raster data in qgis matthew rosencrans tom diliberto
TRANSCRIPT
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Raster Data in QGIS
Matthew RosencransTom DiLiberto
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Outline
• What is Raster Data?• Supported Formats• What can one do with Raster Data?
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Raster Data
• Raster Data = gridded data– NWP Model Output– Imagery– LDAS outputs– LIDAR/RADAR– Interpolated fields from point/line/polygon data
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Raster Data
• QGIS uses GDAL libraries– GDAL = Geospatial Data Abstraction Library
www.gdal.org
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Supported Formats
• ArcInfo Grid (ASCII or Binary)• GeoTIFF/GeoPDF• ERDAS IMAGINE/ ENVI• GIF/JPG/PNG• netCDF HDF4/5• NGSGEOID• USGS DEM• OGC formats• Grib2• http://www.gdal.org/formats_list.html
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Raster Data
• What good is data without interpretation?• Need to colorize/stylize/query
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Symbologies• Singleband • Psuedocolors• Multiband
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Symbologies• Pseudocolor– Scalar fields– Temperature– Precipitation– Satellite Index
Choose colors
Decide your thresholds
Apply
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Creating Raster Data• Interpolation of point data– Multiple methods• Inverse distance weighted (CPC)• Natural Neighbor (used by CPC and USDA)• Kriging
– SAGA GIS - open source for making Gridded Data
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Creating Raster Data• Inverse distance weighted (CPC)– Power Value • higher = emphasis on the nearest points, less smooth• lower = emphasis on further points, smoother
– Control number of points– Non physical
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Creating Raster Data• Natural Neighbor (used by CPC and USDA)– Voronoi (Thiessen) polygons constructed– Sample point, new Thiessen polygon constructed– Weight = proportion of overlap new/initial poly
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Creating Raster Data• Kriging– SAGA GIS – Some issues with software– Assumes distance or direction between sample
points can be used to explain variation in the surface
– forms weights from surrounding values to predict unmeasured locations
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Raster Processing• Raster Processing• Clipping – GDAL Clip Raster By Extent.• Instead of masking out ocean, or able to subset
for country or province• Extracting• Point Sampling • Extracting by value
• Resampling (helps to match resolution)
• Mathematical operations within and across rasters• Raster Addition/subtraction• Creating Totals, Climatologies, Anomalies
• Local Statistics (Zonal Statistics)
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Raster Processing• Raster Processing• Clipping – GDAL Clip Raster By Extent.
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Raster Processing• Raster Processing• Resampling (helps to match resolution)
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Raster Processing• Raster Processing• Extracting• Point Sampling –QGIS Plugin or SAGA tool • Extracting by value
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Raster Processing• Raster Processing• Mathematical operations within and across rasters• Raster Addition/subtraction• Creating Totals, Climatologies, Anomalies
Covered in Day 3Local Statistics (Zonal Statistics)
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Raster Processing• Lab Work will cover• Symbolizing Rasters• Querying rasters• Clipping• Raster Calculator• Calculations• Interpolations• Querying rasters at points