Lec_11_Intro to Raster
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A raster is a matrix of cells that each store a specific value like rainfall, temperature, or elevation. Rasters do not have attribute tables unless one is created, and can contain integer or real number values but not text. Pyramids are downsampled versions of rasters that improve performance by accessing different resolution levels for zooming in and out. They are created individually for each raster dataset. Hill shade effect adds illumination to rasters to enhance visualization by simulating sunlight interaction with the terrain surface.
Lec_11_Intro to Raster
- 1. Intro to Raster Prof. Dr. Sajid Rashid Ahmad sajidpu@yahoo.com Atiqa Ijaz Khan _ Demonstrator atiqa_ss09@yahoo.com
- 2. 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. Tuesday, March 3, 2015Institute of Geology, University of the Punjab 2
- 3. Tuesday, March 3, 2015Institute of Geology, University of the Punjab 3 Satellite Imagery
- 4. Tuesday, March 3, 2015Institute of Geology, University of the Punjab 4 Topo-sheet
- 5. Tuesday, March 3, 2015Institute of Geology, University of the Punjab 5 Aerial Photography
- 6. Raster Attribute Tuesday, March 3, 2015Institute of Geology, University of the Punjab 6 Raster does not have associated attribute table, unless created by some means.
- 7. 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 smaller ones. • Pyramids are created for each raster datasets individually, not as a whole for raster mosaics or catalog. Tuesday, March 3, 2015Institute of Geology, University of the Punjab 7
- 8. • Pyramids are the version of the raster datasets. • They are used to control the speed of the drawing a raster as one zoom in or out. • Generally, two types of resolutions are used: • Coarser: As zoom out. • Finer: As zoom in. Tuesday, March 3, 2015Institute of Geology, University of the Punjab 8
- 9. Tuesday, March 3, 2015Institute of Geology, University of the Punjab 9 Pyramid Performance Zoom In Zoom Out
- 10. 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. Tuesday, March 3, 2015Institute of Geology, University of the Punjab 10
- 11. 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 or IMGs • 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. Tuesday, March 3, 2015Institute of Geology, University of the Punjab 11
- 12. Tuesday, March 3, 2015Institute of Geology, University of the Punjab 12Create Pyramids Pyramids Re-sampling Techniques
- 13. Hill Shade Effect • It adds a hypothetical source of illumination to light up the areas for each cell of raster. • 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 sunlight and surface. • It has integer values from 0 – 255. Tuesday, March 3, 2015Institute of Geology, University of the Punjab 13
- 14. Parameters of Hill Shade Effect • Azimuth • The azimuth is the angular direction of the sun, measured from north in clockwise degrees 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. Tuesday, March 3, 2015Institute of Geology, University of the Punjab 14
- 15. Tuesday, March 3, 2015Institute of Geology, University of the Punjab 15 Azimuth Altitude Azimuth: 315º Altitude: 45º
- 16. Tuesday, March 3, 2015Institute of Geology, University of the Punjab 16