GeoGridLines

is an option for GeoGraphics that specifies what parallels and meridians to show.

Details

	None	no geo grid lines drawn
	Automatic	geo grid lines placed automatically
	n	approximate number of parallels and meridians
	Quantity[…]	geo grid lines placed at multiples of an angular quantity
	{latgrid,longrid}	geo grid lines specified separately for latitude and longitude

With the Automatic setting, geo grid lines are usually placed at points whose coordinates have the minimum number of digits in their decimal representation.
For each latitude/longitude coordinate, the following grid line options can be given:

	None	no geo grid lines drawn
	Automatic	geo grid line positions chosen automatically
	n	approximate number of parallels and meridians
	Quantity[…]	geo grid lines placed at multiples of an angular quantity
	{x₁,x₂,…}	geo grid lines drawn at the specified positions
	{{x₁,style₁},…}	geo grid lines with specified styles
	func	function to be applied to x_min, x_max to get geo grid line option

When a positive integer n is specified for either latitude or longitude, various heuristics are applied to produce "nice" geo grid lines. As a result, the number of grid lines actually shown may not be exactly n.
For GeoGridLines->n or GeoGridLines->Automatic, the number of grid lines shown may differ between parallels and meridians.
For a Quantity specification, only angular quantities are supported.
The geo grid line function func[x_min,x_max] may return any other geo grid line option.
AbsoluteOptions can be used to extract explicit lists of latitudes and longitudes that were used for GeoGridLines specifications given Automatic settings.
Use GeoGridLinesStyle to set styles for geo grid lines.