GIS 101: Introduction to Geographic Information Systems

1
GIS 101 Introduction to Geographic Information
Systems

• Week III
• REVIEW
• Coordinate Systems and Digital Maps

2
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)

3
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

4
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.

5
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.

6
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).

7
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.

8
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

9
Topology - What in the heck is that dang deal?

• Topology is the property that describes the
  adjacency, and connectivity of features.

10
TopologyWhy We Need It

• Topology enables the advanced functions of GIS
• Proximity
• Routing
• Buffering

11
DIGITAL MAPS

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

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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
  space.
• Programmers use hexadecimal as shorthand for
  binary, since two decimal digits correspond to 8
  bits (a byte).

13
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.

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Two Storage Models for GIS

• Raster
• Vector

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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

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RASTER

• A cell has a resolution, given as the cell size
  in ground units.

17
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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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.

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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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Comparison/Contrast

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

21
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.

22
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.

23
Map Structure in the GIS

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

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