Transcript GEOG370_Ch3p2

Map Basics, partII
GEOG 370
Christine Erlien, Instructor
Previously in Ch. 3
 Symbolization
 Simplification/generalization
 Classification
 Scale
 Reference & Thematic Maps
 Major Map Elements
Wrapping up with:
 Projections
 Grid systems
Geographic Data

Features must be
referenced to some
real world location
 Georeferencing
Geographic Data & Position

Important elements must agree:
– scale
– ellipsoid
– datum
– projection
– coordinate system
Geographic Data & Position: Scale

When is this is an issue?
– When data created for use at a particular
scale are used at another

Why is this an issue?
– All features are stored with precise
coordinates, regardless of the precision of
the original source data
– What does this mean?
• Data from a mixture of scales can be displayed
& analyzed in the same GIS project  this can
lead to erroneous or inaccurate conclusions
Geographic Data & Position: Scale

Example:
– Location of same feature at different scales
– (-114.875, 45.675)
(-114.000, 45.000)
• Zoomed out  look like same point
• Zoomed in  look like separate points

Take-home message:
– Be aware of the scale at which data were
collected  metadata
– Measurements will only be as good as
least accurate data source
Geodesy

Study of the Earth’s
– Size
– Shape
– Gravitational fields
Geographic Data & Position:
Ellipsoid
The earth is not flat
 it must be round?!
 Not perfectly round:

– Small irregularities on
the surface such as
mountains, basins,
etc.
– Distortion due to the
Earth’s rotation
– Irregularities due to
variations in gravity
Geographic Data & Position:
Ellipsoid

The earth’s shape is irregular
– Slightly flattened at the poles
– Equator bulges
– Southern Hemisphere slightly larger than
Northern Hemisphere
Geographic Data & Position:
Ellipsoid

Ellipsoid: Hypothetical, non-spherical
shape of earth
– Note: Earth’s ellipsoid is only 1/300 off
from sphere
– Basis for datums
• Datum: Reference for elevation on the earth’s
surface
Reference Ellipsoids

Earth's surface not perfectly
symmetrical, so ellipsoid fitting one
geographical region may not fit another
– Reason for different reference ellipsoids
– Examples:
• Clarke 1866: Used for N. America until recently
• GRS80: Geodetic Reference System of 1980
• WGS84: Developed by US military, refined
version of GRS80
Ellipsoids & Datums: Importance

Differences exist between different
ellipsoids & datums
– Coordinates different in each  can be
significant distance
– Elevation  can be major differences at
large scales

Note: Be aware of the ellipsoid & datum
for datasets you are working with
Geographic Data & Position:
Projection

Projection: Process by which the round
earth is portrayed on a flat map

To project
– Think of a light inside the globe, projecting
outlines of continents onto a piece of paper
wrapped around globe
Process of Map Projection
1. Scale change
– Actual globe  reference globe based on
desired scale (e.g. 1:1,000,000)
2. Reference globe mathematically
projected onto flat surface
Families of Projections

Planar/Azimuthal

Cylindrical

Conical
Cylindrical projections
http://www.progonos.com/furuti/MapProj/Normal/ProjCyl/projCyl.html
Cylindrical projections

General properties:
– Meridians equally spaced
– Spacing between parallels of latitude
increases toward poles
– On globe, longitude lines converge at poles
 cylindrical projection forces them to be
parallel
– The farther away a point is from the
tangent line (where cylinder contacts the
globe), the greater the distortion
– Useful for sailing (No direction distortion)
Cylindrical projections: Distortion
http://www.fes.uwaterloo.ca/crs/geog165/cylproj.htm
Conic Projections
Conic projections are created by setting a cone over a globe
and projecting light from the center of the globe onto the
cone.
Conic Projections

General properties:
– Contact with globe along either 1 or 2 lines
of latitude
– Longitude lines projected onto the conical
surface, meeting at its apex
– Latitude lines projected onto the cone as
rings
– Distance between longitude lines widens
as their distance from the apex increases
– Typically used for mid-latitude zones with an
east-to-west orientation
Conic Projections
From Getting Started with Geographic Information Systems,
Keith C. Clarke
Conic Projections: Distortion
http://www.fes.uwaterloo.ca/crs/geog165/conproj.htm
Azimuthal/Planar Projections
Planar projections, also called azimuthal projections, project
map data onto a flat surface.
When the plane touches the earth at
either the north or south poles
latitude lines appear as concentric
circles and longitude lines radiate
from the pole at their true angle like
the spokes on a wheel. This
particular map projection's light
source originates at the center of the
earth but this is not true for all planar
map projections. (ESRI Press)
Azimuthal/Planar Projections

General properties:
– Tangent to the globe at one point
– North & South Poles  most common
contact points
• Longitude lines converge at the pole
• Distance between longitude lines increases as
the distance from the pole increases
• Latitude lines appear as a series of concentric
circles.
– Used most often to map polar regions
Azimuthal/Planar Projections:Distortion
http://www.fes.uwaterloo.ca/crs/geog165/azproj.htm
Map projections: Distortion
Converting from 3-D globe to flat
surface causes distortion
 Types of distortion

– Shape
– Area
– Distance
– Direction

No projection can preserve all four of
these spatial properties
Map projections: Distortion

Shape
– The ability of a map projection to maintain
shape of geographic features
– Conformal projections: Map projections
that maintain shapes/angles, scale factor
locally
• Best used on small areas  difficult to maintain
true angles for large areas
• Distorts area
• Application: Marine or air navigation
Conformal projections
Example: Mercator
Map projections: Distortion
 Area
– The ability of a map projection to maintain
equal area for geographic features (e.g.,
correct area relative to one another)
– Equal area projections: Map projections
that maintain this property
• Application: Instruction & small-scale
general reference maps
– No map projection can preserve both
conformality and equal area
Equal area projections
Example: Mollweide
Distortion minimal near the intersection of Equator & central
meridian, increases toward the edges of the map
Map projections: Distortion

Distance
– Map projection's ability to maintain true
distance
• Maintained for only certain parallels or
meridians OR
• Maintained in all directions around 1 or 2 points
Equidistant projections
http://www.fes.uwaterloo.ca/crs/geog165/cylproj.htm#Equidistant%20Projections
Map projections: Distortion

Direction
– Map projection's ability to maintain true
direction between geographic locations
– Azimuthal map projections: Maintain
direction with respect to 1 or 2 points
• Angle of a line drawn between any two
locations on the projection gives the correct
direction with respect to true north
• Application: Navigation
Azimuthal projections
Lambert azimuthal
Tangent to North Pole
http://www.warnercnr.colostate.edu/class_info/nr502/lg2/projection_descriptions
/lambert_azimuthal.html