▁within ▁the ▁SI ▁system , ▁its ▁ret ention ▁for ▁na ut ical ▁and ▁av iation ▁use ▁is ▁important ▁because ▁the ▁length ▁of ▁a ▁na ut ical ▁mile , ▁upon ▁which ▁the ▁kn ot ▁is ▁based , ▁is ▁closely ▁related ▁to ▁the ▁longitude / lat itude ▁ge ographic ▁coordinate ▁system . ▁As ▁a ▁result , ▁na ut ical ▁miles ▁and ▁kn ots ▁are ▁convenient ▁units ▁to ▁use ▁when ▁navig ating ▁an ▁aircraft ▁or ▁ship . ▁ ▁Standard ▁na ut ical ▁charts ▁are ▁on ▁the ▁Mer c ator ▁projection ▁and ▁the ▁horizontal ▁( E ast - West ) ▁scale ▁var ies ▁with ▁latitude . ▁On ▁a ▁chart ▁of ▁the ▁North ▁Atlantic , ▁the ▁scale ▁var ies ▁by ▁a ▁factor ▁of ▁two ▁from ▁Florida ▁to ▁Green land . ▁A ▁single ▁graph ic ▁scale , ▁of ▁the ▁sort ▁on ▁many ▁maps , ▁would ▁therefore ▁be ▁useless ▁on ▁such ▁a ▁chart . ▁Since ▁the ▁length ▁of ▁a ▁na ut ical ▁mile , ▁for ▁practical ▁purposes , ▁is ▁equivalent ▁to ▁about ▁a ▁minute ▁of ▁latitude , ▁a ▁distance ▁in ▁na ut ical ▁miles ▁on ▁a ▁chart ▁can ▁easily ▁be ▁measured ▁by ▁using ▁div iders ▁and ▁the ▁latitude ▁scales ▁on ▁the ▁sides ▁of ▁the ▁chart . ▁Rec ent ▁British ▁Admir alty ▁charts ▁have ▁a ▁latitude ▁scale ▁down ▁the ▁middle ▁to ▁make ▁this ▁even ▁easier . ▁ ▁Speed ▁is ▁sometimes ▁incorrectly ▁expressed ▁as ▁" k not s ▁per ▁hour ", ▁which ▁would ▁mean ▁" na ut ical ▁miles ▁per ▁hour ▁per ▁hour " ▁and ▁thus ▁would ▁refer ▁to ▁acceleration . ▁