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Edit File: gdal_tutorial.dox

/* $Id: gdal_tutorial.dox 35222 2016-08-28 06:06:11Z goatbar $ */

/*!
\page gdal_tutorial GDAL API Tutorial

\section gdal_tutorial_open Opening the File

Before opening a GDAL supported raster datastore it is necessary to
register drivers. There is a driver for each supported format. Normally
this is accomplished with the GDALAllRegister() function which attempts
to register all known drivers, including those auto-loaded from .so files
using GDALDriverManager::AutoLoadDrivers(). If for some applications it
is necessary to limit the set of drivers it may be helpful to review
the code from <a href="gdalallregister.cpp.html">gdalallregister.cpp</a>.
Python automatically calls GDALAllRegister() when the gdal module is
imported.

Once the drivers are registered, the application should call the free
standing GDALOpen() function to open a dataset, passing the name of the
dataset and the access desired (GA_ReadOnly or GA_Update).

In C++:
\code
#include "gdal_priv.h"
#include "cpl_conv.h" // for CPLMalloc()

int main()
{
    GDALDataset  *poDataset;

GDALAllRegister();

poDataset = (GDALDataset *) GDALOpen( pszFilename, GA_ReadOnly );
    if( poDataset == NULL )
    {
	...;
    }
\endcode

In C:
\code
#include "gdal.h"
#include "cpl_conv.h" /* for CPLMalloc() */

int main()
{
    GDALDatasetH  hDataset;

GDALAllRegister();

hDataset = GDALOpen( pszFilename, GA_ReadOnly );
    if( hDataset == NULL )
    {
	...;
    }
\endcode

In Python:
\code
    import gdal
    from gdalconst import *

dataset = gdal.Open( filename, GA_ReadOnly )
    if dataset is None:
        ...
\endcode

Note that if GDALOpen() returns NULL it means the open failed, and that
an error messages will already have been emitted via CPLError().  If you want
to control how errors are reported to the user review the CPLError()
documentation.  Generally speaking all of GDAL uses CPLError() for error
reporting.  Also, note that pszFilename need not actually be the name
of a physical file (though it usually is).  It's interpretation is driver
dependent, and it might be an URL, a filename with additional parameters
added at the end controlling the open or almost anything.  Please try not
to limit GDAL file selection dialogs to only selecting physical files.

\section gdal_tutorial_dataset Getting Dataset Information

As described in the <a href="gdal_datamodel.html">GDAL Data Model</a>, a
GDALDataset contains a list of raster bands, all pertaining to the same
area, and having the same resolution. It also has metadata, a coordinate
system, a georeferencing transform, size of raster and various other
information.

In the particular, but common, case of a "north up" image without any rotation
or shearing, the georeferencing transform takes the following form :

\code
    adfGeoTransform[0] /* top left x */
    adfGeoTransform[1] /* w-e pixel resolution */
    adfGeoTransform[2] /* 0 */
    adfGeoTransform[3] /* top left y */
    adfGeoTransform[4] /* 0 */
    adfGeoTransform[5] /* n-s pixel resolution (negative value) */
\endcode

In the general case, this is an affine transform.

If we wanted to print some general information about the
dataset we might do the following:

In C++:
\code
    double        adfGeoTransform[6];

printf( "Driver: %s/%s\n",
            poDataset->GetDriver()->GetDescription(),
            poDataset->GetDriver()->GetMetadataItem( GDAL_DMD_LONGNAME ) );

printf( "Size is %dx%dx%d\n",
            poDataset->GetRasterXSize(), poDataset->GetRasterYSize(),
            poDataset->GetRasterCount() );

if( poDataset->GetProjectionRef()  != NULL )
        printf( "Projection is `%s'\n", poDataset->GetProjectionRef() );

if( poDataset->GetGeoTransform( adfGeoTransform ) == CE_None )
    {
        printf( "Origin = (%.6f,%.6f)\n",
                adfGeoTransform[0], adfGeoTransform[3] );

printf( "Pixel Size = (%.6f,%.6f)\n",
                adfGeoTransform[1], adfGeoTransform[5] );
    }
\endcode

In C:
\code
    GDALDriverH   hDriver;
    double        adfGeoTransform[6];

hDriver = GDALGetDatasetDriver( hDataset );
    printf( "Driver: %s/%s\n",
            GDALGetDriverShortName( hDriver ),
            GDALGetDriverLongName( hDriver ) );

printf( "Size is %dx%dx%d\n",
	    GDALGetRasterXSize( hDataset ),
            GDALGetRasterYSize( hDataset ),
            GDALGetRasterCount( hDataset ) );

if( GDALGetProjectionRef( hDataset ) != NULL )
        printf( "Projection is `%s'\n", GDALGetProjectionRef( hDataset ) );

if( GDALGetGeoTransform( hDataset, adfGeoTransform ) == CE_None )
    {
        printf( "Origin = (%.6f,%.6f)\n",
                adfGeoTransform[0], adfGeoTransform[3] );

printf( "Pixel Size = (%.6f,%.6f)\n",
                adfGeoTransform[1], adfGeoTransform[5] );
    }
\endcode

In Python:
\code
    print 'Driver: ', dataset.GetDriver().ShortName,'/', \
          dataset.GetDriver().LongName
    print 'Size is ',dataset.RasterXSize,'x',dataset.RasterYSize, \
          'x',dataset.RasterCount
    print 'Projection is ',dataset.GetProjection()

geotransform = dataset.GetGeoTransform()
    if not geotransform is None:
	print 'Origin = (',geotransform[0], ',',geotransform[3],')'
	print 'Pixel Size = (',geotransform[1], ',',geotransform[5],')'
\endcode

\section gdal_tutorial_band Fetching a Raster Band

At this time access to raster data via GDAL is done one band at a time.
Also, there is metadata, block sizes, color tables, and various other
information available on a band by band basis.  The following codes fetches
a GDALRasterBand object from the dataset (numbered 1 through GetRasterCount())
and displays a little information about it.

In C++:
\code
	GDALRasterBand  *poBand;
        int             nBlockXSize, nBlockYSize;
        int             bGotMin, bGotMax;
        double          adfMinMax[2];

poBand = poDataset->GetRasterBand( 1 );
	poBand->GetBlockSize( &nBlockXSize, &nBlockYSize );
        printf( "Block=%dx%d Type=%s, ColorInterp=%s\n",
                nBlockXSize, nBlockYSize,
                GDALGetDataTypeName(poBand->GetRasterDataType()),
                GDALGetColorInterpretationName(
                    poBand->GetColorInterpretation()) );

adfMinMax[0] = poBand->GetMinimum( &bGotMin );
        adfMinMax[1] = poBand->GetMaximum( &bGotMax );
        if( ! (bGotMin && bGotMax) )
            GDALComputeRasterMinMax((GDALRasterBandH)poBand, TRUE, adfMinMax);

printf( "Min=%.3fd, Max=%.3f\n", adfMinMax[0], adfMinMax[1] );

if( poBand->GetOverviewCount() > 0 )
            printf( "Band has %d overviews.\n", poBand->GetOverviewCount() );

if( poBand->GetColorTable() != NULL )
            printf( "Band has a color table with %d entries.\n",
                     poBand->GetColorTable()->GetColorEntryCount() );
\endcode

In C:
\code
	GDALRasterBandH hBand;
        int             nBlockXSize, nBlockYSize;
        int             bGotMin, bGotMax;
        double          adfMinMax[2];

hBand = GDALGetRasterBand( hDataset, 1 );
        GDALGetBlockSize( hBand, &nBlockXSize, &nBlockYSize );
        printf( "Block=%dx%d Type=%s, ColorInterp=%s\n",
                nBlockXSize, nBlockYSize,
                GDALGetDataTypeName(GDALGetRasterDataType(hBand)),
                GDALGetColorInterpretationName(
                    GDALGetRasterColorInterpretation(hBand)) );

adfMinMax[0] = GDALGetRasterMinimum( hBand, &bGotMin );
        adfMinMax[1] = GDALGetRasterMaximum( hBand, &bGotMax );
        if( ! (bGotMin && bGotMax) )
            GDALComputeRasterMinMax( hBand, TRUE, adfMinMax );

printf( "Min=%.3fd, Max=%.3f\n", adfMinMax[0], adfMinMax[1] );

if( GDALGetOverviewCount(hBand) > 0 )
            printf( "Band has %d overviews.\n", GDALGetOverviewCount(hBand));

if( GDALGetRasterColorTable( hBand ) != NULL )
            printf( "Band has a color table with %d entries.\n",
                     GDALGetColorEntryCount(
                         GDALGetRasterColorTable( hBand ) ) );
\endcode

In Python (note several bindings are missing):
\code
	band = dataset.GetRasterBand(1)

print 'Band Type=',gdal.GetDataTypeName(band.DataType)

min = band.GetMinimum()
	max = band.GetMaximum()
	if min is None or max is None:
	    (min,max) = band.ComputeRasterMinMax(1)
	print 'Min=%.3f, Max=%.3f' % (min,max)

if band.GetOverviewCount() > 0:
	    print 'Band has ', band.GetOverviewCount(), ' overviews.'

if not band.GetRasterColorTable() is None:
	    print 'Band has a color table with ', \
	    band.GetRasterColorTable().GetCount(), ' entries.'
\endcode

\section gdal_tutorial_read Reading Raster Data

There are a few ways to read raster data, but the most common is via
the GDALRasterBand::RasterIO() method.  This method will automatically take
care of data type conversion, up/down sampling and windowing.  The following
code will read the first scanline of data into a similarly sized buffer,
converting it to floating point as part of the operation.

In C++:
\code
	float *pafScanline;
        int   nXSize = poBand->GetXSize();

pafScanline = (float *) CPLMalloc(sizeof(float)*nXSize);
        poBand->RasterIO( GF_Read, 0, 0, nXSize, 1,
                          pafScanline, nXSize, 1, GDT_Float32,
                          0, 0 );
\endcode

The pafScanline buffer should be freed with CPLFree() when it is no longer used.

In C:
\code
	float *pafScanline;
        int   nXSize = GDALGetRasterBandXSize( hBand );

pafScanline = (float *) CPLMalloc(sizeof(float)*nXSize);
	GDALRasterIO( hBand, GF_Read, 0, 0, nXSize, 1,
                      pafScanline, nXSize, 1, GDT_Float32,
                      0, 0 );
\endcode

The pafScanline buffer should be freed with CPLFree() when it is no longer used.

In Python:

\code
  	scanline = band.ReadRaster( 0, 0, band.XSize, 1, \
                                     band.XSize, 1, GDT_Float32 )
\endcode

Note that the returned scanline is of type string, and contains
xsize*4 bytes of raw binary floating point data. This can be converted
to Python values using the struct module from the standard
library:

\code
	import struct

tuple_of_floats = struct.unpack('f' * b2.XSize, scanline)
\endcode

The RasterIO call takes the following arguments.
\code
CPLErr GDALRasterBand::RasterIO( GDALRWFlag eRWFlag,
                                 int nXOff, int nYOff, int nXSize, int nYSize,
                                 void * pData, int nBufXSize, int nBufYSize,
                                 GDALDataType eBufType,
                                 int nPixelSpace,
                                 int nLineSpace )
\endcode

Note that the same RasterIO() call is used to read, or write based on
the setting of eRWFlag (either GF_Read or GF_Write).  The nXOff, nYOff,
nXSize, nYSize argument describe the window of raster data on disk to
read (or write).  It doesn't have to fall on tile boundaries though access
may be more efficient if it does.

The pData is the memory buffer the data is read into, or written from. It's
real type must be whatever is passed as eBufType, such as GDT_Float32, or
GDT_Byte. The RasterIO() call will take care of converting between the
buffer's data type and the data type of the band. Note that when converting
floating point data to integer RasterIO() rounds down, and when converting
source values outside the legal range of the output the nearest legal value
is used. This implies, for instance, that 16bit data read into a GDT_Byte
buffer will map all values greater than 255 to 255, the data is not
scaled!

The nBufXSize and nBufYSize
values describe the size of the buffer.  When loading data at full resolution
this would be the same as the window size.  However, to load a reduced
resolution overview this could be set to smaller than the window on disk.  In
this case the RasterIO() will utilize overviews to do the IO more efficiently
if the overviews are suitable.

The nPixelSpace, and nLineSpace are normally zero indicating that default
values should be used.  However, they can be used to control access to
the memory data buffer, allowing reading into a buffer containing other
pixel interleaved data for instance.

\section gdal_tutorial_close Closing the Dataset

Please keep in mind that GDALRasterBand objects are owned by their
dataset, and they should never be destroyed with the C++ delete operator.
GDALDataset's can be closed by calling GDALClose() (it is NOT recommended
to use the delete operator on a GDALDataset for Windows users because of known
issues when allocating and freeing memory across module boundaries. See the relevant
<a href="http://trac.osgeo.org/gdal/wiki/FAQMiscellaneous#HowshouldIdeallocateresourcesacquaintedfromGDALonWindows">topic</a> on the FAQ).
Calling GDALClose will result in proper cleanup, and flushing of any pending writes.
Forgetting to call GDALClose on a dataset opened in update mode in a popular format
like GTiff will likely result in being unable to open it afterwards.

\section gdal_tutorial_creation Techniques for Creating Files

New files in GDAL supported formats may be created if the format driver
supports creation.  There are two general techniques for creating files,
using CreateCopy() and Create().
The CreateCopy method involves calling the CreateCopy() method on the
format driver, and passing in a source dataset that should be copied.  The
Create method involves calling the Create() method on the driver, and then
explicitly writing all the metadata, and raster data with separate calls.
All drivers that support creating new files support the CreateCopy() method,
but only a few support the Create() method.

To determine if a particular format supports Create or CreateCopy
it is possible to check the DCAP_CREATE  and DCAP_CREATECOPY metadata on the
format driver object.  Ensure that GDALAllRegister() has been called before
calling GetDriverByName().  In this example we fetch a driver, and determine
whether it supports Create() and/or CreateCopy().

In C++:
\code
#include "cpl_string.h"
...
    const char *pszFormat = "GTiff";
    GDALDriver *poDriver;
    char **papszMetadata;

poDriver = GetGDALDriverManager()->GetDriverByName(pszFormat);

if( poDriver == NULL )
        exit( 1 );

papszMetadata = poDriver->GetMetadata();
    if( CSLFetchBoolean( papszMetadata, GDAL_DCAP_CREATE, FALSE ) )
        printf( "Driver %s supports Create() method.\n", pszFormat );
    if( CSLFetchBoolean( papszMetadata, GDAL_DCAP_CREATECOPY, FALSE ) )
        printf( "Driver %s supports CreateCopy() method.\n", pszFormat );
\endcode

In C:
\code
#include "cpl_string.h"
...
    const char *pszFormat = "GTiff";
    GDALDriverH hDriver = GDALGetDriverByName( pszFormat );
    char **papszMetadata;

if( hDriver == NULL )
        exit( 1 );

papszMetadata = GDALGetMetadata( hDriver, NULL );
    if( CSLFetchBoolean( papszMetadata, GDAL_DCAP_CREATE, FALSE ) )
        printf( "Driver %s supports Create() method.\n", pszFormat );
    if( CSLFetchBoolean( papszMetadata, GDAL_DCAP_CREATECOPY, FALSE ) )
        printf( "Driver %s supports CreateCopy() method.\n", pszFormat );
\endcode

In Python:

\code
    format = "GTiff"
    driver = gdal.GetDriverByName( format )
    metadata = driver.GetMetadata()
    if metadata.has_key(gdal.DCAP_CREATE) \
       and metadata[gdal.DCAP_CREATE] == 'YES':
        print 'Driver %s supports Create() method.' % format
    if metadata.has_key(gdal.DCAP_CREATECOPY) \
       and metadata[gdal.DCAP_CREATECOPY] == 'YES':
        print 'Driver %s supports CreateCopy() method.' % format
\endcode

Note that a number of drivers are read-only and won't support Create()
or CreateCopy().

\section gdal_tutorial_createcopy Using CreateCopy()

The GDALDriver::CreateCopy() method can be used fairly simply as most
information is collected from the source dataset.  However, it includes
options for passing format specific creation options, and for reporting
progress to the user as a long dataset copy takes place.  A simple copy
from the a file named pszSrcFilename, to a new file named pszDstFilename
using default options on a format whose driver was previously fetched might
look like this:

In C++:
\code
    GDALDataset *poSrcDS =
       (GDALDataset *) GDALOpen( pszSrcFilename, GA_ReadOnly );
    GDALDataset *poDstDS;

poDstDS = poDriver->CreateCopy( pszDstFilename, poSrcDS, FALSE,
                                    NULL, NULL, NULL );

/* Once we're done, close properly the dataset */
    if( poDstDS != NULL )
        GDALClose( (GDALDatasetH) poDstDS );
    GDALClose( (GDALDatasetH) poSrcDS );
\endcode

In C:
\code
    GDALDatasetH hSrcDS = GDALOpen( pszSrcFilename, GA_ReadOnly );
    GDALDatasetH hDstDS;

hDstDS = GDALCreateCopy( hDriver, pszDstFilename, hSrcDS, FALSE,
                             NULL, NULL, NULL );

/* Once we're done, close properly the dataset */
    if( hDstDS != NULL )
        GDALClose( hDstDS );
    GDALClose(hSrcDS);
\endcode

In Python:

\code
    src_ds = gdal.Open( src_filename )
    dst_ds = driver.CreateCopy( dst_filename, src_ds, 0 )

# Once we're done, close properly the dataset
    dst_ds = None
    src_ds = None
\endcode

Note that the CreateCopy() method returns a writable dataset, and that it
must be closed properly to complete writing and flushing the dataset to disk.
In the Python case this occurs automatically when "dst_ds" goes out of scope.
The FALSE (or 0) value used for the bStrict option just after the destination
filename in the CreateCopy() call indicates that the CreateCopy() call
should proceed without a fatal error even if the destination dataset cannot
be created to exactly match the input dataset.  This might be because the
output format does not support the pixel datatype of the input dataset, or
because the destination cannot support writing georeferencing for instance.

A more complex case might involve passing creation options, and using a
predefined progress monitor like this:

In C++:
\code
#include "cpl_string.h"
...
    char **papszOptions = NULL;

papszOptions = CSLSetNameValue( papszOptions, "TILED", "YES" );
    papszOptions = CSLSetNameValue( papszOptions, "COMPRESS", "PACKBITS" );
    poDstDS = poDriver->CreateCopy( pszDstFilename, poSrcDS, FALSE,
                                    papszOptions, GDALTermProgress, NULL );

/* Once we're done, close properly the dataset */
    if( poDstDS != NULL )
        GDALClose( (GDALDatasetH) poDstDS );
    CSLDestroy( papszOptions );
\endcode

In C:
\code
#include "cpl_string.h"
...
    char **papszOptions = NULL;

papszOptions = CSLSetNameValue( papszOptions, "TILED", "YES" );
    papszOptions = CSLSetNameValue( papszOptions, "COMPRESS", "PACKBITS" );
    hDstDS = GDALCreateCopy( hDriver, pszDstFilename, hSrcDS, FALSE,
                             papszOptions, GDALTermProgres, NULL );

/* Once we're done, close properly the dataset */
    if( hDstDS != NULL )
        GDALClose( hDstDS );
    CSLDestroy( papszOptions );
\endcode

In Python:

\code
    src_ds = gdal.Open( src_filename )
    dst_ds = driver.CreateCopy( dst_filename, src_ds, 0,
                                [ 'TILED=YES', 'COMPRESS=PACKBITS' ] )

# Once we're done, close properly the dataset
    dst_ds = None
    src_ds = None
\endcode

\section gdal_tutorial_create Using Create()

For situations in which you are not just exporting an existing file to a new
file, it is generally necessary to use the GDALDriver::Create() method (though
some interesting options are possible through use of virtual files or in-memory
files). The Create() method takes an options list much like CreateCopy(),
but the image size, number of bands and band type must be provided explicitly.

In C++:
\code
    GDALDataset *poDstDS;
    char **papszOptions = NULL;

poDstDS = poDriver->Create( pszDstFilename, 512, 512, 1, GDT_Byte,
                                papszOptions );
\endcode

In C:
\code
    GDALDatasetH hDstDS;
    char **papszOptions = NULL;

hDstDS = GDALCreate( hDriver, pszDstFilename, 512, 512, 1, GDT_Byte,
                         papszOptions );
\endcode

In Python:

\code
    dst_ds = driver.Create( dst_filename, 512, 512, 1, gdal.GDT_Byte )
\endcode

Once the dataset is successfully created, all appropriate metadata and raster
data must be written to the file. What this is will vary according to usage,
but a simple case with a projection, geotransform and raster data is covered
here.

In C++:
\code
    double adfGeoTransform[6] = { 444720, 30, 0, 3751320, 0, -30 };
    OGRSpatialReference oSRS;
    char *pszSRS_WKT = NULL;
    GDALRasterBand *poBand;
    GByte abyRaster[512*512];

poDstDS->SetGeoTransform( adfGeoTransform );

oSRS.SetUTM( 11, TRUE );
    oSRS.SetWellKnownGeogCS( "NAD27" );
    oSRS.exportToWkt( &pszSRS_WKT );
    poDstDS->SetProjection( pszSRS_WKT );
    CPLFree( pszSRS_WKT );

poBand = poDstDS->GetRasterBand(1);
    poBand->RasterIO( GF_Write, 0, 0, 512, 512,
                      abyRaster, 512, 512, GDT_Byte, 0, 0 );

/* Once we're done, close properly the dataset */
    GDALClose( (GDALDatasetH) poDstDS );
\endcode

In C:
\code
    double adfGeoTransform[6] = { 444720, 30, 0, 3751320, 0, -30 };
    OGRSpatialReferenceH hSRS;
    char *pszSRS_WKT = NULL;
    GDALRasterBandH hBand;
    GByte abyRaster[512*512];

GDALSetGeoTransform( hDstDS, adfGeoTransform );

hSRS = OSRNewSpatialReference( NULL );
    OSRSetUTM( hSRS, 11, TRUE );
    OSRSetWellKnownGeogCS( hSRS, "NAD27" );
    OSRExportToWkt( hSRS, &pszSRS_WKT );
    OSRDestroySpatialReference( hSRS );

GDALSetProjection( hDstDS, pszSRS_WKT );
    CPLFree( pszSRS_WKT );

hBand = GDALGetRasterBand( hDstDS, 1 );
    GDALRasterIO( hBand, GF_Write, 0, 0, 512, 512,
                  abyRaster, 512, 512, GDT_Byte, 0, 0 );

/* Once we're done, close properly the dataset */
    GDALClose( hDstDS );
\endcode

In Python:

\code
    import osr
    import numpy

dst_ds.SetGeoTransform( [ 444720, 30, 0, 3751320, 0, -30 ] )

srs = osr.SpatialReference()
    srs.SetUTM( 11, 1 )
    srs.SetWellKnownGeogCS( 'NAD27' )
    dst_ds.SetProjection( srs.ExportToWkt() )

raster = numpy.zeros( (512, 512), dtype=numpy.uint8 )
    dst_ds.GetRasterBand(1).WriteArray( raster )

# Once we're done, close properly the dataset
    dst_ds = None
\endcode

\htmlonly

$Id: gdal_tutorial.dox 35222 2016-08-28 06:06:11Z goatbar $

\endhtmlonly