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Introduction to Geographical Data

Kris Ray Confederated Tribes of the

Colville Reservation

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GIS Definitions

Geographic Information System (GIS) used for capturing, storing, analyzing and managing data and associated attributes which are spatially referenced to Earth OR

A computer system capable of holding and using data describing places on Earth's surface OR

Graphic representation of where features are, explicitly and relative to one another

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Components of GIS

Hardware (computer to run program and display data)

Software (to collect and calculate data)

Trained staff (to coordinate above functions)

Data (collected from actual points on earth)

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Foundation of GIS

Geographic data - referenced spatially by coordinate system

Latitude and longitude Universal Transverse Mercator Legal-Survey Description

Attribute data Characteristics of feature or map component

Map legend Links attributes to geographic data

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Geographic Data

Point data - Associated with a single location in space. Example: monitoring sites

Linear data - location is described by a string of spatial coordinates. Examples: rivers, roads and pipelines

• Area data - described by a closed string of spatial coordinates or polygon. Examples: forest stands, soil classification, and administrative boundaries

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Layers

• Representation of a dataset in a digital map environment

• Components of the legend

• Theme

• Overlays

• Composed of visual representation and attribute data

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Visualization of Layers

“Mini-maps” or “sub-maps”come together to make a map that answers a question

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Projection

• Mathematical means of transferring information from a model of Earth, 3-dimensional curved surface to 2-dimensional medium—paper or a computer screen

• Different projections used for different types of maps – each projection particularly suits certain uses

• For example, a projection that accurately represents shapes of continents will distort relative sizes

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Projections

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Datum• Reference specifications of measurement

system, usually a system of coordinate positions on a surface (horizontal datum) or heights above or below a surface (vertical datum)

• North American Datum of 1927 (NAD 27) – Zero based on geographic center of

lower 48, in Kansas• World Geodetic System of 1984 (WGS 84)

and the North American Datum of 1983 (NAD 83) – Zero is based on Earth’s center of gravity

Source: ESRI @ http://support.esri.com/index.cfm?fa=knowledgebase.gisDictionary.search&searchTerm=datum

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Different Data

NAD 27NAD 83 + WGS

84

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Coordinate System• Geographic Coordinates such as latitude

and longitude, 56°27'40" and 116°11'25".

• Map Projection Universal Transverse Mercator (UTM) where coordinates are measured in meters, 545,000.000 and 6,453,254.000 normally reference to a central meridian. Easting's refer to X coordinates while Northings refer to Y coordinates.

• Legal Survey Description, Meridian, Township, Range and Section

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Simple Example

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Complex Example

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Remember

• GIS is a way of analyzing geographic data

• Layers: “mini-maps” or “sub-maps”• Best maps have layers that come

together to answer one specific question

• WGS 84 and NAD 83 work well with GPS

• Always make sure data (plural of datum) match

Source: ESRI @ http://support.esri.com/index.cfm?fa=knowledgebase.gisDictionary.search&searchTerm=datum

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Enjoy the

Wonderful World

of GIS