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

From Getting Started with Geographic Information Systems, Keith C. Clarke

Conic Projections

Conic Projections: Distortion

http://www.fes.uwaterloo.ca/crs/geog165/conproj.htm

Azimuthal/Planar ProjectionsPlanar 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