Understanding Map Coordinates

The accuracy of a map coordinate is based on the number of decimal points. The more decimal points there are, the more accurate the coordinate.

The Royal Observatory in Greenwich, UK, sits where east meets west at Longitude 0°. If you retrieve its coordinates from the marker on Google you get the following result:

51.47685176382669, -0.0005004272995577789

That the 19 decimal places, which is extremely precise. We will explain later why there are 14 decimals in the latitude coordinate vs 19 in the longitude. Most coordinates are quoted with 0-6 decimals. The majority stops at 5-7 decimal places when pinpoint accuracy is required. To understand the implications of this, take a look at the table that follows. It shows the level of accuracy gained or lost, based on the number of decimal places in the coordinates.

The Difference a Decimal Point makes

The table shows the difference between using 8 decimal places versus using none, a difference of 1 millimeter to 1 kilometer.

There is a significant difference in a coordinate when no decimal places are given compared to a coordinate with 5 decimal places. A difference of ~68 miles or 110 kilometers.

Does that mean it is wrong to use less decimal points? It depends on the usage.

There are times when using 0-2 is acceptable. When providing the location of a mountain, for example. But it would not be as acceptable when describing the location of a building, or the location of a car, or a phone.

A coordinate of 3-5 decimal places may be appropriate in describing a building, but the size matters. A building could be as large as several city blocks; an airport terminal or the Pentagon for instance, while another may be a corner store of 150 sq ft, or a kiosk. The smaller the object being mapped, the larger the number of decimal places required to pinpoint accuracy. In the example of the Pentagon, many more decimal places would required to pin point an actual entrance.

Latitude vs Longitude

Latitudes are horizontal lines that measure distance north or south of the equator. The equator lies at latitude 0. North of the equator is represented by a positve number and south of it by a negative number. The North Pole is at 90 degrees latitude (or 90.0° N) and the south poles is at -90 degrees latitude (or 90.0° S).<

Longitudes, also called meridians, are vertical lines that run from the north to south poles. They measure the distance east or west of the prime meridian, which has a longitude of 0 degrees and runs through through Greenwich, England. The Royal Observatory in Greenwich, UK is located on this line. East of the prime meridian is represented by a positive number and west by a negative number.

Decimals in Latitude vs Longitude

There is a reason why the coordinates above only show 14 decimals for the latitude vs 19 for the longitude. The reason follows:

Latitude: One decimal place in latitude represents approximately 111 kilometers (69 miles) at any point on Earth. This distance remains constant regardless of location.

Longitude: The distance represented by one decimal place in longitude varies depending on latitude. Closer to the equator, it's closer to 111 kilometers, but increases as you move towards the poles. For example, at 45° latitude, one decimal place in longitude is about 78 kilometers (48.5 miles).

The implication of this is that the distances in the table above may be slightly different depending on how close the location is to the equator. It's not a big difference, but should be noted.