Resource Mapping

Friday, March 09, 2007


Hi Guys

There are three ways of expressing coordinates

Degrees Minutes and Seconds

This is the "Conventional" system e.g. 33 degree 15 minutes 20 seconds South or in an abbreviated form 33o 15' 20" S

Degrees are expressed using the o , minutes using ' and seconds using "

The four directions North, South, East and West are used. So since we are living in the Southern Hemisphere and East of Greenwich we will use South and East. If you were living in Japan you would use North and East. A person living in San Francisco in the USA would use North and West, whereas a South Ameican would use South and West.

Degrees with decimal minutes (default setting in GPS units)

This is the system that most Global Positioning Systems (GPS) units use as their default. It is actually best to change the settings to Decimal Degrees - see later sections of this posting. In this configuration the degrees are expressed as a decimal so there are no seconds.

33 degrees 15.3333 minutes South or in an abbreviated form 33o 15.3333'S.

I find this system rather awkward to work with so it is best to avoid using it.

Decimal Degrees

All GIS applications use decimal degrees when using unprojected data expresed in Latitudes and Longitudes. In this configuration both minutes and seconds are put into a decimal.

33.255555 degrees South or -33.255555o in an abbreviated form.

You will need to work to six decimal points to get good accuracy. Furthermore, you will need to assign a positive value to East and North hemispheres and negative values to East and South hemispheres respectively. Since we live in the East and South hemispheres our longitudes will get positive values and our latitutudes get negative values.

How do I convert between these three congigurations?

In the first example I will convert Degrees Minutes and Second into Decimal Degrees using the value 33o 15' 20" S.

Leave Degrees as is

Convert Minutes to decimals so it will be (15/60)= 0.250000

Convert Seconds to Minutes and then convert these minutes to decimals so it will be(20/3600)= 0.005555 (working to six places)

Add the converted minutes and the converted seconds to the degrees

30 + 0.250000 + 0.005555 = 30.2555555

Next I will convert Degrees and Decimal minutes into Decimal Degrees using the value 33o 15.33333'S.

Convert the minutes into decimals so it will be (15.33333'S/60)= 0.2555555

Add the converted minutes to the degrees

30 + 0.2555555 = -30.2555555 (it gets a negative since it is in the Southern Hemisphere)

How would I convert decimal degrees back to Degrees, Minutes and Seconds?

Lets take our value -30.2555555

  • The whole units of degrees will remain the same (i.e. in -30.255555° latitude, start with 30°).

  • Multiply the decimal by 60 (i.e. 0.255555 * 60 = 15.33333).

  • The whole number becomes the minutes (15').

  • Take the remaining decimal and multiply by 60. (i.e. .3333 * 60 = 20).

  • Leave Degrees as is

The decimal minute needs to convertered firs to decimal minutes by multiplying by 0.6(0.2555555*0.6)= 0.1533333, the first two represent the minute. You take the last four digits .003333*.6

In Google Maps we find a location

FNB Stadium Johannesburg


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