GTECH 361

Lecture 04

Referencing Data to Real Locations

Today’s Content

Two types of coordinate systems

Geographic

Projected

Today’s Objectives

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

Graticule Latitude Longitude

Prime Meridian

The Earth’s Shape The ancient Greek’s mathematical

harmony

Simplest approximation: the sphere

The Earth as an Ellipsoid

Making of an Ellipsoid

The Earth’s Shape

Why Multiple Datums?

Geodetic Datums

Projected Coordinates

Flattening the Earth

Origin of X, Y Coordinates

Central Parallel

False Easting/northing

Coordinates in ArcGIS 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

Map units are determined by GCS or PCS GCS in degrees or decimal degrees PCS usually in feet or meters

Display units are determined by you They are defined as part of the data

frame

(Decimal) Degrees

Converting from degrees to decimal degrees1. 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

Cylindrical Conical Planar

Cylindrical Projections

Conic Projections

Planar Projections

Understanding Distortion

Distortion cannot be avoided; we have to choose from distortion of Shape

Area

Distance

Direction

Preserving Properties If two properties are to be preserved

then one is always direction

These properties are incompatible:

Shape Property

Conformal

Non-conformal

Area Property

Distance Property

Direction Property

Direction Property

Azimuthal map with shortest distance

Mercator with rhumb line or loxodrome

Tissot Indicatrices

Equatorial (normal) Aspect

Transverse Aspect

Oblique Aspect

Aspects for Planar Projections

PolarGnomic

Stereographic Orthographic

Aspects for Planar Projections

Equatorial AspectGnomic

Stereographic Orthographic

Aspects for Planar Projections

Oblique AspectGnomic

Stereographic Orthographic

Aspects for Conic Projections

Normal aspect

Polyconic Projection

Hassler, 1820sUS Coastal Survey

Perspective

Position of thelight source

Perspectives

Classifying Projections

Classifying Projections

Cylindrical straight parallels; straight meridians

Pseudo-cylindrical

straight parallels, curved meridians

Conicpartial concentric circles for parallels; straight meridians

Pseudo-conicpartial concentric circles for parallels; curved meridians

PlanarConcentric circles for parallels; straight meridians

Modified planarNo common appearance of parallels and meridians

Choosing a Map Projection

Conformal (shape-preserving) maps Topographic and cadastral Navigation Civil engineering Weather

Choosing a Map Projection

Area-preserving maps Population density Land use Quantitative attributes

Choosing a Map Projection

Scale-preserving mapsno map preserves true distance for all measurements 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

An angular unit of measure

A prime meridian

A datum, which includes a spheroid

Planar Coordinate Systems

Cartesian Coordinates

Calculate distance A-B

Universal Transverse Mercator

UTM zones

UTM Zones

.. as seen from the North Pole

UTM Projections Each zone uses a custom Transverse

Mercator projection with its own central meridian

Universal Polar Stereographic

Fills the holes of UTM in polar regions

State Plane Coordinate System

SPC

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 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