Introduction to GIS SGO 1910/4930 September 19, 2006.

Introduction to GIS

SGO 1910/4930

September 19, 2006

Announcements

• Review lecture on Thursday (21.09.06) 12.00 - 14.00 in 221HH).

• Midterm quiz next week (26.09.06)25 questions (multiple choice, true-false)

Georeferencing

Georeferencing

Geographic information contains either an explicit geographic reference (such as latitude and longitude coordinates), or an implicit reference such as an address, road name, or postal code.

Geographic references allow you to locate features for analysis.

Time is optional in a GIS, whereas location is essential.

Without location, data are non-spatial or aspatial and have little value within a GIS.

Georeferencing

Is essential in GIS, since all information must be linked to the Earth’s surface

The method of georeferencing must be:Unique, linking information to exactly one location

Shared, so different users understand the meaning of a georeference

Persistent through time, so today’s georeferences are still meaningful tomorrow

Uniqueness

A georeference may be unique only within a defined domain, not globally

There are many instances of Storgatas in Norway, but only one in any city

The meaning of a reference to Greenwich may depend on context, since there are cities and towns called Greenwich in several parts of the world

Georeferences as Measurements Some georeferences are metric

They define location using measures of distance from fixed places

E.g., distance from the Equator or from the Greenwich Meridian

Others are based on ordering E.g. street addresses in most parts of the world

order houses along streets Others are only nominal

Placenames do not involve ordering or measuring

Place names

The earliest form of georeferencing And the most commonly used in

everyday activities Many names of geographic features

are universally recognized Others may be understood only by

locals Names work at many different

scales From continents to small villages and

neighborhoods

Persistence through time

Changes can lead to confusion (Peking to Beijing, St. Petersburg to Leningrad)

Place names can be disassociated with location over time (e.g., Atlantis, Camelot)

Example: South Africa

Since the first democratic election in South Africa in 1994, a number of changes have been made to geographical names in the country. It can get a bit confusing, as mapmakers struggle to keep up, and roadsigns aren't immediately changed. In many instances, the 'new' names were existing ones used by parts of the population; others are new municipal entities. All name changes have to be approved by the South African Geographical Names Council, which is responsible for standardising geographical names in South Africa.

Name changes in South Africa

Redivision of the Provinces in South AfricaOne of the first major changes was the redivision of the country into eight provinces, rather than the existing four (Cape Province, Orange Free State, Transvaal, and Natal ). The Cape Province divided into three (Western Cape, Eastern Cape, and Northern Cape), the Orange Free State became the Free State, Natal was renamed KwaZulu-Natal, and the Transvaal was divided into Gauteng, Mpumalanga (initially Eastern Transvaal), Northwest Province, and Limpopo Province (initially Northern Province). Renamed Towns in South AfricaAmong the towns renamed were some named after leaders significant in Afrikaner history. So Pietersburg, Louis Trichard, and Potgietersrust became, respectively, Polokwane, Makhoda, and Mokopane (the name of a king). Warmbaths changed to Bela-Bela, a Sesotho word for hot spring. Names Given to New Geographical EntitiesSeveral new municipal and megacity boundaries have been created. The City of Tshwane Metropolitan Municipality covers cities such as Pretoria, Centurion, Temba, and Hammanskraal. The Nelson Mandela Metropole covers the East London/Port Elizabeth area. Colloquial City Names in South AfricaCape Town is known as eKapa. Johannesburg is called eGoli, literally meaning "the place of gold". Durban is called eThekwini, which translates as "In the Bay" (although some controversy was caused when several eminent Zulu linguists claimed that the name actually means "the one-testicled one" referring to the shape of the bay). Changes to Airport Names in South AfricaThe names of all South African airports were changed from politician's names to simply the city or town they're located in. Cape Town International Airport needs no explanation, whereas who but a local would know where DF Malan Airport was? Johannesburg International Airport may change to O.R. Tambo International Airport.

Postal Addresses and Postcodes

Every dwelling and office is a potential destination for mailDwellings and offices are arrayed along streets, and numbered accordinglyStreets have names that are unique within local areasLocal areas have names that are unique within larger regionsIf these assumptions are true, then a postal address is a useful georeference

Where Do Postal Addresses Fail as Georeferences?

In rural areasUrban-style addresses have been extended recently to many rural areas

For natural featuresLakes, mountains, and rivers cannot be located using postal addresses

When numbering on streets is not sequential

E.g. in Japan

Postcodes as Georeferences

Defined in many countriesE.g. ZIP codes in the US

Hierarchically structuredThe first few characters define large areas

Subsequent characters designate smaller areas

Coarser spatial resolution than postal address

Useful for mapping

ZIP code boundaries are a convenient way to summarize data in the US. The dots on the left have been summarized as a density per square

mile on the right

Linear Referencing

A system for georeferencing positions on a road, street, rail, or river network

Combines the name of the link with an offset distance along the link from a fixed point, most often an intersection

Users of Linear Referencing

Transportation authorities To keep track of pavement quality,

signs, traffic conditions on roads

Police To record the locations of accidents

Problem Cases

Locations in rural areas may be a long way from an intersection or other suitable zero point

Pairs of streets may intersect more than once

Measurements of distance along streets may be inaccurate, depending on the measuring device, e.g. a car odometer

Cadasters

Maps of land ownership, showing property boundaries

The Public Land Survey System (PLSS) in the US and similar systems in other countries provide a method of georeferencing linked to the cadaster

In the Western US the PLSS is often used to record locations of natural resources, e.g. oil and gas wells

Portion of the Township and Range system (Public Lands Survey System) widely used in the western US as the basis of land ownership. Townships are laid out in six mile squares on either side of an accurately surveyed Principal

Meridian. The offset shown between townships 16N and 17N is needed to accommodate the Earth’s curvature (shown much exaggerated). The square mile sections within each township are numbered as shown in (A) east of the

Principal Meridian, and reversed west of the Principal Meridian.

T15N

T16N

T17N

T18N

R1W R1E

T14N

T19N

R2W R2E

1 2 3 4 5 6

12 11

10

9 8 7

13 14

15

16

17

18

24 23

22

21

20

19

25 26

27

28

29

30

36 35

34

33

32

31

Latitude and Longitude

The most comprehensive and powerful method of georeferencing Metric, standard, stable, unique

Uses a well-defined and fixed reference frame Based on the Earth’s rotation and center

of mass, and the Greenwich Meridian

Geographic Coordinates Geographic coordinates are the earth's latitude

and longitude system, ranging from 90 degrees south to 90 degrees north in latitude and 180 degrees west to 180 degrees east in longitude.

A line with a constant latitude running east to west is called a parallel.

A line with constant longitude running from the north pole to the south pole is called a meridian.

The zero-longitude meridian is called the prime meridian and passes through Greenwich, England.

A grid of parallels and meridians shown as lines on a map is called a graticule.

Geographic Coordinates

Parallels

EquatorPri

me

Mer

idia

n

Pri

me

Mer

idia

nMer

idia

ns

Geographic Coordinates as Data

Oslo, Norway

59o56’ N. Latitude

10o45’ E. Longitude

North Pole

Greenwich

Equator

Definition of longitude. The Earth is seen here from above the North Pole, looking along the Axis, with the Equator forming the outer circle. The location

of Greenwich defines the Prime Meridian. The longitude of the point at the center of the red cross is determined by drawing a plane through it and the axis, and measuring the angle between this plane and the Prime Meridian.

Definition of Latitude

Requires a model of the Earth’s shape

The Earth is somewhat elliptical The N-S diameter is roughly 1/300 less

than the E-W diameter More accurately modeled as an ellipsoid

than a sphere An ellipsoid is formed by rotating an

ellipse about its shorter axis (the Earth’s axis in this case)

Earth Shape: Sphere and Ellipsoid

The History of Ellipsoids

Because the Earth is not shaped precisely as an ellipsoid, initially each country felt free to adopt its own Ellipsoid as the most accurate approximation to its own part of the Earth

Today an international standard has been adopted known as WGS 84 Its US implementation is the North American

Datum of 1983 (NAD 83) Many US maps and data sets still use the North

American Datum of 1927 (NAD 27) Differences can be as much as 200 m

Cartography and GIS

Understanding the way maps are encoded to be used in GIS requires knowledge of cartography.

Cartography is the science that deals with the construction, use, and principles behind maps.

Cartography

How can a flat map be used to describe locations on the earth’s curved surface?

Projections and Coordinates

There are many reasons for wanting to project the Earth’s surface onto a plane, rather than deal with the curved surface

The paper used to output GIS maps is flatFlat maps are scanned and digitized to create GIS databasesRasters are flat, it’s impossible to create a raster on a curved surfaceThe Earth has to be projected to see all of it at onceIt’s much easier to measure distance on a plane

Distortions

Any projection must distort the Earth in some way

Two types of projections are important in GIS

Conformal property: Shapes of small features are preserved: anywhere on the projection the distortion is the same in all directionsEqual area property: Shapes are distorted, but features have the correct areaBoth types of projections will generally distort distances

Map Projections

A transformation of the spherical or ellipsoidal earth onto a flat map is called a map projection.

The map projection can be onto a flat surface or a surface that can be made flat by cutting, such as a cylinder or a cone.

If the globe, after scaling, cuts the surface, the projection is called secant. Lines where the cuts take place or where the surface touches the globe have no projection distortion.

Map Projections (ctd) Projections can be based on axes parallel to the

earth's rotation axis (equatorial), at 90 degrees to it (transverse), or at any other angle (oblique).

A projection that preserves the shape of features across the map is called conformal.

A projection that preserves the area of a feature across the map is called equal area or equivalent.

No flat map can be both equivalent and conformal. Most fall between the two as compromises.

To compare or edge-match maps in a GIS, both maps MUST be in the same projection.

“no flat map can be both equivalent and conformal.”

Cylindrical Projections

Conceptualized as the result of wrapping a cylinder of paper around the Earth

The Mercator projection is conformal

Conic Projections

Conceptualized as the result of wrapping a cone of paper around the Earth

Standard Parallels occur where the cone intersects the Earth

The “Unprojected” Projection

Assign latitude to the y axis and longitude to the x axis

A type of cylindrical projectionIs neither conformal nor equal areaAs latitude increases, lines of longitude are much closer together on the Earth, but are the same distance apart on the projection

Also known as the Plate Carrée or Cylindrical Equidistant Projection

The Universal Transverse Mercator (UTM) Projection

A type of cylindrical projection Implemented as an internationally standard

coordinate systemInitially devised as a military standard

Uses a system of 60 zonesMaximum distortion is 0.04%

Transverse Mercator because the cylinder is wrapped around the Poles, not the Equator

Zones are each six degrees of longitude, numbered as shown at the top, from W to E

Implications of the Zone System

Each zone defines a different projection Two maps of adjacent zones will not fit

along their common border Jurisdictions that span two zones must

make special arrangementsUse only one of the two projections, and accept the greater-than-normal distortions in the other zoneUse a third projection spanning the jurisdictionE.g. Italy is spans UTM zones 32 and 33

UTM Coordinates

In the N Hemisphere define the Equator as 0 mN

The central meridian of the zone is given a false Easting of 500,000 mE

Eastings and northings are both in meters allowing easy estimation of distance on the projection

A UTM georeference consists of a zone number, a six-digit easting and a seven-digit northing

E.g., 14, 468324E, 5362789N

State Plane Coordinates

Defined in the US by each stateSome states use multiple zones

Several different types of projections are used by the system

Provides less distortion than UTMPreferred for applications needing very high accuracy, such as surveying

Converting Georeferences

GIS applications often require conversion of projections and ellipsoids

These are standard functions in popular GIS packages

Street addresses must be converted to coordinates for mapping and analysis

Using geocoding functions

Placenames can be converted to coordinates using gazetteers

GIS Capability

A GIS package should be able to move between

map projections,

coordinate systems,

datums, and

ellipsoids.

Data Acquisition:Getting the Map into the Computer

GIS maps are digital

Real maps: traditional paper maps that can be touched

Virtual maps: an arrangement of information inside the computer; the GIS can be used to generate the map however and whenever necessary.

GIS Data Conversion

Traditionally the most time-consuming and expensive part of a GIS project

Involves a one-time cost

Digital maps can be reused and shared.

Requires maintenance (eg. updating)

GIS data can be

Purchased.

Found from existing sources in digital form.

Captured from analog maps by GEOCODING.

Finding Existing Map Data

Map libraries

Reference books

State and local agencies

Federal agencies

Commercial data suppliers

Existing Map Data

Existing map data can be found through a map library, via network searches, or on media such as CD-ROM and disk.

Many major data providers make their data available via the Internet.

Statenskartverkhttp://ngis.statkart.no/katalog/java/katalog.asp

Rasterdata

Temakart

Vektordata

Primærdata

Prosjekter

Data Collection

One of most expensive GIS activitiesMany diverse sourcesTwo broad types of collection

Data capture (direct collection)Data transfer

Two broad capture methodsPrimary (direct measurement)Secondary (indirect derivation)

Data Collection Techniques

Raster Vector

Primary Digital remote sensing images

GPS measurements

Digital aerial photographs

Survey measurements

Secondary Scanned maps Topographic surveys

DEMs from maps Toponymy data sets from atlases

GEOCODING

Geocoding is the conversion of spatial information into digital form.

Geocoding involves capturing the map, and sometimes also capturing the attributes.

Primary Data Capture

Capture specifically for GIS use

Raster – remote sensinge.g. SPOT and IKONOS satellites and aerial photography

Passive and active sensors

Resolution is key considerationSpatial

Spectral

Temporal

Secondary Geographic Data CaptureData collected for other purposes can be converted for use in GIS

Raster conversionScanning of maps, aerial photographs, documents, etc

Important scanning parameters are spatial and spectral (bit depth) resolution

Vector Primary Data Capture

SurveyingLocations of objects determines by angle and distance measurements from known locations

Uses expensive field equipment and crews

Most accurate method for large scale, small areas

GPSCollection of satellites used to fix locations on Earth’s surface

Differential GPS used to improve accuracy

Vector Secondary Data CaptureCollection of vector objects from maps, photographs, plans, etc.Digitizing

Manual (table) Heads-up and vectorization

Photogrammetry – the science and technology of making measurements from photographs, etc.COGO – Coordinate Geometry

Managing Data Capture ProjectsKey principles

Clear plan, adequate resources, appropriate funding, and sufficient time

Fundamental tradeoff between Quality, speed and price

Two strategiesIncremental‘Blitzkrieg’ (all at once)

Alternative resource optionsIn houseSpecialist external agency

Summary

Data collection is very expensive, time-consuming, tedious and error proneGood procedures required for large scale collection projectsMain techniques

PrimaryRaster – e.g. remote sensingVector – e.g. field survey

SecondaryRaster – e.g. scanningVector – e.g. table digitizing

Digitizing

Captures map data by tracing lines from a map by hand

Uses a cursor and an electronically-sensitive tablet

Result is a string of points with (x, y) values

Digitizer

The Digitizing Tablet

Digitizer cursor transmitsa pulse from an electomagneticcoil under the view lens.

Pulse is picked up bynearest grid wires undertablet surface.

Result is sent tocomputer afterconversion tox and y units.

map

Digitizing

Stable base map

Fix to tablet

Digitize control

Determine coordinate transformation

Trace features

Proof plot

Edit

Clean and build

Selecting points to digitize

Scanner

Scanning

Places a map on a glass plate, and passes a light beam over it

Measures the reflected light intensity

Result is a grid of pixels

Image size and resolution are important

Features can “drop out”

Scanning example

This section of map was scanned, resulting in a file in TIF format that was bytes in size. This was a file of color intensities between 0 and 255 for red, green, and blue in each of three layers spaced on a grid 0.25 millimeter apart. How much data would be necessary to capture the features on your map as vectors? Would it be more or less than the grid (raster) file?

Field data collection

Pen Portable PC and GPS

Data Transfer

Buy v build is an important questionMany widely distributed sources of GIKey catalogs include

US NSDI Clearinghouse networkGeography Network

Access technologiesTranslationDirect read

Attribute data

Logically can be thought of as in a flat file

Table with rows and columns

Attributes by records

Entries called values.

Database Management Systems

Data definition module sets constraints on the attribute values

Data entry module to enter and correct values

Data management system for storage and retrieval

Data definitions can be listed as a data dictionary

Database manager checks values with this dictionary, enforcing data validation.

The Role of Error

Map and attribute data errors are the data producer's responsibility, but the GIS user must understand error.

Accuracy and precision of map and attribute data in a GIS affect all other operations, especially when maps are compared across scales.

Quick review:

Geographic information contains either an explicit geographic reference (such as latitude and longitude coordinates), or an implicit reference such as an address, road name, or postal code.

Geographic references allow you to locate features for analysis.