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GIS 101 Introduction to Geographic Information
Systems

• Cartographic Principles

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Cartography

• The art, science, and craft of mapmaking.
• Cartography is the science that deals with the
  construction, use, and principles behind maps.

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Elements of Cartography

• Map itself to be most prominent feature
• Legend - Explaining the map features
• Title Subtitles - Descriptive Clear
• Neatlines - Separating map elements
• Scale bar - Describing the scale
• North Arrow - Indicating direction

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Cartographic Principles

• Attribute Information
• Map Scale
• Map Projections
• Coordinate Systems
• Geographic Information

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Attribute Information

• Information can be organized as lists, numbers,
  tables, text, pictures, maps, or indexes.
• Clusters of information called data can be stored
  together as a database.
• A database is stored in a computer as files and
  in files as tables - thus Tabular.

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Attribute Information

• In a database, we store attributes as column
  headers and records as rows.
• The contents of an attribute for one record is a
  value. A value can be numerical or text.
• Data in a GIS must contain a geographic reference
  to a map, This reference is known as coordinates.

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Bringing It All together

• The GIS cross-references attribute data with the
  map data, allowing searches based on either or
  both.
• Understanding the way maps are encoded to be used
  in GIS requires knowledge of cartography.
• Getting these maps into Digital form is called
  DIGITIZING.

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Bringing It All together

• Cartography is the science that deals with the
  construction, use, and principles behind maps.

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Map Scale and Projections

• The earth can be modeled as a sphere, an oblate
  ellipsoid, or a geoid.
• The sphere is about 40 million meters in
  circumference. (24000 Miles)
• An ellipsoid is an ellipse rotated in three
  dimensions about its shorter axis.
• The earth's ellipsoid is only about 1/297 off
  from a sphere.

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Map Scale and Projections

• Many ellipsoids have beep measured, and maps
  based on each.
• Examples are WGS83 and GRS80.An ellipsoid gives
  the base elevation for mapping, called a datum.
• Examples are NAD27 and NAD83.

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Map Scale and Projections

• The geoid is a figure that adjusts the best
  ellipsoid and the variation of gravity locally.
  It is the most accurate, and is used more in
  geodesy than GIS and cartography.

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Map Scale and Projections

• Map scale is based on the representative
  fraction, the ratio of a distance on the map to
  the same distance on the ground.
• To compare or edge-match maps in a GIS, both maps
  MUST be at the same scale and have the same
  extent.
• The metric system is far easier to use for GIS
  work. But going between imperial and metric
  measurements can be a juggling act.

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Map Scale and Projections

• Geographic coordinates are the earth's latitude
  and longitude system ranging from 90 degrees
  south to 90 degrees north in latitude 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.

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Map Scale and Projections

• The zero-longitude meridian is called 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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Map Scale and 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.
• Projections can be based on axis parallel to the
  earth's rotation (equatorial), or at 90 degrees
  to it (transverse), or at any other (oblique).

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Map Scale and Projections

• All map projections representing all or part of
  the Earths surface as a flat map, create
  distortions in distance, area, shape, or
  direction.

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Map Scale and Projections

• A projection that preserves the shape of features
  across the map is conformal.
• A projection that preserves the area of a feature
  across the map is equal area or equivalent.
• No flat map can be both equivalent and conformal.
  Most fall between two as compromises.

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Coordinate Systems

• The Coordinate Plane
• (or Cartesian Coordinate System)
• The plane uses two axis 1 horizontal (x),
  representing east-west, and 1 vertical (y),
  representing north-south.
• The point at which they intersect is called the
  ORIGIN.
• Most modern map projections use positive x,y
  coordinates

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Coordinate Systems

• A coordinate system is a standardized method for
  assigning code locations so that locations can be
  found using the codes alone.
• Standardized coordinate systems use absolute
  locations - Not Relative.
• A map captured in the units of the paper sheet on
  which it is printed is based on relative
  locations on the map.
• In a coordinate system, the x-direction value is
  the easting and the y-direction value is the
  northing. Most systems make both values positive.

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Coordinate Systems

• Some standard coordinate systems used in the
  United States are
• Geographic coordinates
• Universal Transverse Mercator system
• Military grid
• State plane system
• A GIS package should be able to move between map
  projections, coordinate systems, datums, and
  ellipsoids.

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Coordinate Systems and Projections Work Together

• To compare or edge-match maps in a GIS, both maps
  MUST be in the same coordinate system.
• To compare or edge-match maps in a GIS, both maps
  MUST be in the same projection.

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Geographic Information

• Geographic Information has the characteristics of
  volume,dimensionality, and continuity.
• Simple geographic features can be used to build
  more complex ones.
• Areas are made up of lines which are made up of
  points represented by their coordinates.

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Geographic Information

• Geographic features collectively have the
  properties of size, distribution, pattern,
  contiguity, neighborhood, shape, scale, and
  orientation.
• Much of GIS analysis and description consists of
  investigating the properties of geographic
  features and determining the relationships
  between them.

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DIGITAL MAPS

• GIS is computer based, this necessitates storing
  spatial and tabular data as numbers.

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Two Storage Models for GIS

• Vector
• Raster

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VECTOR

• A vector data model uses points stored by their
  real coordinates.
• lines and areas are built from sequences of
  points in order.
• lines have a direction to the ordering of the
  points.
• Polygons can be built from points or lines.
• Vectors can store information about topology.

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RASTER

• A raster data model uses a grid.
• One grid cell is one unit, it holds one, and only
  one attribute.
• Every cell has a value, even if it is "missing."
  - NULL VALUE
• A cell holds a number and the number can be used
  as an index value representing an attribute
• A cell has a resolution, given as the cell size
  in ground units.

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Raster As a Grid

• Grids are poor at representing points, lines and
  areas, but good at representing surfaces.
• Grids are good only at very localized topology,
  and weak otherwise.
• Grids are a natural for scanned or remotely
  sensed data.
• Grids must often include redundant or missing
  data.

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Comparison/Contrast

• Vectors work well with pen and light plotting
  devices, and tablet digitizers.
• Vectors are not good at continuous coverages or
  plotters that fill areas.
• Rasters are easy for the computer to understand
  and store, easy to read and write, and easy to
  draw on the screen.
• Changing vector to raster is easy, raster to
  vector is hard.

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Transforming Vector to Raster

• Points and lines in raster format have to move to
  a cell center.
• Lines can become fat. Areas may need separately
  coded edges.
• Each cell can be owned by only one feature.
• As data, all cells must be able to hold the
  maximum cell value.

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Comparison/Contrast

• Vectors are easier for Humans to Understand, draw
  and conceptualize.
• They represent the real world more than raster.

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Tabular Data - How the Attribute Information is
Stored

• Attribute data are stored logically in files.
• A file is represented in table form as a matrix
  of numbers and values stored in rows and columns,
  like a spreadsheet.
• DBMSs use many different methods to store and
  manage files in physical files.

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Map Structure in the GIS

• A GIS map is a scaled down digital representation
  of point, line, area, and volume features.
• A Raster maps directly onto a programming
  computer memory structure called an array.

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Topology and the Arc/Node Model

• In the Arc/Node model, an area consists of lines
  and a line consists of points.
• Points, lines, and areas can each be stored in
  their own files, with links between them.
• The topological vector model uses the line (arc)
  as a basic unit. Areas (Polygons) are built up
  from arcs.

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Topology and the Arc/Node Model

• The end point of a line (arc) is called a node.
  Arc junctions are only at nodes.
• Stored with the arc is the topology, i.e. the
  connecting arcs and left and right polygons.

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Graphic Editing

• Snapping
• Dangle
• Slivers
• Undershoots
• Overshoots
• Node
• Vertex

Additional Terms...

• Georeferenced.
• Orthopphotography
• TIN
• TIF, JPEG, BIL - Formats for Imagery(Raster)

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TopologyWhy We Need It

• Topology allows automated error detection and
  elimination.
• Digitized or imported data must have
  topologically built.
• A GIS has to be able to build topology from
  unconnected arcs.
• Nodes that are close together can be "snapped" to
  establish a connection.
• Slivers due to double digitizing and overlay are
  eliminated.

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TopologyWhy We Need It

• Topology allows many GIS operations to be done
  without accessing the point files.
• Topology enables the advanced functions of GIS
• Proximity
• Routing
• Buffering

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Formats for GIS Data

• Most GIS systems can import different data
  formats, or use utility programs to convert
  them.
• Data formats can be industry standard, commonly
  accepted or standard. Shapefiles DXF(Data
  Exchange Format)

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Formats for GIS Data

• Vector GIS data formats are DLG (Digital Line
  Graph) and TIGER, which have topology.
• Most digital images are raster
• Orthophoto - def pp204
• Georeferenced.
• Most GIS accept TIF, GIF, JPEG or encapsulated
  PostScript, which are not georeferenced.
• DEMs (Digital Elevation Models are true raster
  data formats.

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Data Exchange

• Efficient data exchange is important for the your
  GIS.
• Data exchange by translation (export and import)
  can lead to significant errors in attributes and
  in geometry.
• In the United States, the SDTS was evolved to
  facilitate data transfer.
• Both DLG and TIGER data are available in SDTS
  format.

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Data Exchange Bottom line

• Understand what the systems are and know what
  your GIS package accepts.
• To transfer data it is necessary to know
• What coordinates your data are in
• What projection your data are in
• What the datum is
• What units the data are in.

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