Coordinates and height

A position arrives as 35°40'12.5", the map wants 35.670139, and a survey drawing speaks in X and Y. Some of these are only different ways of writing the same thing. Others rest on a different foundation, and mixing those up moves you hundreds of metres.

Degrees, minutes, seconds and decimal degrees

These are two notations for the same position. Divide the minutes by 60 and the seconds by 3600, then add: 35°40'12.5" becomes 35 + 40/60 + 12.5/3600 = 35.670139°. Going back, take the fraction and multiply by 60 twice.

There is a third notation you will meet in GNSS and at sea: degrees and decimal minutes, written without seconds. 3540.1234 means 35° 40.1234'. The first two digits are degrees for latitude and the first three for longitude.

Reading 3540.1234 as 35.40° is the single most common mistake here. It puts you about 30 km south of where you are.

A different datum moves you 400 metres

Latitude and longitude only mean something once you decide which ellipsoid stands for the Earth and where it sits. That decision is the datum, and countries have changed theirs.

Japan switched in 2002. The same physical place differs by roughly 12 arcseconds in both latitude and longitude between the old and the current datum, which is 400 to 450 metres on the ground.

So a coordinate from an old map, an old register or an old paper cannot be typed into a modern map as it stands. The clue is the year: anything from before the change deserves suspicion. If the source does not say, convert one known landmark and see whether it lands where it should.

Plane rectangular coordinates and UTM

Latitude and longitude are angles on a curved surface, so distances and areas are awkward to compute with them. Surveying and design therefore work in metres on a plane.

Japan divides the country into 19 zones, each with its own origin. Two things catch people out. X runs north and Y runs east, the opposite of a maths graph. And the same pair of numbers means a different place in a different zone, so a drawing without its zone number is unusable.

Outside Japan, UTM is the common scheme: the world is cut into 6-degree bands of longitude and each is flattened separately.

The height a receiver reports is not the height on a map

GNSS measures height above the ellipsoid, the mathematical surface of the datum. Maps print height above mean sea level. The difference between the two surfaces is the geoid height, and in Japan it runs 30 to 40 metres.

elevation = ellipsoidal height − geoid height

The difference varies from place to place, so you cannot subtract one fixed number. National agencies publish a geoid model and conversion software reads it.

Whether your receiver is showing one or the other depends on the device and its settings. In NMEA the two are separate fields, so keep both and you will not be stuck later.

Mistakes that look correct

Latitude and longitude swapped. In Japan latitude runs 20 to 46 and longitude 122 to 154; outside those ranges they are the wrong way round.

Signs forgotten. South and west are negative. Working only with domestic data, you never notice.

X and Y swapped in plane coordinates. North and east change places.

Ellipsoidal height used as elevation. Thirty to forty metres of error, and every number still looks plausible.

Every one of these produces values that look right. So after any conversion, put one point on a map and look at it. A single check catches all of them.