Lecture 2 Introduction to GIS, Data models, Data structures.
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Transcript of Lecture 2 Introduction to GIS, Data models, Data structures.
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Lecture 2
Introduction to GIS, Data models, Data structures
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REPRESENTATION AND DATA STRUCTURESCoordinates and Attributes
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Common Data Models
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REPRESENTATION AND
DATA STRUCTURES
•Most common data
models define thematic layers
•Typically, layers, one
layer for each distinct view of a theme
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Part 1: Coordinates (Cartesian)
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Spherical coordinate can be expressed two ways
e.g.,
44o or
44.51o
30’ 35”(DMS)
(DD)
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We Can Convert
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Three Types of Vector Features
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One-to-one,because of Attributes
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Enforced Uniformity
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Planar Topology – no overlaps
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On a blank sheet of paper, draw a
topologically correct (not necessarily
geometrically correct) rendition of the 6
coastal (name counties, if you can).
Identify all the nodes.
Create a table corresponding to the
geographic data you drew (make the
linkages clear).
Include an attribute for county area, in
square kilometers, and square miles
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Rasters – Fixed Cell Size, Grid Orientation
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Connecting data, contrast with vector
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Raster, one-to-one correspondence, how
many rows in the attribute table?
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Number of cells = 100!Raster, one-to-one correspondence, rarely used
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Many-to-one much more common, to tame the attribute table
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How can we mimic a vector one-to-one relationship between individual polygon codes and table rows, when using a raster, without keeping track of individual cells?
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Raster – The Mixed Pixel Problem
Landcover map – Two classes, land or water
Cell A is straightforward
What category to assignFor B, C, or D?
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Resampling Ambiguity
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Changing Resolution (resampling) with Categorical
Data Assignment Method Matters!
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Orientation and/or Cell Size May Differ
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Resampling - Distance-weighted averagingcontinuous data:
bilinear interpolation
z
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No Decision is Final – We Can Convert
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Data and File Structures
Data are stored as binary numbers
Bits are 0 or 1
Bytes are 8 bits
Data (e.g., raster cells)are often references as 1 byte, two byte, etc.
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Cells Have a Type, Size
Type – e.g., Real, unsigned integer, signed integer, text
Size – 8 bit, 32 bit, 64 bit, long, short, character width
These control the size of the datasets, and type of data that may be stored
Mixing types, sizes, often requires some care
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Raster data set properties
We can store a number upTo 2 or4,293,967,296 in a cell
32
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Data andFile Structures
Data often have specificorganization to
•reduce size
•speed access
•ease updates
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Example: ESRI ShapefilesLandcover dataset, wash_lcis a cluster of files,
wash_lc.shp - containing the coordinates wash_lc.dbf - containing the attributes wash_lc.shx - containing linkages, other infowash_lc.prj - optional, containing projection information wash_lc.sbn - an optional indexing file
Thursday, August 22, 13
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Data and File Structures CompressionReducing size – e.g., raster run-length
coding
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Rasters – Discrete or Continuous
Features Can we compress either
without loss?discretecontinuous