Portraying Earth Data types File types Lecture 2 Portraying Earth Data types File types

Portraying Earth Globes vs. Maps – 3D vs. 2D Shape – size Lat/Long: what is it what units Projections: why do we need them what do they do Scale Map essentials

Size & Shape of Earth Diameter = 8000 miles Radius = 4000 Circumference = 25,000 Earth slightly flattened at poles - wider at equator

Latitude/Longitude Latitude and Longitude define site location on the surface of the earth Latitude is used to express distance north or south of the equator Longitude shows east-west distance from the Greenwich meridian @ 0° to the International Date Line @ 180° Locations are given in degrees, minutes, seconds or degrees and fractions of degrees For example: Boston, MA: Lat 42.37N – Long 71.03W Boston, MA: Lat 42° 22’ 11” - Long 71° 1’ 47” Lawrence, MA: Lat 42.72N – Long 71.12W Lawrence, MA: Lat 42° 43’ 12” - Long 71° 7’ 12”

Map Projection A system based on mathematical computation It transforms the round surface (3D) of the Earth to display on a flat surface (2D), i.e. a map. Called ‘projection’ because it was originally a question of ‘projecting’ the grid (lat & long lines) onto a sheet of paper graphically. It is impossible to portray a curved (3D) surface on a flat (2D) map perfectly There is always distortion There is NO perfect projection

Why Projections? The main objective is to preserve either the shape of the land or the area of the features - you can’t have it both ways! The following slides show different projections

Conic Projection Globe with light at center, projects images onto paper cone – when paper laid out flat, conic projection.

Plane Projection Azimuthal projection Plane projection Globe with light at center, projects images onto adjacent paper plane (flat surface) – when paper is removed, you have: Azimuthal projection Plane projection Zenithal projection

Cylindrical Projection A globe with light in center projects onto paper cylinder – when paper laid flat, you have a cylindrical projection.

Equivalence vs. Conformality The central problem, when constructing a map is choosing the projection. We must think about what is being shown; what part or how much of the earth is to be mapped? Equivalence = size Conformality = shape

Equivalence Equivalent projection is also called an equal area (size) projection Misleading impressions of size are avoided Good for showing distributions of geographic features Problem: on small scale (large area) world maps - shapes of land masses are disfigured

Conformality Angular relationships are maintained Shape of land mass on map is same as shape on Earth All meridians and parallels cross at 90° angles – right angles. Problem: the size of an area is considerably distorted to portray the proper shape.

Mercator Projection is the most well known projection is a special purpose map projection used for navigation has area distortion is conformal, i.e. preserves shape, not size is the only map projection that shows true compass bearings for navigation straight lines on a Mercator projection will always intersect longitude lines at the same angle - Mercator is the only map projection that does that

MERCATOR PRO JECTION

Note the size of South America and Greenland South America = 17,819,000 sq km Greenland = 2,166,086 sq km Note the size of South America and Greenland Conformal projection — preserves shapes

How does North America compare between these three projections? Mercator Conic Note how the lat-long lines are aligned in each of the projections. Azimuthal

Equal Area Projection Why is it used here? How do South America & Greenland compare in area? (p. 92)

Shows distribution of habitable and non-habitable land Equal Area Projection Shows distribution of habitable and non-habitable land

Large and Small Scale Maps A small scale map shows a larger area with little detail and has a large denominator 1:250,000 A large scale map shows a smaller area with more detail has a small denominator 1:500 or 1/500

A map with a scale of 1:10,000,000 would be considered small scale when compared with a map of 1:1,000. This is a small scale map – there is no detail shown. There is such great area – the whole earth – it must be small scale. i.e. 1 inch must be equal to millions of inches on the earth!

Data types Two basic types of data: Vector Points Lines Polygons Raster Grid composed of cells i.e. an array pixels arranged in rows and columns each with its own value

Vector data A vector based system displays graphical data as points, lines or curves, or areas with attributes.

Raster data A raster based system displays, locates, and stores graphical data by using a matrix or grid of cells.

File types & extensions Shapefiles .shp Layer files .lyr Raster files .jpeg .tiff .gif .sid Database files .dbf ArcMap document .mxd

Databases Excell Access

Acronyms & terms Lat/long Large scale vs. small scale Raster vs. vector .shp .dbf .lyr .mxd