Transcript Document

GTECH 361
Lecture 04
Referencing Data to Real Locations
Today’s Content
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Two types of coordinate systems
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Geographic
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Projected
Today’s Objectives
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name two types of coordinate systems
identify components of each type of coordinate system
assign coordinate system information to a dataset
set display units for a data frame and measure distances on
a map
explain what a map projection is
list the major categories of map projections
list spatial properties that may be distorted when different
map projections are applied
change the map projection for a data frame and describe its
effects
Geographic Coordinates
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Graticule
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Latitude
Longitude
Prime Meridian
The Earth’s Shape
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The ancient Greek’s mathematical harmony
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Simplest approximation: the sphere
The Earth as an Ellipsoid
Making of an Ellipsoid
The Earth’s Shape
Why Multiple Datums?
Geodetic Datums
Projected Coordinates
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Flattening the Earth
Origin of X, Y Coordinates
Central Parallel
False Easting/northing
Coordinates in ArcGIS
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All geographic data have geographic
coordinates (lat/lon)
Some may have projected coordinates
in addition to the geographic ones
ArcGIS assigns the coordinate system
to a map based on the GCS or PCS of
the first dataset loaded
Subsequent datasets are converted
on-the-fly
Map and Display Units
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Map units are determined by GCS or PCS
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GCS in degrees or decimal degrees
PCS usually in feet or meters
Display units are determined by you
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They are defined as part of the data frame
(Decimal) Degrees
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Converting from degrees to decimal degrees
1. Divide each value by the number of minutes
(60) or seconds (3600) in a degree
2. Add up the degrees to get the answer
Map Projection Types
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Cylindrical
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Conical
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Planar
Cylindrical Projections
Conic Projections
Planar Projections
Understanding Distortion
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Distortion cannot be avoided; we have
to choose from distortion of
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Shape
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Area
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Distance
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Direction
Preserving Properties
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If two properties are to be preserved then
one is always direction
These properties are incompatible:
Shape Property
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Conformal
Non-conformal
Area Property
Distance Property
Direction Property
Direction Property
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Mercator with rhumb line
or loxodrome
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Azimuthal map
with shortest distance
Tissot Indicatrices
Equatorial (normal) Aspect
Transverse Aspect
Oblique Aspect
Aspects for Planar Projections
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Polar
Gnomic
Stereographic
Orthographic
Aspects for Planar Projections
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Equatorial Aspect
Gnomic
Stereographic
Orthographic
Aspects for Planar Projections
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Oblique Aspect
Gnomic
Stereographic
Orthographic
Aspects for Conic Projections
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Normal aspect
Polyconic Projection
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Hassler, 1820s
US Coastal Survey
Perspective
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Position of the
light source
Perspectives
Classifying Projections
Classifying Projections
Cylindrical
straight parallels; straight meridians
Pseudo-cylindrical
straight parallels, curved meridians
Conic
partial concentric circles for parallels;
straight meridians
Pseudo-conic
Planar
Modified planar
partial concentric circles for parallels;
curved meridians
Concentric circles for parallels; straight
meridians
No common appearance of parallels and
meridians
Choosing a Map Projection
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Conformal (shape-preserving) maps
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Topographic and cadastral
Navigation
Civil engineering
Weather
Choosing a Map Projection
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Area-preserving maps
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Population density
Land use
Quantitative attributes
Choosing a Map Projection
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Scale-preserving maps
no map preserves true distance for all measurements
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Airline distances
Distance from
epicenter of an
earthquake
Cost calculations
Choosing a Map Projection
Characteristics of Map Projections
Projection
Category
Properties
Common Uses
Conformal
Preserves local shapes and
angles
Topographic maps,
navigation charts,
weather maps
Equal Area
Preserves areas
Dot density maps,
thematic maps
Equidistant
Preserves distance from one
or two specified points to all
other points on the map
Maps of airline
distances,
seismic maps showing
distances from an
earthquake epicenter
Azimuthal
All directions are true from a
single specified point (usually
the center) to all other points
on the map
Navigation and route
planning maps
Compromise
No point is completely
distortion free; distortion is
minimized near the center
and along the equator
World maps
Components of a GCS
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An angular unit of measure
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A prime meridian
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A datum, which includes a spheroid
Planar Coordinate Systems
Cartesian Coordinates
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Calculate distance A-B
Universal Transverse Mercator
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UTM zones
UTM Zones
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.. as seen from the North Pole
UTM Projections
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Each zone uses a custom Transverse Mercator
projection with its own central meridian
Universal Polar Stereographic
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Fills the holes of UTM in polar regions
State Plane Coordinate System
SPC
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N-S zones use Transverse Mercator
E-W zones use Lambert Conformal Conic
Maximal scale error is 1:10,000
NAD27 or NAD83 datum
Public Land Survey System
PLS are shown in purple
PLS
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It is used to locate areas, not points
It is not rigorous enough for spatial
analysis like the calculation of distance
or direction
It is not a grid imposed on a map
projection (a system invented in a
room), but lines measured on the
ground by surveyors
PLS Meridians and Baselines
PLS Area Unit Hierarchy
PLS Township Sections
A township is divided into 36 sections,
each a square mile (640 acres)
A section is divided into 160-acre
quarters, which can be further divided
into halves, quarters, and so on