Waymarks in this category present one of the great lines listed below!
A. Major circles of latitude:
1. Arctic Circle (66° 33' 39'' N)
2. Tropic of Cancer (23° 26' 22'' N)
3. Equator (0° latitude)
4. Tropic of Capricorn (23° 26' 22'' S)
5. Antarctic Circle (66° 33' 39'' S)
B. Equal degree of latitude:
For example 45 degrees.
Some others are mentioned in Wikipedia site of circles of latitudes.
C. Equal degree of longitudes, except Greenwich / Prime meridian that already has its own category.
D. Historical Meridians:
See Wikipedia site of historical meridians
Here are some examples of great lines of earth markers:
Image from Wikipedia (180 th meridian marker in Taveuni, Fiji)

Image from wikipedia (Tropic of cancer monument in northeast of Villa de Cos, Zacatecas, Mexico.)