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GIS 101: Introduction to Geographic Information Systems


Map scale is based on the representative fraction, the ratio of a distance on ... The map projection can be onto a flat surface or a surface that can be made flat ... – PowerPoint PPT presentation

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Title: GIS 101: Introduction to Geographic Information Systems

GIS 101 Introduction to Geographic Information
  • Week III
  • Coordinate Systems and Digital Maps

Map Projections
  • Geoid- 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.
  • Ellipsoid- An ellipsoid is an ellipse rotated in
    three dimensions about its shorter axis.
  • Sphere- The sphere of the earth is about 40
    million meters in circumference. (24000 Miles)

Map 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

Map 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.

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
  • The metric system is far easier to use for GIS
    work. But going between imperial and metric
    measurements can be a juggling act.

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 axes parallel to the
    earth's rotation (equatorial), or at 90 degrees
    to it (transverse), or at any other (oblique).

Coordinate Systems
  • A coordinate system is an ordered set of data
    values that specifies a location may be absolute
    or relative.
  • 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.

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
  • Most modern map projections use positive x,y

Topology - What in the heck is that dang deal?
  • Topology is the property that describes the
    adjacency, and connectivity of features.

TopologyWhy We Need It
  • Topology enables the advanced functions of GIS
  • Proximity
  • Routing
  • Buffering

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

Methods to Store the Data
  • Files can be written in binary or as ASCII text.
  • Binary is faster to read and smaller, ASCII can
    be read by humans and edited, but uses more
  • Programmers use hexadecimal as shorthand for
    binary, since two decimal digits correspond to 8
    bits (a byte).

Map Structure in the GIS
  • A GIS map is a scaled down digital representation
    of point, line, area, and volume features.
  • While most GIS systems can handle both raster and
    vector data, only one is used for the internal
    organization of spatial data.

Two Storage Models for GIS
  • Raster
  • Vector

  • 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."
  • 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.

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

  • 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
  • Polygons can be built from points or lines.

  • 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.

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

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.

Tabular Data - How the Attribute Information is
  • 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.

Map Structure in the GIS
  • A GIS map is a scaled down digital representation
    of point, line, area, and volume features.

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 is in
  • What projection your data is in
  • What the datum is
  • What units the data are in.
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