C6

• Two fundamental data structures for storing data in GIS

  • RASTER
  • VECTOR

  
• Two fundamental approaches for representing spatial data

  • VECTOR DATA
  • RASTER DATA

  
• Raster and vector are two terms frequently mentioned by people dealing with geospatial technologies

  
• As a student of spatial analysis, you must be familiar with these terms and how they are used
• It is not necessary to understand all the programming and process that a GIS uses with data, but understanding the fundamentals will allow the student to differentiate between methods of analysis
• Data format


  How data (attribute data and location data) is stored and displayed in GIS
• Attribute data


  The characteristics of an object, such as trunk diameter for a tree or pH for a soil sample
• Locational data


  The coordinates necessary to place an object on a map in its correct geographic position
• Vector data


  A connect the dots drawing, composed of dots (points), a series of connected dots (lines), or a series of dots that create an enclosed shape (polygons)

  
• All mappable objects need to be identified by one of three types of vectors: POINTS, LINES OR POLYGONS
• Vector data


  • Objects or conditions in the real world are represented by points, lines and polygons that define their boundaries, much as if they were being drawn on a map
  • The position of each object is defined by its placement in a map space that is organized by a coordinate reference system

  
• Raster data


  • The space is regularly subdivided into cells (usually square in shape)
  • The location of geographic objects or conditions is defined by the row and column positions of the cells they occupy
  • The area that each cell represents defines the spatial resolution available
  • The value stored for each cell indicates the type of object or condition that is found at that location in the raster model, and the homogeneous units are the cells

  
• Vertexes


  The actual dots within the GIS map
• Three different types of features that all use vertexes

  • Point feature (single vertex)
  • Line feature (at least two vertexes with a straight line connecting each pair)
  • Polygon feature (minimum of three vertexes connected by three straight lines and enclosed)

  
• Node


  A vertex that intersects another feature, i.e. shared between the features
• Vertexes and lines serve as the link between the locational and attribute data
• Data in vector format

  • Each vertex has a latitude and longitude associated with it that provides its location
  • When any vertex or line within a line feature or a vertex, line, or area within an enclosed polygon is selected, the GIS is able to find the record in the database that is associated with that feature
  • The attribute data from that feature's record can be displayed, reviewed or used to label the feature
• Two main sources of vector data

  • GPS
  • Digitizing
• Using GPS


  • GPS can be used to collect both locational and attribute data
  • Locational data is a series of vertexes
  • A data dictionary is the outline format of features to be mapped, type of attribute to be collected, and the specification of attribute values as either nominal, numerical or ordinal, discrete or continuous
• Digitizing data


  • Vector data can be created by digitizing a scanned image within GIS software
  • Aerial digital orthophotographs can be used as the basis for digitizing
• Vector data from the internet

  The user can download map data that has been placed on the internet
• Raster data formats


  • Store data in a grid - a series of rows and columns that form a grid
  • Each cell can provide locational data and has one attribute value

  
• Raster image


  • A georeferenced digital photograph
  • Locational data is not included directly with each grid cell, but must be associated with the image's grid cell by identifying the coordinate in the upper-left-hand-grid cell, defining the grid cell size horizontally and vertically, and any rotation that might be needed for the image
  • This information is stored in a simple text file called a world file
  • Attribute data within a raster image cell is a color value

  
• Raster grids provide valuable information and the color represents specific attributes such as elevation data, yield data, nutrient data and distance from a specific object

  
• Comparison of raster and vector analysis

  • Vector data work well for purposes of record keeping
  • Raster data works better for spatial analysis because raster surfaces cover the complete study area and the grid cells line up

  
• Advantages of raster data

  • Good for complex analysis
  • Efficient for overlays
  • Data structure common for imagery

  
• Advantages of vector data

  • Compact data structure
  • Efficient for encoding topology
  • True representation of shape
• Disadvantages of raster data

  • Large datasets
  • Topology hard to represent
  • Maps less "realistic"
• Disadvantages of vector data

  • Complex structure
  • Overlay operations difficult
  • Might imply false sense of accuracy