Kalmar "gonic line" yawanci tana nufin layin da aka zayyana a saman duniya inda magnetic declination ya zama sifili. Ma'ana, layi ne wanda allurar compass ke nunawa arewa ta gaskiya, maimakon magnetin arewa. An samo kalmar daga kalmar Helenanci "agon," ma'ana takara ko gwagwarmaya, saboda layin yana wakiltar iyaka tsakanin yankunan da ke da bambancin tasirin maganadisu kuma yana iya zama da amfani a kewayawa.