**What are the geographic coordinate systems?**

Copy your degree symbol from here.

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The geographic coordinate system (HSC) uses a three-dimensional spherical surface to determine its location on Earth. HSCs are often incorrectly called datums, although datums are only part of HSCs. GSK includes an angular unit of measure, an initial meridian and a datum (based on a spheroid).

The point on the spheroid is determined by the latitude and longitude. Latitude and longitude are angles, the apex of which is located in the center of the Earth, and one of the sides passes through a point on the earth’s surface. Angles are usually measured in degrees (or in degrees). Earth in the form of a globe, which shows the values of latitude and longitude.

In a spherical system, “horizontal lines” or lines corresponding to the east-west direction are lines of equal latitude or parallel. Vertical lines are lines of equal longitude or meridians. These lines encircle the globe and form a network of geographic coordinates, called a degree grid.

The line of latitude, which is located in the middle between the poles, is called the equator. It corresponds to a line of zero latitudes. The line of zero longitudes is called the zero (or initial) meridian. For most geographic coordinate systems, the prime meridian is the line of longitude passing through the Greenwich Observatory in England. Some countries use longitude lines passing through Berne, Bogotá or Paris as the prime meridians. The starting point of the cartographic grid (0,0) is determined by the intersection of the equator and the zero meridians. Then the globe is divided into four geographical quadrants (quarters of the globe), which are determined by the compass at the starting point. North and south are located, respectively, above and below the equator, and west and east, respectively, to the left and to the right of the prime meridian.

## Latitude and longitude

Latitude and longitude values are usually measured either in decimal degrees or in degrees, minutes, seconds (DMS). Latitude values are measured relative to the equator and can vary from 90 ° at the South Pole to + 90 ° at the North Pole. Longitude values are relative to the prime meridian. They can vary from -180 ° when moving west from the zero meridians and up to 180 ° when moving east from the zero meridians. If Greenwich is taken as the prime meridian, then the coordinates in Australia, located south of the equator and east of Greenwich, will have positive longitude values and negative latitude values.

It is convenient to count the values of longitude along the X-axis, and latitudes along the Y-axis. In the geographical coordinate system, data is displayed as if the degree is a linear unit of measurement. This method is essentially no different from the Plate Carree projection.

Although latitude and longitude values are used to determine the exact position of a point on the surface of a ball, these quantities are not universal units. Only at the equator is the distance corresponding to one degree of longitude approximately equal to the distance corresponding to one degree of latitude. This is due to the fact that the equator is the only parallel whose length corresponds to the length of the meridian (double). (Circles with the same radius as the Earth’s spheroid are called large circles. These are the equator and all meridians).

Above and below the equator, the circles that define the parallels become gradually shorter and shorter, until they turn into a point at the North and South poles at which the meridians converge. As the meridians converge to the poles, the distance corresponding to one degree of latitude decreases to zero. On Clark’s 1866 spheroid, one degree of latitude at the equator is 111.321 km, while at latitude 60 ° it is only 55.802 km. Since degrees of latitude and longitude do not have a standard length, you cannot accurately measure distances or areas or easily display data on a flat map or computer screen.

### Convert degree-minute-second to decimal degrees

We present a simple equation for converting coordinates of the form “degree-minute-second” to “decimal fractions of a degree”.

Image Example to change manually

DD = (Seconds/3600) + (Minutes/60) + Degrees

If the degree value is negative, the conversion should be done differently. Here is one of the options:

DD = – (Seconds/3600) – (Minutes/60) + Degrees

The instructions below show how to convert a table field with longitude or latitude data in degrees-minute-second angular units to “decimal fractions of a degree” using the Field Calculator. The code is written in VBScript, but it is easy to convert to other programming languages. It is assumed that the degree-minute-second is stored as a text string with spaces between numbers and does not contain other characters. For example, the data was saved as:

32 37 24.5 where 32 is degrees, 37 is minutes and 24.5 is seconds.

- The result should be saved in a numeric field.
- Add a table to Arc Map.
- Right-click on the table in the table of contents and click Open.
- Press the Options (Options), and then click Add Field (Add Field).
- Enter Lat2 in the Name field.
- Click the down arrow Type and select Double from the list.
- If you already have a field named Lat2, select another name that is not used.
- Click OK.
- Right-click the field Lat2 and select the field calculator (Field Calculator).
- If a message box appears, click Yes.
- Check the box next to the option Extras (Advanced).
- Paste the code into the box:

Dim Degrees Dim Minutes Dim Seconds Dim DMS Dim DD
DMS = Split([Latitude]) Degrees = CDbl(DMS(0)) Minutes = CDbl(DMS(1)) Seconds = CDbl(DMS(2)) If Degrees < 0 Then DD = -(Seconds/3600) – (Minutes/60) + DegreesElse DD = (Seconds/3600) + (Minutes/60) + DegreesEnd If |

In the sixth line starting with **DMS =**, the text in square brackets [] should contain the field name allocated for latitude data. Replace the word **Latitude** in the code with the name of the field (in your table) that contains degrees of latitude with minutes and seconds.

- Enter the following code into the box at the bottom of the Lat2 = dialog box :
- CDbl(DD)
- Click OK.
- Repeat steps 3 to 12 for longitude.