lec_11_intro to raster
TRANSCRIPT
Intro to Raster
Prof. Dr. Sajid Rashid Ahmad
Atiqa Ijaz Khan _ Demonstrator
Raster
• “A raster is a matrix of identically squared size cells.”
• Each cell store a specific value.
• Like: rainfall, temperature, elevation etc.
• It could be from integers or real numbers.
• Raster does not have associated attribute table, unless created by some means.
• It can never have text data in it.
• Examples:
• Satellite images, scanned map, aerial photographs etc.
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Satellite Imagery
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Topo-sheet
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Aerial Photography
Raster Attribute
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Raster does not have associated attribute table, unless
created by some means.
Pyramids
• Pyramids are used to improve the performance.
• They are down-sampled version of the original raster.
• Each successive layer is down-sampled at scale of 2:1.
• Every successive layer is down-sampled at a fixed resolution.
• Only that particular resolution is accessed for display.
• This process speeds up the drawing.
• Larger datasets required more time to create pyramids as compare to smallerones.
• Pyramids are created for each raster datasets individually, not as a whole forraster mosaics or catalog.
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• Pyramids are the version of the raster datasets.
• They are used to control the speed of the drawing a raster as one zoom inor out.
• Generally, two types of resolutions are used:
• Coarser: As zoom out.
• Finer: As zoom in.
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Pyramid Performance
Zoom In
Zoom Out
Types of Pyramids
• Two types of pyramids files:
• Reduced Resolution Datasets (.rrd) created in ERDAS (a Remote Sensing software)
• Overview (.ovr) created in ArcGIS 10.1
• ArcGIS can read both of these files.
• But only write Overview file.
• An overview file is created in two cases:
• File format is not an ERDAS IMAGINE file (.img)
• Pyramids are created in ArcGIS 10.1 or higher.
• Benefit over (.rrd) file:
• Overview file (.ovr) controls the compression type and quality of pyramids.
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Pyramids Re-sampling Techniques
• 3 types of techniques are available with pyramids.
• Nearest:• Uses the values of the closest cell to assign value. It is by Default.
• Examples: Discrete raster data, like land use map, scanned map etc.
• Bilinear:• Uses the weighted average distance for the 4 nearest cells to assign value.
• Examples: Continuous data, like satellite images, or aerial photography, 1-bit TIFFs orIMGs
• Cubic:• Uses a smooth curve to pass through the 16 nearest cell’s centers to assign value.
• Examples: Continuous data, like satellite images, or aerial photography.
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12Create Pyramids
Pyramids Re-sampling
Techniques
Hill Shade Effect
• It adds a hypothetical source of illumination to light up the areas for each cell ofraster.
• It usually enhances the visualization for display.
• It is also known as “Shaded Relief”.
• It simulates how the terrain will look like as with the interaction between sunlightand surface.
• It has integer values from 0 – 255.
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Parameters of Hill Shade Effect
• Azimuth
• The azimuth is the angular direction of the sun, measured from north in clockwisedegrees from 0 to 360.
• An azimuth of 90º is east.
• The default azimuth is 315º (NW).
• Altitude
• The altitude is the slope or angle of the illumination source above the horizon.
• The units are in degrees, from 0 (on the horizon) to 90 (overhead).
• The default is 45 degrees.
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Azimuth Altitude
Azimuth: 315º
Altitude: 45º
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