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G492 GIS for Earth Sciences
Map Projections and Coordinate Systems
I.
Introduction
A.
Fundamental Concern with Map Work
1.
Maps / GIS
a.
2-D representation of Earth surface
b.
locations of map features on cartesian coordinate system
(1)
Data Points
(2)
x,y coordinates in 2-D
2.
Earth Globe = 3-D
a.
spherical trigonometry / geometry
(1)
longitude / latitude system
3.
Map Projection - mathematical / geometric method for projecting spherical positions on a
2-D grid system
a.
transformation of geographic grid (lat / long) to 2-D cartesian coordinate system
B.
GIS and Map Layers
1.
Layer Concept = portion of Earth's surface divided into separate "coverages" or thematic
layers
a.
Layer examples for a locality
(1)
geology, soils, topography, vegetation, roads, streams, buildings, etc.
(a)
each theme = layer
2.
Goal of Map Layers
a.
to overlay each layer on top of one another in proper geographic coordinate space
(1)
layers must precisely overlay on top of one another
3.
Problem with projections and layers: if separate layers are in different projections, maps
will not properly align
NOTE: You must know the system of projection for each of the map layers in GIS, if they are not properly
coordinated, layers will not properly align with one another, or with real world positions.
C.
Basic Terminology
1.
Georeference system - coordinate system used to plot position of points, lines, and
polygons in map space
2.
Map Registration - proper alignment of map coordinates with real world coordinates of
features on the Earth's surface
3.
Map Projection - converting digital maps from lat-long (geographic coordinates based on
3-D spherical geometry) to 2-D coordinates
II.
Map Scale and Resolution
A.
Fractional Scaling
1.
1:24,000 - common large scale used on topo sheets
a.
covers small area in great detail
2.
1:1,000,000 - common small scale used on world maps
a.
covers large area in minimal detail