How to Read GPS Coordinates: Formats and Safe Sharing

How do you read GPS coordinates?

Read GPS coordinates by identifying the format, the latitude and longitude, and their hemisphere letters or signs. Latitude describes north or south; longitude describes east or west. Keep the coordinate format and map datum with the numbers when sharing them. A correctly copied position does not establish a safe or legal route to it: use current official information, appropriate navigation skills and backup navigation.

You do not need to become a surveyor to make this less confusing. Start by separating three questions: what do these numbers mean, how are they written, and what position does the device actually represent?

Which number is latitude, and which is longitude?

Latitude measures position north or south of the equator. Longitude measures position east or west of the prime meridian. The NOAA latitude explanation describes degrees, minutes and seconds; its longitude explanation explains the east-west coordinate.

In signed decimal degrees, north and east are positive; south and west are negative. Thus a negative longitude indicates west, not south. A minus sign belongs to its own number.

Label the values when you share them. Google Maps search expects latitude first and longitude second, but the receiving system's field labels and documentation should determine how you enter a coordinate. Do not assume that an unlabeled pair from another file uses the same order.

A useful first check is whether the values are possible:

• Latitude must be between -90 and 90 degrees.
• Longitude must be between -180 and 180 degrees.
• In degrees-minutes-seconds notation, minutes and seconds must each be less than 60.

The USGS coordinate data dictionary documents those limits. Passing them only means a coordinate is numerically possible. Swapped numbers can sometimes both fall within the allowed ranges while pointing somewhere entirely different.

What are the three common coordinate formats?

Degrees can be subdivided in different ways. One degree contains 60 minutes, and one minute contains 60 seconds. These are angular units, not elapsed time.

The following is an arithmetic-only training example. It is not a trail recommendation, meeting point or place to visit. All three rows express the same coordinate pair.

The location of the decimal point matters. In DDM, the decimal belongs to the minutes. In DD, it belongs to the degrees. Never remove the degree and minute symbols and join the remaining digits together.

A value such as 40.30 degrees means 40 degrees plus 0.30 of a degree. It therefore equals 40 degrees 18 minutes, not 40 degrees 30 minutes. That difference comes from multiplying 0.30 by 60.

Keep the original text or screenshot before changing formats. If something goes wrong, you need the source coordinate, not several successive conversions whose starting point has disappeared.

How do you convert degrees, minutes and seconds?

For the magnitude of a coordinate, use:

Decimal degrees = degrees + minutes ÷ 60 + seconds ÷ 3,600.

Then apply the hemisphere: south or west becomes negative in signed decimal notation. Apply that sign to the whole result.

For the example latitude:

40 + 30 ÷ 60 + 18 ÷ 3,600 = 40 + 0.5 + 0.005 = 40.505.

For the example longitude:

105 + 15 ÷ 60 + 36 ÷ 3,600 = 105 + 0.25 + 0.01 = 105.26.

Because that longitude is west, its signed result is -105.26.

To obtain DDM instead, convert only the seconds into a fraction of a minute. Eighteen seconds becomes 0.300 minutes; 36 seconds becomes 0.600 minutes. Add those fractions to the existing minutes, leaving the degrees unchanged.

You can check the latitude conversion backwards: 0.505 × 60 gives 30.3 minutes. The whole minutes are 30; the remaining 0.3 × 60 gives 18 seconds. This is a useful desk check, not a task to prioritize during an emergency.

When using a converter, select the input format explicitly. A tool that accepts your numbers without an error has not necessarily interpreted them as you intended.

How should you enter a coordinate into a map?

Use this check before relying on a saved waypoint:

1. Copy the original coordinate, including hemisphere letters or minus signs.
2. Identify DD, DDM or DMS. Check which unit contains the decimal.
3. Record the stated datum, if supplied.
4. Read the destination fields or search instructions. Enter each value in its required format and order.
5. Check the plotted region, then compare the position with relevant mapped landmarks.
6. Keep the original alongside the saved waypoint so another person can verify it.

Google Maps' computer instructions accept DD, DDM and DMS searches. They specify latitude before longitude and decimal points rather than decimal commas. Use the documented syntax; a comma separating the two coordinates is different from a comma inside a decimal number.

Those instructions also explain how to right-click a map position to retrieve coordinates. That produces the coordinate of the selected map point. It does not establish that you are standing there.

If a result falls in an unexpected country, hemisphere or part of a valley, stop and recheck the source. Do not repeatedly change signs or swap numbers until a pin looks plausible. A plausible pin can still be wrong.

Does changing the coordinate format change the datum?

No. Changing between DD, DDM and DMS changes how you write the angles. A datum or reference framework concerns how positions relate to the Earth.

Ordnance Survey distinguishes conversion from transformation: changing the representation is not the same operation as changing the reference framework. Its examples include GPS's WGS84 framework and the OSGB36 framework associated with the British National Grid.

Look for the datum in the map margin, file documentation or app's relevant settings. Preserve the stated datum when sharing coordinates. If it is unknown, say so rather than labeling the numbers WGS84 because they came from a digital device.

If the source and destination use different datums, use an appropriate documented transformation or get help from the mapping authority. Simply changing a label does not transform the position. The USGS explanation of NAD27 and NAD83 shifts illustrates why this is not merely a formatting detail.

A grid reference is also not automatically latitude and longitude. If your map supplies grid coordinates, retain its reference-system information and use instructions for that system.

Do more decimal places mean a more accurate position?

No. Extra decimal places describe how finely a number is written; they do not prove that the underlying position was measured accurately. Adding zeros changes neither the position nor the quality of the measurement.

A correctly converted coordinate can still represent a misplaced pin, an old location or an uncertain GPS fix. Check what the app is displaying: a selected map point, a saved waypoint, a current device position or a last-known position. Use the app's documentation where those indicators are unclear.

The archived GPS.gov accuracy explanation identifies blocked and reflected satellite signals among sources of positioning error. Buildings and trees can affect reception. A detailed display should not be treated as proof that these effects are absent.

Do not move toward an edge, into traffic or onto unsafe terrain to improve reception. Remain in a safe position and use other navigation information. If the coordinate and your surroundings disagree, treat the disagreement as unresolved.

What should you include when sharing a hiking position?

A useful location message identifies the numbers and their context. Copy this structure into your trip notes:

• Latitude and longitude, explicitly labeled.
• Format: DD, DDM or DMS.
• Datum, if known; otherwise marked unknown.
• Time of the reading, including the relevant time zone.
• Whether it is your current position, last-known position or a selected map point.
• Trail or park name, nearby identifiable landmarks and any uncertainty.

This separates the intended destination from the sender's actual position. A waypoint named “meeting place” and a live location reading answer different questions.

Before the trip, agree how your group will exchange positions and practice interpreting them. Check a familiar location from a safe place, using the original coordinate and the receiving map. The purpose is to catch misunderstandings before you need the information outdoors.

What if you need to give coordinates in an emergency?

Seek help promptly; do not delay an emergency call to convert a coordinate. In the United States, call 911 when emergency assistance is needed; elsewhere use the local emergency number. Read the display carefully, including labels, signs or hemisphere letters, and follow the dispatcher's instructions.

The National Park Service emergency guide advises sharing coordinates, landmarks and trip information when lost. Explain whether your position is current or last known and whether you are uncertain about it. Also give the trail or park name and other requested details.

Its guidance for someone lost who cannot safely retrace their steps includes staying put unless there is an immediate threat to life. Follow emergency responders' directions. This coordinate guide is not a complete emergency plan.

What should you prepare beyond the coordinates?

A position is one part of navigation. It does not identify a permitted approach, confirm a bridge is open or show that a path is passable today.

Use our GPX checking guide to examine a route file separately, and offline map preparation guide to test your map coverage before departure.

The National Park Service Ten Essentials includes navigation equipment and knowing how to use it. Carry appropriate backup navigation and power, check current official conditions and leave a trip plan with a trusted person. Learn and practice the necessary map and compass skills before relying on them outdoors.

Sources

• NOAA: What is latitude?
• NOAA: What is longitude?
• USGS: Commercial data coordinate dictionary
• Google Maps: Find or search for coordinates
• Ordnance Survey: Geodesy and positioning FAQ
• USGS: NAD27 to NAD83 shift
• GPS.gov archive: GPS accuracy
• National Park Service: Outdoor emergency plan
• National Park Service: Ten Essentials