map basics lecture #3, intro to gis spring 2006. topics map definitions types of maps map...

Map Basics

Lecture #3, Intro to GIS

spring 2006

Topics

• Map definitions

• Types of maps

• Map projections

• Geodetic Datums

• Coordinate Systems

What are maps?

• Maps are models of reality

• Attempts to represent the two-dimensional curved surface of the Earth or other body on a plane (flat surface)

• main data source for GIS

The beginnings of Cartography

• Ancient Greece:

Ptolemy (90 - 168 BC)’s Geographia– bases of coordinate systems– maps– list of places

Ptolemy’s world map

Sri Lankaoversized

Muhammed Al’Idrisi’s view of the

world (1154)

What’s strange about this map?

Kepler’s map of the world

Types of maps

1) Topographic map:

"Topography”: pertaining to the shape of the surface, represented by contours and/or shading, but topographic maps also show roads and other prominent features

Topographic maps

Thematic map (choropleth):

show geographical concepts such as the distribution of population

climate

land use etc.

Thematic maps

Map elements

• Map features: points, lines, areas, …

• Scale

• Accuracy

• Extent

Scale

the amount of reduction in the representation of a real world geographic phenomenon on a map.

or, the ratio of map distance to earth distance

1:24,000

1 inch = 2000 ft

1 2000 ft

verbal scale

representative fraction

bar scale

Accuracy:what is it?

• Quality of the source data

• Map scale

• mapping skills

• distortions from flattening Earth’s surface to fit on a sheet of paper

Construction of a map: summary

Steps:

1. Select projection2. Select a model for the shape of the earth (sphere

or ellipsoid)

3. Transform geographic coordinates (longitude and latitude) to plane coordinates (eastings and northings or x,y)

4. Reduction of scale

Map Projections

•Used to display locations on the curved surface of the earth on a flat sheet or surface according to some set of rules

•Involve distortions in:

shape (of a region)

distance (between two points)

direction (bearing from one point to another)

area (of a region)

Properties of map projections

1. Equal-area: correctly represents areas sizes of the sphere on the map

e.g. Lambert cylindrical equal-area projection

2. Equidistant: correctly represents distances

e.g. Plate Carrée projection

3. Conformal: represents angles and shapes correctly at infinitely small locations.

e.g. Mercator projection

The classification of Map Projections

A. Planar (azimuthal)• flat sheet is placed in contact with a globe,

and points are projected from the globe to the sheet

Ex: Lambert's azimuthal equal-area projection

• Preserves area• Distorts shapes & distances

B. Conical projections

• distorts scale and distance except along standard parallels

Ex: Alber’s equal area conical projection

C. Cylindrical projections

• project the sphere onto a cylinder tangent to a central meridian

• meridians and parallels intersect at right angles

E.g.Mercator projection (conformal)

•Preserves angles

•Distorts scale, distance, direction and area

•distortions increase away from the central meridian

•used in sailing (direction more important than distance)Note an obvious distortion?

Plate carrée (geographic projection)

• latitude and longitude coordinates• many distortions; mostly used in thematic mapping•Compare the size of Greenland

D. “Compromise” projections

• Distortions are balanced

• Make things "look right”

• shape distorted more in the polar regions than at the equator

E.g. Robinson projection

Mainly used for geographic maps, e.g National Geographic

Earth’s gravitational field• Gravity = force of

attraction between two bodies with a mass

• Earth is not a perfect sphere

• density not uniform

• local topography (e.g presence of mountains) and geology (the density of rocks) also influence the gravitational field.

Geoid and Ellipsoid

Geoid = Earth’s shape (minus topographic relief)

Ellipsoid = approximation to the shape of the geoid, defined mathematically

Ellipsoid parameters

Geoid-ellipsoid relationships

Mean Sea Level (MSL) = an approximation to the geoid,

used as reference surfaces for height measurements

(orthometric heights).

Common datums:

Why the need for a common Reference System?For historical reasons each country has its own geodetic network and national geodetic reference frame.

•World Geodetic System (WGS84): GLOBAL•NORTH AMERICAN DATUM OF 1927 •NORTH AMERICAN DATUM of 1983

Projections demo:ArcIMS

Projections Demo: ArcGIS

• Latitude, Longitude, Map Projections and Great Circles