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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.

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Introduction to gis l.jpg

Introduction to GIS

SGO 1910/4930

September 19, 2006


Announcements l.jpg
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)



Georeferencing4 l.jpg
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.


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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 l.jpg
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 l.jpg
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


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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


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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)


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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.


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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.


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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


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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 l.jpg
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


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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 l.jpg
Linear Referencing in the US. The dots on the left have been summarized as a density per square mile on the right

  • 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


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Users of Linear Referencing in the US. The dots on the left have been summarized as a density per square mile on the right

  • Transportation authorities

    • To keep track of pavement quality, signs, traffic conditions on roads

  • Police

    • To record the locations of accidents


Problem cases l.jpg
Problem Cases in the US. The dots on the left have been summarized as a density per square mile on the right

  • 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


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Cadasters in the US. The dots on the left have been summarized as a density per square mile on the right

  • 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


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1 in the US. The dots on the left have been summarized as a density per square mile on the right

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T19N

T18N

T17N

T16N

T15N

T14N

R2W

R1W

R1E

R2E

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.


Latitude and longitude l.jpg
Latitude and Longitude in the US. The dots on the left have been summarized as a density per square mile on the right

  • 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 l.jpg
Geographic Coordinates in the US. The dots on the left have been summarized as a density per square mile on the right

  • 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.


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Parallels in the US. The dots on the left have been summarized as a density per square mile on the right

Meridians

Geographic Coordinates

Prime Meridian

Equator

Prime Meridian


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Geographic Coordinates as Data in the US. The dots on the left have been summarized as a density per square mile on the right


Oslo norway l.jpg

Oslo, Norway in the US. The dots on the left have been summarized as a density per square mile on the right

59o56’ N. Latitude

10o45’ E. Longitude


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North Pole in the US. The dots on the left have been summarized as a density per square mile on the right

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 l.jpg
Definition of Latitude in the US. The dots on the left have been summarized as a density per square mile on the right

  • 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 l.jpg
Earth Shape: Sphere and Ellipsoid in the US. The dots on the left have been summarized as a density per square mile on the right


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The History of Ellipsoids in the US. The dots on the left have been summarized as a density per square mile on the right

  • 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 l.jpg
Cartography and GIS in the US. The dots on the left have been summarized as a density per square mile on the right

  • 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 l.jpg
Cartography in the US. The dots on the left have been summarized as a density per square mile on the right

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


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Projections and Coordinates in the US. The dots on the left have been summarized as a density per square mile on the right

  • 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 l.jpg
Distortions in the US. The dots on the left have been summarized as a density per square mile on the right

  • 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 l.jpg
Map Projections in the US. The dots on the left have been summarized as a density per square mile on the right

  • 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 l.jpg
Map Projections (ctd) in the US. The dots on the left have been summarized as a density per square mile on the right

  • 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.


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“no flat map can be both equivalent and conformal.” in the US. The dots on the left have been summarized as a density per square mile on the right


Cylindrical projections l.jpg
Cylindrical Projections in the US. The dots on the left have been summarized as a density per square mile on the right

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

  • The Mercator projection is conformal


Conic projections l.jpg
Conic Projections in the US. The dots on the left have been summarized as a density per square mile on the right

  • 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 l.jpg

Assign latitude to the in the US. The dots on the left have been summarized as a density per square mile on the righty 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 “Unprojected” Projection


The universal transverse mercator utm projection l.jpg
The Universal Transverse Mercator (UTM) Projection in the US. The dots on the left have been summarized as a density per square mile on the right

  • 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



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Implications of the Zone System at the top, from W to E

  • 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 l.jpg
UTM Coordinates at the top, from W to E

  • 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 l.jpg
State Plane Coordinates at the top, from W to E

  • 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


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Converting Georeferences at the top, from W to E

  • 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 l.jpg
GIS Capability at the top, from W to E

  • A GIS package should be able to move between

    • map projections,

    • coordinate systems,

    • datums, and

    • ellipsoids.


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Data Acquisition: at the top, from W to EGetting the Map into the Computer


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GIS maps are digital at the top, from W to E

  • 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.


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GIS Data Conversion at the top, from W to E

  • 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)


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GIS data can be at the top, from W to E

  • Purchased.

  • Found from existing sources in digital form.

  • Captured from analog maps by GEOCODING.


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Finding Existing Map Data at the top, from W to E

  • Map libraries

  • Reference books

  • State and local agencies

  • Federal agencies

  • Commercial data suppliers


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Existing Map Data at the top, from W to E

  • 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.


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Statenskartverk at the top, from W to Ehttp://ngis.statkart.no/katalog/java/katalog.asp

  • Rasterdata

  • Temakart

  • Vektordata

  • Primærdata

  • Prosjekter


Data collection l.jpg
Data Collection at the top, from W to E

  • 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 l.jpg
Data Collection Techniques at the top, from W to E


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GEOCODING at the top, from W to E

  • Geocoding is the conversion of spatial information into digital form.

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


Primary data capture l.jpg
Primary Data Capture at the top, from W to E

  • 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


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Secondary Geographic Data Capture at the top, from W to E

  • 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


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Vector Primary Data Capture at the top, from W to E

  • 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


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Vector Secondary Data Capture at the top, from W to E

  • 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


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Managing Data Capture Projects at the top, from W to E

  • 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


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Summary at the top, from W to E

  • 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


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Digitizing at the top, from W to E

  • 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


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Digitizer at the top, from W to E


The digitizing tablet l.jpg

map at the top, from W to E

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.

The Digitizing Tablet


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Digitizing at the top, from W to E

  • Stable base map

  • Fix to tablet

  • Digitize control

  • Determine coordinate transformation

  • Trace features

  • Proof plot

  • Edit

  • Clean and build


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Selecting points to digitize at the top, from W to E


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Scanner at the top, from W to E


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Scanning at the top, from W to E

  • 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”


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Scanning example at the top, from W to E

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?


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Field data collection at the top, from W to E


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Pen Portable PC and GPS at the top, from W to E


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Data Transfer at the top, from W to E

  • 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


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Attribute data at the top, from W to E

  • Logically can be thought of as in a flat file

  • Table with rows and columns

  • Attributes by records

  • Entries called values.


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Database Management Systems at the top, from W to E

  • 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.


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The Role of Error at the top, from W to E

  • 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.


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Quick review: at the top, from W to E

  • 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.