Longitude and latitude are measures of locations on the globe. If you know how to read longitude and latitude on a map, you can determine the geographic coordinates of any point on the map. While online maps will make it easy to determine longitude and latitude in just one click, sometimes working on them on paper can help. To be able to read longitude and latitude correctly first understand the concepts behind these measurements. After understanding the basics, learn how to identify longitude and latitude on a map and pinpoint the exact point of a location on earth.
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Part 1 of 2: Understanding Longitude and Latitude
Step 1. Get to know the concept of latitude well
Latitude is an imaginary line that determines the distance of a location north or south of the equator, which is an imaginary horizontal line around the earth's midpoint between the poles. The Earth is divided into 180 latitudes on both sides of the equator, which are called parallels. These parallels extend horizontally around the earth parallel to the equator. Half of these 180 lines are north of the equator, while the remaining half are south.
Step 2. Learn the definition of longitude
Longitude is an imaginary line that determines the distance of a location to the east or west of the prime meridian, which is a vertical imaginary line that extends down the center of the globe from the North Pole to the South Pole. Longitude is a series of vertical lines from the North Pole to the South Pole, which are also called meridians because every place touched by the same meridian has the same midday. There are 360 meridians on both sides of the prime meridian; half is east of the prime meridian, and the remaining half is west of the prime meridian.
The meridian on the opposite side of the earth from the prime meridian is called the antimeridian
Step 3. Check the units of measurement used for longitude and latitude
The units for longitude and latitude are usually degrees (°), minutes (′), or seconds (″). The entire distance from one parallel to the other is 1°. To produce more accurate measurements, each degree can be further divided into 60 minutes, and each minute can be divided into 60 seconds (for a total of 3,600 seconds per degree).
Longitude and latitude are measured in degrees instead of absolute units of measurement (e.g. kilometers or miles) because the earth is spherical. Although the distance between degrees of latitude is always constant (111, 112 km or 60 nautical miles), the shape of the earth causes the distance between degrees of longitude to decrease as you get closer to the poles
Step 4. Measure the longitude and latitude relative to point 0
When measuring latitude in both directions, the equator is the starting point, aka 0° latitude. Similarly, the prime meridian is the starting point of longitude, aka 0° longitude. Each longitude and latitude measurement is expressed according to the distance from the starting point for both directions.
- For example, the North Pole is 90° N, which means it is 90° north of the equator.
- The antimeridian is 180° from either the west or east of the prime meridian.
- The Great Sphinx of Giza in Egypt is at 29°58′31″N, 31°8′15″E. This means that this historic building is 30° north of the equator, and about 31° east of the prime meridian.
Part 2 of 2: Determining Longitude and Latitude Coordinates on a Map
Step 1. Find a map with longitude and latitude
Not all maps include longitude and latitude. You're more likely to find it on a large area map, such as an atlas, or on a small map designed to display the terrain with great accuracy, such as a topographical map. If you are in the US, you can find detailed topographic maps for most areas through the US Geological Survey.
Step 2. Determine the location point you want to know
Look at the map and find the area or feature you want to know the coordinates for. Mark the dot with a small pencil or pin.
Step 3. Find the longitude and latitude markers
Latitude is depicted on a map as a series of equidistant horizontal lines extending from one side of the map to the other, while longitude is shown as a series of equidistant vertical lines extending from top to bottom. Look for numbers along the edges of the map to find the coordinates of each line. These numbers are called “graticule”.
- The latitude graticule is marked along the western and eastern edges of the map. The longitude graticule is marked along the northern and southern edges of the map.
- Depending on the scale of the map, the graticule may be written as a fraction of a degree instead of a full degree. For example, there may be a Graticule mark every minute in degrees instead of every degree (for example, 32°0′, 32°1′, and so on).
- The map also tells us that the latitude and longitude listed are relative to the prime and equatorial meridians, respectively (for example, North or South, East or West).
- Try not to confuse longitude and latitude with UTM lines, another type of coordinate system commonly found on maps. UTM numbers are usually written in small text (and without the degree symbol) along the edges of the map, and UTM gridlines can be written using a different color than latitude and longitude.
Step 4. Use a ruler to mark the latitude of the measured point
Take a ruler and pencil, and draw a horizontal line from the point to the east or west edge of the map, which is the closest distance. Make sure that the line you draw is parallel to the nearest latitude on the map.
Step 5. Draw another line to create a point longitude
Start at the same point, and use a ruler and pencil to draw a straight vertical line to the north or south edge of the map, which is closest. Make sure the line is parallel to the nearest longitude.
Step 6. Estimate the longitude and latitude of the point using the graticule
Depending on the scale of the map, you can estimate the coordinates of points down to seconds. See the point where the longitude and latitude intersect with the coordinates on the edge of the map, and estimate the coordinates of the point you are looking for according to its position relative to the nearest graticule.
- If the map displays seconds, find the nearest second on the scale at the edge of the map that intersects the latitude and longitude. For example, if your latitude falls at 5″ above 32°20′N, your point's latitude is approximately 32°20′5″N.
- If the map shows minutes instead of seconds, it means that you can estimate latitude or longitude in 6 seconds by dividing the distance between each graticule by 1/10. If the line falls at 2/10 to the left of the line 120°14′E, it means that the longitude of your point is approximately 120°14′12″E.
Step 7. Combine the measurement results to determine the coordinates
Geographic coordinates are where longitude and latitude intersect at one point. Look at the numbers for longitude and latitude obtained, and put them together (for example, 32°20′5″N, 120°14′12″E).
