Introduction to GIS Slides by pptfiles

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									Lecture 6


    Raster data
Raster layers
n   It’s all cells
A matrix of cells




                                      Column
Grid origin
       0, 0   0   1   2   3   4   5    6       7
          0
          1
          2
     o
    Rw    3
                                                     Grid cell
          4                                          3, 6
          5
          6
          7                                        Square cells


                                                                  From Clarke
Resolution (cell size)
n   ½ cell size = 2x
    the space
Cell values
n   Each cell has a value: Integer, real number, or NoData
n   Cells can store raw numbers (elevation, temperature, slope)
    q   1200
    q   1130
    q   1000
    q   990
                                 1   1   1   1   1   4   4   4
    q   430
                                 1   2   1   2   2   4   4   4
n   Or index values
    q 1 = water                  1   1   1   3   3   3   3   3

    q 2 = land                   1   1   2   2   2   2   2   3
    q 3 = road                   2   2   1   2   2   2   2   3
    q 4 = building
                                 4   4   2   1   2   2   2   3
                                 8   4   3   2   1   1   2   3
                                 3   3   3   2   2   2   1   1
                                                                  From Clarke
Categories of raster data
n   Continuous
    q   Elevation
    q   Rainfall



n   Discrete          Land
                      Water
    q   Landuse       Bridge
    q   Tree type

n   Imagery
    q   Air photo
    q   Sateilte
The mixed pixel problem
Water dominates   Winner takes all   Edges separate
   W W G            W G G               W E     G
   W W G            W W G               W E     G
   W W G            W G G               E   E   G




                                                    From Clarke
Raster to vector conversion


 Vector…     to raster…   back to vector




How can you improve
      these results?



                                           From Clarke
Raster overlay
n   Cells from multiple layers
n   Same location
n   Like a shish kabob
n   Write equations with maps as variables – map algebra




      Value in layer 1                     Rainfall 1998   10
    + Value in layer 2                     Rainfall 1999   21
    + Value in layer 3                     Rainfall 2000    9
    + Value in layer 4                     Rainfall 2001   11
                                                equals     51
Map algebra
n   Use math operations +, -, \, and *
n   Cells overlap each other
n   Math performed on overlapping cells

                             More or less rainfall?
                                  7-4=3
     7   9   2   5   8         4    3   4   4   3        3   6 -2 1    5
     9   4   4   4   8         6    5   4   4   5        3   -1 0 0    3
     5
     1
         6
         3
             4
             3
                 1
                 3
                     9
                     6
                         -     2
                               7
                                    2
                                    4
                                        1
                                        0
                                            1
                                            1
                                                1
                                                1
                                                    =    3 4 3
                                                         -6 -1 3
                                                                   0
                                                                   2
                                                                       8
                                                                       5
     1   1   1   5   6         5    5   0   0   0        -4 -4 1   -5 -6

     Rain 1999                     Rain 2000            Rain difference
Map algebra
n       Logical operations (and, or)
n       Cells overlap each other
n       Fast math – It’s all Zeros and ones
n       1- good, 0 - bad

                               Find best Ski areas

    1    1   0   0   0           0   0   0    0   0       0   0   0   0   0
    1    1   1   0   0           0   1   0    1   1       0   1   0   0   0
    0    1   0   1   0   AND     0   1   1    1   1   =   0   1   0   1   0
    0    0   0   0   0           0   0   0    1   1       0   0   0   0   0
    0    0   0   0   0           0   0   0    0   0       0   0   0   0   0

Slope > 15%                          No trees
Multiple rasters and transparency
Metadata
n   Data about data
n   Date information
n   Who made it
n   Location
n   Scale
n   Intended use
n   Storage formats
    q   FGDC
    q   FAQ
    q   ISO
    q   XML
TIN: Triangulated Irregular Network
n   3D vector data
n   Triangles
n   More efficient than a
    grid
Elevations with TIN
3D Visualization
3D Visualization
World view – zoom to Mt Everest
Raster data exercise

								
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