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1
Introduction to Geographical Data
Kris Ray Confederated Tribes of the
Colville Reservation
2
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