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Introduction to GIS

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These are standard functions in popular GIS packages ... e.g. SPOT and IKONOS satellites and aerial photography. Passive and active sensors ... – PowerPoint PPT presentation

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Title: Introduction to GIS


1
Introduction to GIS
  • SGO 1910/4930
  • September 19, 2006

2
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)

3
Georeferencing
4
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.

5
  • 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.

6
Georeferencing
  • Is essential in GIS, since all information must
    be linked to the Earths 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 todays georeferences
    are still meaningful tomorrow

7
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

8
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

9
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

10
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)

11
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.

12
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.

13
Postal Addresses and Postcodes
  • Every dwelling and office is a potential
    destination for mail
  • Dwellings and offices are arrayed along streets,
    and numbered accordingly
  • Streets have names that are unique within local
    areas
  • Local areas have names that are unique within
    larger regions
  • If these assumptions are true, then a postal
    address is a useful georeference

14
Where Do Postal Addresses Fail as Georeferences?
  • In rural areas
  • Urban-style addresses have been extended recently
    to many rural areas
  • For natural features
  • Lakes, mountains, and rivers cannot be located
    using postal addresses
  • When numbering on streets is not sequential
  • E.g. in Japan

15
Postcodes as Georeferences
  • Defined in many countries
  • E.g. ZIP codes in the US
  • Hierarchically structured
  • The first few characters define large areas
  • Subsequent characters designate smaller areas
  • Coarser spatial resolution than postal address
  • Useful for mapping

16
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
17
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

18
Users of Linear Referencing
  • Transportation authorities
  • To keep track of pavement quality, signs, traffic
    conditions on roads
  • Police
  • To record the locations of accidents

19
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

20
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

21
 
 
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 Earths 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.
22
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 Earths rotation and center of mass,
    and the Greenwich Meridian

23
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.

24
Geographic Coordinates
Prime Meridian
Equator
Prime Meridian
25
Geographic Coordinates as Data
26
Oslo, Norway
  • 59o56 N. Latitude
  • 10o45 E. Longitude

27
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.
28
Definition of Latitude
  • Requires a model of the Earths 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 Earths axis in this
    case)

29
Earth Shape Sphere and Ellipsoid
30
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

31
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.

32
Cartography
  • How can a flat map be used to describe locations
    on the earths curved surface?

33
Projections and Coordinates
  • There are many reasons for wanting to project the
    Earths surface onto a plane, rather than deal
    with the curved surface
  • The paper used to output GIS maps is flat
  • Flat maps are scanned and digitized to create GIS
    databases
  • Rasters are flat, its impossible to create a
    raster on a curved surface
  • The Earth has to be projected to see all of it at
    once
  • Its much easier to measure distance on a plane

34
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 directions
  • Equal area property Shapes are distorted, but
    features have the correct area
  • Both types of projections will generally distort
    distances

35
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.

36
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.

37
no flat map can be both equivalent and
conformal.
38
Cylindrical Projections
  • Conceptualized as the result of wrapping a
    cylinder of paper around the Earth
  • The Mercator projection is conformal

39
Conic Projections
  • Conceptualized as the result of wrapping a cone
    of paper around the Earth
  • Standard Parallels occur where the cone
    intersects the Earth

40
The Unprojected Projection
  • Assign latitude to the y axis and longitude to
    the x axis
  • A type of cylindrical projection
  • Is neither conformal nor equal area
  • As 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

41
The Universal Transverse Mercator (UTM) Projection
  • A type of cylindrical projection
  • Implemented as an internationally standard
    coordinate system
  • Initially devised as a military standard
  • Uses a system of 60 zones
  • Maximum distortion is 0.04
  • Transverse Mercator because the cylinder is
    wrapped around the Poles, not the Equator

42
Zones are each six degrees of longitude, numbered
as shown at the top, from W to E
43
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 arrangements
  • Use only one of the two projections, and accept
    the greater-than-normal distortions in the other
    zone
  • Use a third projection spanning the jurisdiction
  • E.g. Italy is spans UTM zones 32 and 33

44
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

45
State Plane Coordinates
  • Defined in the US by each state
  • Some states use multiple zones
  • Several different types of projections are used
    by the system
  • Provides less distortion than UTM
  • Preferred for applications needing very high
    accuracy, such as surveying

46
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

47
GIS Capability
  • A GIS package should be able to move between
  • map projections,
  • coordinate systems,
  • datums, and
  • ellipsoids.

48
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49
Data AcquisitionGetting the Map into the
Computer
50
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.

51
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)

52
GIS data can be
  • Purchased.
  • Found from existing sources in digital form.
  • Captured from analog maps by GEOCODING.

53
Finding Existing Map Data
  • Map libraries
  • Reference books
  • State and local agencies
  • Federal agencies
  • Commercial data suppliers

54
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.

55
Statenskartverkhttp//ngis.statkart.no/katalog/ja
va/katalog.asp
  • Rasterdata
  • Temakart
  • Vektordata
  • Primærdata
  • Prosjekter

56
(No Transcript)
57
Data Collection
  • One of most expensive GIS activities
  • Many diverse sources
  • Two broad types of collection
  • Data capture (direct collection)
  • Data transfer
  • Two broad capture methods
  • Primary (direct measurement)
  • Secondary (indirect derivation)

58
Data Collection Techniques
59
GEOCODING
  • Geocoding is the conversion of spatial
    information into digital form.
  • Geocoding involves capturing the map, and
    sometimes also capturing the attributes.

60
Primary Data Capture
  • Capture specifically for GIS use
  • Raster remote sensing
  • e.g. SPOT and IKONOS satellites and aerial
    photography
  • Passive and active sensors
  • Resolution is key consideration
  • Spatial
  • Spectral
  • Temporal

61
Secondary Geographic Data Capture
  • Data collected for other purposes can be
    converted for use in GIS
  • Raster conversion
  • Scanning of maps, aerial photographs, documents,
    etc
  • Important scanning parameters are spatial and
    spectral (bit depth) resolution

62
Vector Primary Data Capture
  • Surveying
  • Locations 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
  • GPS
  • Collection of satellites used to fix locations on
    Earths surface
  • Differential GPS used to improve accuracy

63
Vector Secondary Data Capture
  • Collection 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

64
Managing Data Capture Projects
  • Key principles
  • Clear plan, adequate resources, appropriate
    funding, and sufficient time
  • Fundamental tradeoff between
  • Quality, speed and price
  • Two strategies
  • Incremental
  • Blitzkrieg (all at once)
  • Alternative resource options
  • In house
  • Specialist external agency

65
Summary
  • Data collection is very expensive,
    time-consuming, tedious and error prone
  • Good procedures required for large scale
    collection projects
  • Main techniques
  • Primary
  • Raster e.g. remote sensing
  • Vector e.g. field survey
  • Secondary
  • Raster e.g. scanning
  • Vector e.g. table digitizing

66
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

67
Digitizer
68
The Digitizing Tablet
69
Digitizing
  • Stable base map
  • Fix to tablet
  • Digitize control
  • Determine coordinate transformation
  • Trace features
  • Proof plot
  • Edit
  • Clean and build

70
Selecting points to digitize
71
Scanner
72
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

73
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?
74
Field data collection
75
Pen Portable PC and GPS
76
Data Transfer
  • Buy v build is an important question
  • Many widely distributed sources of GI
  • Key catalogs include
  • US NSDI Clearinghouse network
  • Geography Network
  • Access technologies
  • Translation
  • Direct read

77
Attribute data
  • Logically can be thought of as in a flat file
  • Table with rows and columns
  • Attributes by records
  • Entries called values.

78
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.

79
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.

80
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.
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