entire planet. There is no such system for space. Without a clear reference grid, it becomes more accurate to describe the four dimensions as towards/away
Physical Geography for Students and Teachers (页面存档备份,存于互联网档案馆),UK National Grid For Learning(英文) M. Pidwirny《自然地理学基础》,第2版,2006. [2008-08-19]. (原始内容存档于2009-05-01)
population data (PDF). ESPON project 1.4.3 Study on Urban Functions. European Spatial Planning Observation Network: 119. 2007-03 [2010-02-22]. (原始内容 (PDF)存档于2015-09-24)