From Ocean Teacher Library
Geolocation by Latitude and LongitudeBackground
The excellent references below, in Wikipedia, give the scientific background for how latitude and longtitude are defined and measured on the earth. The material below concentrates on practical aspects important to marine data managers.
How Do Latitudes and Longitudes Work?
- Geo-coordinates are the X and Y values that we use to locate a position on the earth surface, just like locating a point on a Cartesian graph. The figure below shows the standard system most widely used today.
- The center of the system (0,0) is a point in the Gulf of Guinea, offshore west Africa. The maximum and minimum values of the longitude and latitude are shown. Latitude +90 is the North Pole; latitude -90 is the South Pole. The maximum and minimum longitudes (+180 and -180) are along the same north-south line through the middle of the Pacific Ocean, because the earth is round.
- The north-south line where longitude = 0 is called the Greenwich Meridian or Prime Meridian.
- The units of measurement are DEGREES of longitude and latitude, symbolized by a small raised circle (°).
- Geo-coordinates should always be written as "signed decimal degrees", meaning:
- Fractional parts of a degree must be written as decimals.
- The sign of the longitude or latitude should be specified, but if it is positive it can be omitted (because it is universally understood to be positive).
Where are the positive and negative coordinates located on the earth?
- Above the Equator and East of the Prime Meridian, the longitudes and latitudes are all positive. This includes most of Europe and Asia.
- Below the Equator and East of the Prime Meridian, the longitudes are positive and the latitudes are negative. This includes much of Africa, Oceania, and Australia.
- Below the Equator and West of the Prime Meridian, the longitudes and latitudes are negative. This includes much of South America.
- Above the Equator and West of the Prime Meridian, the longitudes are negative and the latitudes are positive. This includes Central America and North America.
How do degrees, minutes and seconds fit in with this system?
- Historically, data locations were based on degrees, minutes and seconds (the DMS system). A minute (usually specified with a ' symbol) is 1/60th of a degree, and a second (usually specified with a " symbol) is 1/60th of a minute (or 1/3600th of a degree).
- A DMS location can be converted to signed decimal degrees by the following steps
- Longitudes specified to be WEST are given a negative sign, and the term WEST (or W) is dropped.
- Latitudes specified to be SOUTH are given a negative sign, and the term SOUTH (or S) is dropped.
- Minutes are converted to degrees by dividing the value of the minutes by 60. If the original degrees value is negative, then a negative sign is also given to this fractional result.
- Seconds are converted to degrees by dividing the value of the seconds by 3600. If the original degrees value is negative, then a negative sign is also given to this fractional result.
- The whole and fractional results are added together.
- Example: 123° 24' 30.8" W Longitude
- Whole degrees = -123
- Fractional degrees from minutes = 24/60 = -0.40
- Fractional degrees from seconds = 30.8/3600 = -0.00855
- Total = (-123)+(-0.40)+(-0.00855) = -123.40855°
What other coordinate notations exist?
- The DMS notation system is almost extinct from a data management perspective, but huge amounts of old data are still given with these units.
- Many different methods of defining the Cartesian framework exist. A sampling of them can be viewed in the article Non-Standard Latitude-Longitude Notations.
How do you use coordinates to describe a general area, rather than a specific point?
- Geo-coordinates are frequently used to describe the areas in which datasets have been obtained, especially when formal metadata descriptions of the datasets are being written.
- Sophisticated systems can deal with complex polygonal shapes that conform to the actual shape of the study area. We will leave that subject to the user, and merely state that it is an extension of the simple rectangular schema discussed below.
- The most common geo-coordinate description of a study area is a simple rectangle that contains the data points. The resulting box is recorded in the metadata as a set of four values:
- TOP = a latitude value (appropriately signed)
- BOTTOM = a latitude value (appropriate signed)
- LEFT = a longitude value (appropriately signed)
- RIGHT = a longitude value (appropriately signed)
- The order of the sides is not important, as long as they are specified
- Notice there is no mention of north, south, east or west, just numbers; even the use of the words latitude and longitude is optional
- NOTE: If you have a rectangle in the middle of the Pacific, it is possible for the left side to be a positive longitude and the right side to be a negative longitude!
Additional Resources
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Information about this article
Short title: Lat/Lon Geolocation
Description: none
Expertise level: beginner
Author: Murray.Brown
Approval status: approved
Approved by: Murray.Brown
Last change: 2008-10-3
Subsection of: Coordinate Systems for the Earth Surface
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