Presented by:
Sabya Sachi

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Geospatial Data Abstraction Library(GDAL)
Introduction to GDAL
• GDAL command line utilities
• What is geospatial data?
• Data or information identifying the geographic location of features and boundaries on Earth
• Accessed, manipulated and analysed by Geographic Information System
Two types:
• Raster and Vector
Raster data
• Contains rows and columns of data cells(pixels)
• Each pixel stores a single value consisting of colour and some additional information
• Stored in various formats- from tif and jpeg to BLOB as in RDBMS
Vector data
• Geographical features expressed as vectors
• Coordinates, points, lines, arcs and polygons
• Geometries linked to database describing the attributes
• Can be made to respect spatial attributes
• Introduction to GDAL
 A open source library for reading and writing raster geospatial data formats
 Includes OGR library in its source tree which supports vector formats
 Has various command line utilities for data translation and processing
 Supports over 50 Raster formats
 Primarily developed by Frank Warmerdam until version 1.3.2 when maintainership was transferred to GDAL/OGR Project management committee under Open Source geo spatial foundation
• Crossplatform- supported on Linux, Windows, Mac
• Supports various softwares to read and write multiple data formats
▫ Google Earth
▫ Delta GIS
▫ GRASS GIS
▫ Orfeo Toolbox
▫ ……….
• Has all command line utilities compiled by Frank Warmerdam into a single software FWTools
GDAL Utility programs
 22 utilities
 gdalinfo: report information about a file
 gdal_translate: Copy a raster file with control of output format
 gdaladdo: Add overview to a file
 gdalwarp: Warp an image into new coordinate system
 gdaltindex: Build a mapserver raster tile index
 gdalbuildvrt: Build a VRT from a list of datasets
 gdal_countours: Countours from DEM
 gdaldem: Tools to analyze and visualize DEMs
 rgbgdal_merge.py: Build a quick mosaic from a set of images
 2pct.py: Convert a 24 bit RGB to 8 bit paletted.
 pct2rgb.py: Convert a 8 bit paletted to 24 bit RGB
 gdal2tiles.py: Create a TML tile structure,KML and simple web viewer
 gdal_rasterize: Converts simple vector to raster
 gdaltransform: Transform coordinates
 nearblack: Convert nearly black/white border to exact value
 gdal_retile.py: Retiles a set of tiles or build tiled pyramid levels
 gdal_grid: Create raster from scatter data
 gdal_proximity.py: Compute a raster proximity map
 gdal_polygonize.py: Generate polygons from raster
 gdal_sieve.py: Raster sieve filter
 gdal_fillnodata.py: Interpolate in non data regions
 gdal-config:Get options required to build software using GDAL
gdalinfo
• Lists various information about GDAL supported dataset
• Eg. Format driver, raster size, coordinate system, band data types, band min/max values, descriptions, nodata value etc…
• Syntax:
• An example output
gdal_translate
• Converts data from one format into another
• Can also perform rescaling and resampling
• Syntax:
gdaladdo
• Build or rebuild overview images for most of the raster data formats
• Supports multiple resampling algorithms including nearest resampling cubic resampling etc.
• Syntax:
gdalwarp
• A utility for image mosaicing, reprojection and warping
• Mosiacing: Correcting geometric deformations
• Warping: Mapping a point to another in an mage without changing the data value at that point, results in distortion sometimes, aids in mosaicing.
• Syntax:
• Supports image mosaicing only if there is existing output image
• Doesn’t adjust existing output image spatial extent
gdaltindex
• Builds a shapefile with a record for each input raster file
• Shapefile: Contain nontopological geometry and attribute information for the spatial features in a data set.
• If a shapefile already exists, new data is appended to it.
• Used with mapserver
• Syntax:
gdaldem
• Used for analyzing and visualizing DEMs- Digital Elevation models
• Has seven modes of operation:
▫ Hillshade
▫ Slope
▫ Aspect
▫ Color relief
▫ TRI(Terrain Ruggedness Index)
▫ TPI(Topographic Position Index)
▫ Roughness
• Example:
Original dem image
100% 255 255 255
75% 235 220 175
50% 190 185 135
25% 240 250 150
0% 50 180 50
• gdaldem color-relief clip.tif ramp.txt colored.tif
Coloured relief raster
• gdaldem slope clip slope.tif -s 111120
Slope map
• gdaldem aspect clip.tif aspect.tif
Aspect map
• gdaldem hillshade clip.tif hillshade.tif -z 5 -s 111120
Hillshade- colour relief
gdal_merge
• A python utility
• Automatically mosaic a set of images into a single one
• Images must be in same coordinate system
• Images have must have matching raster bands
• Resolution can be different
• Can overlap
• Syntax:
• Example :
Conclusion
• GDAL is currently a major project in open source community and commercial GIS community
“I see GDAL as the glibc/glibc++ of the geospatial software world. It's open, it provides core functionality, I can't understand how anybody gets anything done without it.”
- Howard Butler