Three Kinds of North True North – Along Meridians Magnetic North – What a compass detects – Important in field – Not important in most GIS Grid North – Along N-S grid lines – Minor importance for compass work – Be aware there’s a difference! – Military uses exclusively
Cautions About UTM Military maps use two letter codes for each 100-km square, but maps will have information to enable conventional UTM With military grid references, number of digits indicates level of precision Older maps will sometimes have obsolete grids Datum, datum, datum!
Military Grid System 45N 89 W = E 342,369, N 4,984,896 Zone 16T Digraph = CQ 1 km accuracy = 16T CQ 42 84 100 m accuracy = 16T CQ 423 848 10 m accuracy = 16T CQ 4236 8489 1 m accuracy = 16T CQ 42369 84896 Ticks on USGS topo map = 3 42,000, 4,9 84,000
Why a Grid? Latitude /LongitudeGrid System North varies from place to place on the map Grid north is always the same direction Angular units differ in scale between N-S and E-W Grid scale the same in all directions E-W angular units vary in scale with latitude Grid squares are always the same size and shape Hexadecimal ScaleDecimal Scale
Public Land Survey System AKA Congressional System Established 1785 Does not apply to: – 13 Original Colonies – Derivative States (VT, KY, TN, ME, WV) – Texas (Former independent country) – Hawaii (Uses Kingdom of Hawaii system) Land Division – 6 x 6 mile townships, 36 sections, quarter sections
Limitations of the Congressional Land Survey System
Congressional Land Survey System Not an accurate grid! Locations within subdivisions may be imprecise Data points used in GIS may be tied to system Authoritative surveys are forever – French strips in LA, MO, WI – Spanish and Mexican land grants
Oh, Canada Eastern Canada: Metes and Bounds, Long Lot Northern Ontario: 6 and 10-mile townships Western Canada: Dominion Land Survey – Modeled on PLSS – 6-mile townships – Townships numbered N-S with Arabic numerals – Ranges numbered E-W with Arabic or Roman numerals – Road allowances between sections – Sections zigzag from 1 in SE to 36 in NE (Opposite US) – ¼-1/4 sections numbered from 1 in SE to 16 in NE