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Coordinate Systems. Global Coordinate System – Latitude, Longitude and elevation UTM – eastings and northings, reference points are the equator and the central meridians (60 of them for 60 zones) Extends from 84 ° N to 80 °S

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coordinate systems
Coordinate Systems
  • Global Coordinate System – Latitude, Longitude and elevation
  • UTM – eastings and northings, reference points are the equator and the central meridians (60 of them for 60 zones) Extends from 84° N to 80 °S
  • State Plane – State specific, northings and eastings NAD 27 is based on feet, NAD 83 is based on meters.
  • There are many many more.
reference ellipsoids and datums
Reference Ellipsoids and Datums
  • Clark 1866 spheroid – NAD 27

(Sometimes Clark 1880?)

  • GRS 80 spheroid – NAD 83
  • WGS 84 spheriod – NAD 83
  • Sphere – for world projections
  • Many many more.
types of projections with examples
Types of Projections with examples
  • Planar (Azimuthal) - North Polar Stereographic
  • Conic – Lambert Conformal Conic, Albers Equal Area, Polyconic
  • Cylindrical – Mercator and Transverse Mercator

GIS is composed of layers

  • Layers
    • land/water
    • roads
    • urban areas
    • pollution levels
  • Data can be represented by VECTORS, or
  • Data can be represented by RASTERS

Vector GIS

  • Every object is given a location
  • Database consists of points, lines and polygons

Raster GIS

  • Every location given an object
  • Grid
    • Cells or pixels

Creating A Raster

  • Overlay grid on real world
  • give each cell a value

Cell Values

  • Assume only one value per cell in any given layer
  • How do you decide what value to give the cell?
    • Value of greatest proportion?
    • Value of the most important feature?
    • Some systems allow for percent composition, edge effect

Cell Values

  • Integer
  • Real Numbers
  • Alphabetic
  • Interpret using legend

Characteristics of a Raster

  • Resolution
    • smallest unit discernible
    • often grid size, but not always
  • Orientation
    • angle that grid makes with true north
  • Value
    • information stored in cell

Characteristics of a Raster

  • Zones
    • areas of continuous values
      • e.g. county, land parcels, etc.
  • Class - term used to describe all the zones of same value in a layer

Characteristics of a Raster

  • Location
    • each cell has a unique location
      • often defined by bottom left corner
      • X, Y coordinate

Storing the Information

  • Full length encoding
    • store every single cell’s value individually

Storing the Information

  • Run Length encoding
    • compress data

Storing the Information

  • Quadtree Encoding – compress data

Sources of Raster Data

  • Scanned Images
    • Aerial photographs
    • maps
  • Satellite images
  • Many packages work on RECTIFYING these images
    • photograph’s scale is not constant across image

Vector GIS

  • Every object is given a location
  • Database consists of points, lines and polygons

Creating a Vector GIS

  • Define Points
    • fundamental property of a vector GIS
    • no dimensions, but they have a location
    • can have attributes associated with it
    • used for utility poles, sampling points, wells etc.

Creating a Vector GIS

  • Define Arcs (lines)
    • line that joins points
    • also called chains and edges
    • has length and direction
    • attributes can include what is on right and left side
    • used to represent road, utility lines, rivers, etc.

Creating a Vector GIS

  • Two Methods of Defining Polygons
    • Polygon Storage
    • Arc Storage
    • Used to represent lakes, landuse categories, forest stands, etc.

Polygon Storage

  • Store Polygon as series of points, starting and ending at same point
  • Each line encoded twice
  • Difficult to dissolve boundary

Polygon Storage - Use Arcs(more common)

  • Every arc stored as a sequence of points
  • Every polygon stored as series of arcs
  • Boundaries only stored once
  • Nodes are points where arcs meet or end
  • The geometric relationship between objects located in space.
    • Adjacency
    • Containment
    • Connectivity
raster and vector summary
Raster and Vector Summary
  • Vectors have advantage of accuracy but not good with continuous fields
  • Vectors were used first - digitizing
  • Earliest include ASCII (x,y coordinates but got too large) then binary took over.
  • Raster not good with lines or points but good with continuous coverage areas.
  • Raster has the mixed pixel problem.

Raster Advantages:

  • Simple data structure
  • Compatible with remotely sensed or scanned data
  • Simple spatial analysis procedures
  • Raster Disadvantages:
  • Requires greater storage space on computer
  • Depending on pixel size, graphical output may be less pleasing
  • Projection transformations are more difficult
  • More difficult to represent topological relationships

Vector Advantages

  • Requires less disk storage space
  • Topological relationships are readily maintained
  • Graphical output more closely resembles hand-drawn maps
  • Vector Disadvantages
  • More complex data structure
  • Not as compatible with remotely sensed data
  • Software and hardware are often more expensive
  • Some spatial analysis procedures may be more difficult
  • Overlaying multiple vector maps is often time consuming
maps as numbers
Maps as numbers
  • Binary 0000 1111
    • Eight bits in a row are termed a byte
    • 256 conbinations or 0 to 255
  • ASCII text- American Standard Code for Information Interchange - 256 standard meanings for the values that fall into one byte. (letters, numbers, special characters)
vector data formats
Vector Data Formats
  • DXF Digital Exchange Format (Autocad) ASCIIw/binary code mantains layers
  • ArcView Shapefiles 9 (.shp)
  • HPGL Hewlett-Packard Graphics Language
    • A device-specific but industry standard language for defining vector graphics in page coordinates (ASCII) - no topology
  • Adobe PostScript ASCII
vector data formats1
Vector Data Formats
  • DLG Digital Line Graph - USGS (1:100,000 & 1:24,000) ASCII UTM
  • TIGER U.S. Census Bureau

(Topologically Integrated Geographic Encoding and Referencing)

    • Geocoded block address-matching
    • Topology correct, but accuracy problems
raster data formats
Raster Data Formats
  • DRG Digital Raster Graph
  • TIF Tagged Interchange Format
  • GIF Graphics Interchange Format
  • JPEG Joint Photographic Experts Group
  • GeoTiff – Geographical referenced TIF Files