Latitude and Longitude

Part of Cartography & Surveying

How to measure and use geographic coordinates to locate positions on the Earth’s surface and connect local maps to the global framework.

Why This Matters

Latitude and longitude are the universal address system of the planet. Every point on Earth has a unique pair of values that cannot be confused with any other location. When two communities far apart need to coordinate — sharing maps, comparing observations, planning a meeting point — geographic coordinates make this possible without a shared local reference.

Beyond communication, latitude and longitude connect to navigation and astronomy in ways that local grid systems cannot. Your latitude tells you which stars pass overhead, the angle of the sun at noon, the length of day at the solstices, and the behavior of the seasons. Your longitude, once determined, allows you to synchronize with distant timekeepers. These connections give geographic coordinates a power that goes beyond cartography.

For communities at the technological frontier, the ability to measure latitude and longitude independently — without any electronic infrastructure — represents a high level of technical capability with broad practical benefits.

Understanding the System

The Earth is treated as a sphere (approximately) divided by two families of circles:

Parallels of latitude: Imaginary horizontal circles parallel to the equator. The equator is 0°. The poles are 90°N and 90°S. Each degree of latitude represents about 111 kilometers on the Earth’s surface, regardless of where you are. This consistency makes latitude straightforward to measure and use.

Meridians of longitude: Imaginary vertical circles running from pole to pole. The prime meridian (0°) was historically set through Greenwich, England, but any meridian can serve as local zero. Longitude extends from 180°W to 180°E. Unlike latitude, the distance represented by one degree of longitude varies by latitude: at the equator it is about 111 km; at 45° it is about 79 km; at the poles it collapses to zero.

Measuring Latitude

Latitude can be measured accurately with simple instruments anywhere in the world.

Northern hemisphere — Polaris method: Polaris (the North Star) sits within about 1° of the true north celestial pole. Its altitude above the horizon equals your latitude, accurate to within 1° without compensation for Polaris’s slight offset from true pole.

To measure:

1. Construct a simple quadrant: a quarter circle (90°) of wood or metal with a degree scale, a plumb bob or plumb line hanging from the center, and a straight sighting edge.
2. At night, sight along the straight edge to align with Polaris.
3. Read the angle between the sighting edge and the plumb vertical.
4. This angle is your latitude.

For better accuracy, measure at the moment when Polaris is directly above or below the celestial pole (called upper and lower transit) and average the two readings. The correction for Polaris’s offset from the true pole is approximately 0.7°, varying in a 26,000-year cycle — for practical purposes, treat Polaris as the true pole.

Solar noon method (any hemisphere):

1. Find the exact time of solar noon (when shadows are shortest and point due north or south).
2. Measure the sun’s altitude at that moment using a sextant, astrolabe, or simple shadow-stick gnomon method.
3. Latitude = 90° − solar altitude + solar declination.

Solar declination varies throughout the year (the sun’s position north or south of the equator). It is 0° at the equinoxes, +23.5° at the June solstice, and −23.5° at the December solstice. A table of solar declination by date is essential for this method.

Simple shadow method: Build a vertical pole (gnomon) of known height. At noon, measure the length of its shadow. The angle of the sun above the horizon = arctan(pole height / shadow length). Adjust for solar declination to get latitude. Accurate to about 1°.

Measuring Longitude

Longitude measurement is fundamentally a problem of time. The Earth rotates 15° of longitude per hour, so a 1-hour difference in local time corresponds to a 15° difference in longitude.

The challenge: You need to know the exact time at a reference meridian (e.g., Greenwich) while simultaneously observing local solar time. Without an accurate clock, this is very difficult.

Lunar distance method (historical): The Moon moves against the background stars predictably. By measuring the angular distance from the Moon to a known star and comparing it to a precomputed table, you can determine the time at Greenwich to within a few minutes, giving longitude to within 1°. This method requires a sextant, a table of lunar distances (precomputed for the current year), and careful arithmetic. It was the method used by navigators before accurate chronometers.

Pendulum clock method: A well-regulated pendulum clock keeps time accurately enough to measure longitude if wound carefully and compared with solar noon observations at each location. The clock is set to noon at a known location, then transported (carefully — pendulum disturbed by movement). At the new location, solar noon is observed. The time on the clock at the new local noon gives the longitude difference.

Practical reality for local mapping: For most community mapping purposes, precise longitude is unnecessary. A local grid oriented to true north serves all practical needs. Longitude determination requires significant investment in instruments and skill, and pays off only when coordinating with distant communities or reconnecting to a global knowledge system.

Degrees, Minutes, and Seconds

Geographic coordinates are expressed in degrees, minutes, and seconds (DMS) or decimal degrees.

DMS: 1 degree = 60 minutes; 1 minute = 60 seconds. A location at 41°15’37”N, 44°48’12”E is 41 degrees, 15 minutes, 37 seconds north latitude and 44 degrees, 48 minutes, 12 seconds east longitude.

Decimal degrees: 41.2603°N, 44.8033°E. Useful for arithmetic. Convert: decimal degrees = degrees + (minutes/60) + (seconds/3600).

Precision: 1 degree of latitude ≈ 111 km. 1 minute ≈ 1.85 km. 1 second ≈ 31 m. For most practical purposes, recording to the nearest minute (1.85 km precision) is adequate for community-level planning. Property surveys and precise engineering require arc-second precision.

Recording and Using Geographic Coordinates

Format consistency: Always record North/South for latitude and East/West for longitude explicitly. A coordinate written as “41.26, 44.80” is ambiguous without hemisphere labels.

Local maps: Mark your community’s geographic coordinates prominently on every local map, along with the map’s coordinate system. This allows future users to relate your local grid to the global system.

Communication: When sending geographic coordinates to another community by messenger or radio, use a phonetic protocol to prevent transcription errors: “Four-one degrees, one-five minutes north; four-four degrees, four-eight minutes east.” Read back and confirm.

Star tables and almanacs: For precise astronomical methods of latitude and longitude measurement, tables of star positions, solar declination, and lunar distances are invaluable. Preserving or reconstructing such tables should be a priority for any community with serious navigational ambitions. They can be computed from first principles by those with advanced mathematical knowledge, or extracted from pre-collapse publications.

The ability to locate yourself precisely on the planet — using the sky as your instrument — is one of the clearest markers of scientific civilization.