5.1"N. The integer-degree component is typically not zero-padded for latitude (one or two digits) but is zero-padded to three digits for longitude in some contexts (073°59'08.52"W).

Degrees, decimal minutes (DDM). Same degree symbol and prime as DMS, with a decimal arcminute: 40°44.904'N. Zero-pad the integer- minute component to two digits. The minute decimals typically run to three places (1.85 m precision at the equator).

UTM. Zone number followed immediately by band letter, no space: 18T. Then easting in metres, then northing in metres, separated by spaces: 18T 585628 4511322. Always positive values (UTM uses false northing on the southern hemisphere to keep all values positive). Whole metres is the convention; centimetre sub-metre is acceptable for surveying.

MGRS. Zone + band + 100-km square letters, all uppercase. Spaces between groups are optional but help readability: 18TWL8562811322 or 18T WL 85628 11322. The total digit count must be even (5+5 = 1 m, 4+4 = 10 m, 3+3 = 100 m, 2+2 = 1 km, 1+1 = 10 km, 0 = 100 km).

Plus Code. 8 alphanumeric characters, + separator, 2–7 more characters. Uppercase only. Alphabet is 23456789CFGHJMPQRVWX — the omitted letters avoid visual ambiguity (no 0/O, no 1/I/L, no lowercase). The + is always after the 8th character, never elsewhere.

Worked examples by domain

A row in a CSV database

id,name,latitude,longitude
1,Empire State Building,40.7484,-73.9857
2,Eiffel Tower,48.8584,2.2945
3,Sydney Opera House,-33.8568,151.2153

Decimal degrees. Signed numbers. Five decimal places — about 1 m precision at the equator, adequate for most database purposes, saving storage relative to higher precision while supporting whatever downstream formatting is needed. The header row uses spelled-out latitude and longitude to make the order unambiguous.

A coordinate in a journalistic article

The library is at 40.7484°N, 73.9857°W (40°44'54"N, 73°59'09"W).

Either form alone is reader-friendly. The DMS form may resonate with older readers and with maps; the DD form is shorter and supports easy copy-paste into a tool. Including both is acceptable in long-form journalism; in headlines, pick one.

A waypoint in a flight plan

WPT: KJFK (John F. Kennedy International)
LAT: 40°38.4'N
LON: 073°46.7'W

DDM, the FAA convention. Longitude is zero-padded to three digits (073, not 73). Arcminute decimal at one place — appropriate precision for flight navigation; aviation accuracy doesn't require sub-metre at the operational level.

A search-and-rescue datum point

Initial datum point: 18T WL 8562 1132

MGRS. The 4+4-digit form gives 10 m precision — appropriate for the operational tempo and the way the coordinate will be spoken aloud over a radio. The two-letter 100-km square (WL) plus the spoken format (“one-eight-tango whiskey-lima eight-five-six-two one-one-three-two”) is shorter than the equivalent DD and unambiguous over a noisy channel.

A scientific paper coordinate

The seismometer is at 35.6764°N, 139.6500°E (WGS84).

Five-decimal DD, hemisphere letters for readability, datum stated explicitly. The datum statement matters for scientific reproducibility — a future replication relies on the reader being able to interpret the angular values against the same reference frame. In everyday journalism the datum statement can be omitted; in scientific publication it cannot.

A GeoJSON Feature

{
  "type": "Feature",
  "geometry": {
    "type": "Point",
    "coordinates": [-73.9857, 40.7484]
  },
  "properties": { "name": "Empire State Building" }
}

GeoJSON uses longitude first, contrary to ISO 6709 — this is the RFC 7946 convention that comes from X-then-Y compatibility with older GIS systems. Documenting the order in the GeoJSON spec is the only defence against integration bugs; the order is hard-coded into the format.

Common gotchas when writing

Flipping latitude and longitude. The single most common bug. Test: if your “latitude” value exceeds 90 in magnitude, the pair is flipped (longitudes can be up to 180; latitudes are bounded to ±90). Adding explicit lat: and lon: labels in machine-readable formats helps; spelling out latitude,longitude in CSV headers helps. The deep treatment lives in /learn/why-latitude-comes-first (when shipped).

Smart-quote autocorrect. Word processors auto-correct ASCII apostrophes and quotes to curly typographic forms (the autocorrected ' ' " "). These curly quotes are not valid DMS notation and many coordinate parsers reject them silently. Disable autocorrect in the editor or paste through a plain-text utility before producing DMS coordinates.

Mixing hemisphere letters and signs. +73.9857°W is contradictory — the positive sign claims east, the W claims west. Pick one convention per dataset and stick to it. Mixed-convention coordinates that pass through one parser may break another.

False precision. A 7-decimal smartphone-GPS reading carries decoration in the 7th place. Truncate to 5 or 6 decimals before sharing. The full treatment is in /learn/precision-vs-accuracy-in-coordinates (when shipped).

Missing hemisphere indicator. 40.7484, 73.9857 without sign or letter is ambiguous — it could be in any of four hemispheres. Always include a sign or a letter.

Datum unstated when it matters. For surveying, legal-boundary, or scientific work, state the datum explicitly (WGS84, NAD83, etc.). For everyday use, WGS84 is the safe default and the statement can be omitted, but be aware that any reader who needs the datum will assume WGS84 silently.

Decimal separator locale mismatch. English: 40.7484. Continental European locales: 40,7484. The pair separator then has to be something other than comma — typically a semicolon or pipe. Be explicit about locale when sharing data across borders, and prefer the English convention for international datasets.

Zone forgotten on UTM. Without the zone, “easting 585628 northing 4511322” is ambiguous: the same metric coordinates describe different physical points in different zones. Always include the zone + band prefix when writing UTM.

Common misconceptions

More decimal places does not mean a more accurate coordinate. It means a more precise representation, which may or may not exceed the source's actual accuracy. A smartphone-recorded 40.7484170°N, -73.9857140°W is precise to centimetres but accurate to ~5 m — the last two digits are computational artifacts.

There is no universal “correct” format. ISO 6709 specifies three angular formats (DD, DMS, DDM); UTM, MGRS, and Plus Codes are governed by separate specifications. The right format for your data depends on the downstream consumer, not on any global standard ranking.

Decimal degrees is not “modern” and DMS is not “old.” Both are actively used today. DMS appears on every USGS topographic quadrangle, in every surveying record produced today, and in many scientific publications.

Coordinates always carry a datum context, even when unstated. The implicit datum is WGS84 in modern contexts. A coordinate without a stated or implicit datum is uninterpretable for any precise use.

Spaces and punctuation matter to parsers, even when they don't matter to readers. A parser may accept 40°44'54.24"N and reject 40 ° 44 ' 54.24 " N even though the two look semantically identical to a human. Match the input expectations of your downstream consumer, especially when generating data programmatically.