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Rasters

The Basics

  • An "image" is digital as opposed to a "picture" which you take with an analog camera
  • Images are made up of pixels which is short for "Picture Element"
  • Pixels contain values (numbers)
  • The more pixels etc. the larger the image file size.

 

 

Color spaces

  • RGB
  • CMY
  • CMYK
  • HSV
  • Grayscale
  • 1-bit

 

Additive vs. subtractive mixing

  • Additive color theory
    • Acts like light
  • Subtractive color theory
    • Acts similar to painting

 

RGB (red-green-blue)

  • Composite of 3 layers / channels (red,green,blue)

Resolution

Images have dimensions

  • Horizontal and vertical dimensions

 

Resolution (DPI)

  • DPI = dots per inch
  • The greater the DPI per equivalent areas
  • Average screen resolution is 72 DPI
  • Typical printer resolution is 300 DPI

 

Spatial resolution

  • When an image refers to something in the "real world" we say it has spatial resolution
  • This refers to the unit of measure in the "real world" that a pixel represents in the image
  • e.g., 30 meter digital elevation models (DEM)

 

Remote Sensing System

What is a remote sensing system

  • Energy source
  • Platform
  • Sensor
  • Data recording / transmission
  • Ground receiving station
  • Data processing
  • Expert interpretation / data users

 

Types of platforms:

  • Airborne (like planes)
  • Satellites
    • Conducted from the space shuttle or more commonly, from satellites
    • Most are at near polar orbit (200 - 1000 km altitude)
    • Allows the same spot to be mapped over time

 

 

Active sensor:

  • Beams energy and detects the reflection

 

 

Spatial data resolution problem

  • Trade-off pixel size vs. spatial coverage
  • Quantization and data volume
  • Data merge from different sources
  • Grid dislacement in time
  • Information content of different resolutions
  • Raster-vector conversion

 

Geometric registration

  • Where does an image belong on the globe?
  • Before this step, data is just expensive pictures

 

 

Simple IP techniques

  • These techniques are accomplished by applying mathematical algorithms to individual pixel values
  • e.g. Brightness simply adds a constant value to each pixel
  • Convolution filters
    • A matrix of multipliers applied to each pixel as it is moved across the image
    • They are typically moved from left to right as you would read a book

 

Image interpretation

  • Interpretation: data -> information
  • Visual interpretation: uses visual methods to interpret analog data (maps)
  • Digital interpretation: uses computer-based methods to interpret digital data

 

Image enhancement

  • Usually done to more effectively display or record the data for subsequent visual interpretation
    • Contrast stretching
    • Filtering
    • Edge detection

 

Image transformation

  • Arithmetic operations done to combine and transform the original bands into "new" images which better display or highlight certain features

 

Image classification

  • To categorize all pixels in an image into land cover classes or themes
  • Multi-spectral data are used to perform classification
  • Spectral patterns present within data used as numerical basis for categorization

 

Integrating remotely sensed data with GIS

  • What GIS has to offer remote sensing:
    • Control points
    • Themes
    • Training Sites
  • What remote sensing has to offer GIS
    • Rapid updates
    • Changes detection
    • Vegetation indices
  • What the future may hold:
    • Fully integrated systems
    • Transparent data integration