INTERNAL: Calculating Standard Parallels and Central Meridian

Setting up projection parameters may sometimes require calculating standard parallels and then a meridian. This article demonstrates how.

Standard Parallels

• Calculate the difference between the minimum and maximum latitude values
• Multiply that difference by 60 to convert the value into minutes instead of degrees
• Divide the minutes by 6
• Divide the previous result by 60
• Add the previous result to the southern latitude and subtract the previous result from the northern latitude
• The two values from the above addition and subtraction are the standard parallels

      69º - 60º    = 9º
      9º x 60'   = 540'
      540' / 6    = 90'
      90' / 60'   = 1.5º or 1º30'
      60º + 1º30' = 61º30' standard parallel south
      69º - 1º30' = 67º30' standard parallel north

Central Meridian

• Calculate the difference between the minimum and maximum longitude values
• Multiply that difference by 60 to convert the value into minutes instead of degrees
• Divide the minutes by 2
• Divide the previous result by 60
• Subtract the previous result from the maximum longitude
• The value from the above subtraction is the central meridian

      120º - 104º   = 16º
      16º x 60'   = 960'
      960' / 2    = 480'
      480' / 60'   = 8º
      120º - 8º    = 112º central meridian