Introduction to GIS SGO 19104930 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 221 HH). 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 Africa One 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 Kwa. Zulu-Natal, and the Transvaal was divided into Gauteng, Mpumalanga (initially Eastern Transvaal), Northwest Province, and Limpopo Province (initially Northern Province). Renamed Towns in South Africa Among 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 Entities Several 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 Africa Cape Town is known as e. Kapa. Johannesburg is called e. Goli, literally meaning "the place of gold". Durban is called e. Thekwini, 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 Africa The 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 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

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

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

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

1 2 3 4 5 6 12 11 10 9 8 7 13 14 15 16 17 18 24 23 22 21 20 19 T 17 N 25 26 27 28 29 30 T 16 N 36 35 34 33 32 31 T 15 N T 19 N T 18 N T 14 N R 2 W R 1 E R 2 E 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 16 N and 17 N 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.

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.

Equator Prime Meridian Parallels Meridians Prime Meridian Geographic Coordinates

Geographic Coordinates as Data

Oslo, Norway 59 o 56’ N. Latitude 10 o 45’ E. Longitude

North Pole Equator Greenwich 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 flat Flat maps are scanned and digitized to create GIS databases Rasters are flat, it’s impossible to create a raster on a curved surface The Earth has to be projected to see all of it at once It’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 directions Equal area property: Shapes are distorted, but features have the correct area Both 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 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

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

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 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

UTM Coordinates § In the N Hemisphere define the Equator as 0 m. N § The central meridian of the zone is given a false Easting of 500, 000 m. E § 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, 468324 E, 5362789 N

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

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 CDROM and disk. Many major data providers make their data available via the Internet.

Statenskartverk http: //ngis. statkart. no/katalog/java/katalog. asp Rasterdata Temakart Vektordata Primærdata Prosjekter

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)

Data Collection Techniques Raster Primary Secondary Vector Digital remote sensing images GPS measurements Digital aerial photographs Survey measurements 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 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

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

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 Earth’s surface Differential GPS used to improve accuracy

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

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

Summary Data collection is very expensive, timeconsuming, 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

Digitizing Captures map data by tracing lines from a map by hand Uses a cursor and an electronicallysensitive tablet Result is a string of points with (x, y) values

Digitizer

The Digitizing Tablet map Pulse is picked up by nearest grid wires under tablet surface. Result is sent to computer after conversion to x and y units. Digitizer cursor transmits a pulse from an electomagnetic coil under the view lens.

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

Field data collection

Pen Portable PC and GPS

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

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.