/*! \page osr_tutorial

<center>
<title>OGR Projections Tutorial</title>
</center>

<h2>Introduction</h2>

The OGRSpatialReference, and OGRCoordinateTransformation classes provide
services to represent coordinate systems (projections and datums) and to
transform between them.  These services are loosely modelled on the 
OpenGIS Coordinate Transformations specification, and use the same 
Well Known Text format for describing coordinate systems. 

Some background on OpenGIS coordinate systems and services can be found
in the Simple Features for COM, and Spatial Reference Systems Abstract Model
documents available from <a href="http://www.opengis.org/">www.opengis.org</a>.
The GeoTIFF Projections Transform List
(http://www.remotesensing.org/geotiff/proj_list) may also be of assistance in 
understanding formulations of projections in WKT.  The 
<a href="http://www.petroconsultants.com/products/geodetic.html">EPSG</a> 
Geodesy web page is also a useful resource. 

<h2>Defining a Geographic Coordinate System</h2>

Coordinate systems are encapsulated in the OGRSpatialReference class.  There
are a number of ways of initializing an OGRSpatialReference object to a
valid coordinate system.  There are two primary kinds of coordinate systems.
The first is geographic (positions are measured in long/lat) and the second
is projected (such as UTM - positions are measured in meters or feet).  

A Geographic coordinate system contains information on the datum (which implies
an spheroid described by a semi-major axis, and inverse flattening), prime 
meridian(normally Greenwich), and an angular units type which is normally 
degrees.  The following code initializes a geographic coordinate system 
on supplying all this information along with a user visible name for the
geographic coordinate system. 

<pre>
	OGRSpatialReference oSRS;

	oSRS.SetGeogCS( "My geographic coordinate system",
	                "WGS_1984", 
			"My WGS84 Spheroid", 
			SRS_WGS84_SEMIMAJOR, SRS_WGS84_INVFLATTENING, 
			"Greenwich", 0.0, 
			"degree", SRS_UA_DEGREE_CONV );
</pre>

Of these values, the names "My geographic coordinate system", "My WGS84 
Spheroid", "Greenwich" and "degree" are not keys, but are used for display
to the user.  However, the datum name "WGS_1984" is used as a key to identify
the datum, and there are rules on what values can be used.  NOTE: Prepare
writeup somewhere on valid datums! 

The OGRSpatialReference has built in support for a few well known coordinate
systems, which include "NAD27", "NAD83", "WGS72" and "WGS84" which can be
defined in a single call to SetWellKnownGeogCS(). 

<pre>
	oSRS.SetWellKnownGeogCS( "WGS84" );
</pre>

Furthermore, any geographic coordinate system in the EPSG database can
be set by it's GCS code number if the EPSG database is available. 

<pre>
	oSRS.SetWellKnownGeogCS( "EPSG:4326" );
</pre>

For serializization, and transmission of projection definitions to other
packages, the OpenGIS Well Known Text format for coordinate systems is
used.  An OGRSpatialReference can be initialized from well known text, or
converted back into well known text.

<pre>
	char	*pszWKT = NULL;

	oSRS.SetWellKnownGeogCS( "WGS84" );
	oSRS.exportToWkt( &pszWKT );
	printf( "%s\n", pszWKT );
</pre>

gives something like:

<pre>
GEOGCS["WGS 84",DATUM["WGS_1984",SPHEROID["WGS 84",6378137,298.257223563,
AUTHORITY["EPSG",7030]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG",6326]],
PRIMEM["Greenwich",0,AUTHORITY["EPSG",8901]],UNIT["DMSH",0.0174532925199433,
AUTHORITY["EPSG",9108]],AXIS["Lat",NORTH],AXIS["Long",EAST],AUTHORITY["EPSG",
4326]]
</pre>

or in more readable form:

<pre>
GEOGCS["WGS 84",
    DATUM["WGS_1984",
        SPHEROID["WGS 84",6378137,298.257223563,
            AUTHORITY["EPSG",7030]],
        TOWGS84[0,0,0,0,0,0,0],
        AUTHORITY["EPSG",6326]],
    PRIMEM["Greenwich",0,AUTHORITY["EPSG",8901]],
    UNIT["DMSH",0.0174532925199433,AUTHORITY["EPSG",9108]],
    AXIS["Lat",NORTH],
    AXIS["Long",EAST],
    AUTHORITY["EPSG",4326]]
</pre>

The OGRSpatialReference::importFromWkt() method can be used to set an 
OGRSpatialReference from a WKT coordinate system definition.

<h2>Defining a Projected Coordinate System</h2>

A projected coordinate system (such as UTM, Lambert Conformal Conic, etc) 
requires and underlying geographic coordinate system as well as a definition
for the projection transform used to translate between linear positions
(in meters or feet) and angular long/lat positions.  The following code
defines a UTM zone 17 projected coordinate system with and underlying 
geographic coordinate system (datum) of WGS84.

<pre>
	OGRSpatialReference	oSRS;

	oSRS.SetProjCS( "UTM 17 (WGS84) in northern hemisphere." );
	oSRS.SetWellKnownGeogCS( "WGS84" );
	oSRS.SetUTM( 17, FALSE );
</pre>

Calling SetProjCS() sets a user
name for the projected coordinate system and establishes that the system
is projected.  The SetWellKnownGeogCS() associates a geographic coordinate
system, and the SetUTM() call sets detailed projection transformation 
parameters.  At this time the above order is important in order to 
create a valid definition, but in the future the object will automatically
reorder the internal representation as needed to remain valid.  For now
<b>be careful of the order of steps defining an OGRSpatialReference!</b>

The above definition would give a WKT version that looks something like
the following.  Note that the UTM 17 was expanded into the details 
transverse mercator definition of the UTM zone.  

<pre>
PROJCS["UTM 17 (WGS84) in northern hemisphere.",
    GEOGCS["WGS 84",
        DATUM["WGS_1984",
            SPHEROID["WGS 84",6378137,298.257223563,
                AUTHORITY["EPSG",7030]],
            TOWGS84[0,0,0,0,0,0,0],
            AUTHORITY["EPSG",6326]],
        PRIMEM["Greenwich",0,AUTHORITY["EPSG",8901]],
        UNIT["DMSH",0.0174532925199433,AUTHORITY["EPSG",9108]],
        AXIS["Lat",NORTH],
        AXIS["Long",EAST],
        AUTHORITY["EPSG",4326]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["latitude_of_origin",0],
    PARAMETER["central_meridian",-81],
    PARAMETER["scale_factor",0.9996],
    PARAMETER["false_easting",500000],
    PARAMETER["false_northing",0]]
</pre>

There are methods for many projection methods including SetTM() (Transverse
Mercator), SetLCC() (Lambert Conformal Conic), and SetMercator().

<h2>Querying Coordinate System</h2>

Once an OGRSpatialReference has been established, various information about
it can be queried.  It can be established if it is a projected or 
geographic coordinate system using the OGRSpatialReference::IsProjected() and 
OGRSpatialReference::IsGeographic() methods.  The 
OGRSpatialReference::GetSemiMajor(), OGRSpatialReference::GetSemiMinor() and 
OGRSpatialReference::GetInvFlattening() methods can be used to get 
information about the spheroid.  The OGRSpatialReference::GetAttrValue()
method can be used to get the PROJCS, GEOGCS, DATUM, SPHEROID, and PROJECTION
names strings.  The OGRSpatialReference::GetProjParm() method can be used to 
get the projection parameters.  The OGRSpatialReference::GetLinearUnits() 
method can be used to fetch the linear units type, and translation to meters.

The following code (from ogr_srs_proj4.cpp) demonstrates use
of GetAttrValue() to get the projection, and GetProjParm() to get projection
parameters.  The GetAttrValue() method searches for the first "value" 
node associated with the named entry in the WKT text representation.  
The #define'ed constants for projection parameters (such as 
SRS_PP_CENTRAL_MERIDIAN) should be used when fetching projection parameter
with GetProjParm().   The code for the Set methods of the various projections
in ogrspatialreference.cpp can be consulted to find which parameters apply to 
which projections.  

<pre>
    const char *pszProjection = poSRS->GetAttrValue("PROJECTION");

    if( pszProjection == NULL )
    {
	if( poSRS->IsGeographic() )
            sprintf( szProj4+strlen(szProj4), "+proj=longlat " );
	else
            sprintf( szProj4+strlen(szProj4), "unknown " );
    }
    else if( EQUAL(pszProjection,SRS_PT_CYLINDRICAL_EQUAL_AREA) )
    {
        sprintf( szProj4+strlen(szProj4),
           "+proj=cea +lon_0=%.9f +lat_ts=%.9f +x_0=%.3f +y_0=%.3f ",
                 poSRS->GetProjParm(SRS_PP_CENTRAL_MERIDIAN,0.0),
                 poSRS->GetProjParm(SRS_PP_STANDARD_PARALLEL_1,0.0),
                 poSRS->GetProjParm(SRS_PP_FALSE_EASTING,0.0),
                 poSRS->GetProjParm(SRS_PP_FALSE_NORTHING,0.0) );
    }
    ...
</pre>

<h2>Coordinate Transformation</h2>

The OGRCoordinateTransformation class is used for translating positions
between different coordinate systems.  New transformation objects are
created using OGRCreateCoordinateTransformation(), and then the
OGRCoordinateTransformation::Transform() method can be used to convert
points between coordinate systems.

<pre>
        OGRSpatialReference oSourceSRS, oTargetSRS;
        OGRCoordinateTransformation *poCT;
        double			x, y;
            
        oSourceSRS.importFromEPSG( atoi(papszArgv[i+1]) );
        oTargetSRS.importFromEPSG( atoi(papszArgv[i+2]) );
            
        poCT = OGRCreateCoordinateTransformation( &oSourceSRS,
                                                  &oTargetSRS );
        x = atof( papszArgv[i+3] );
        y = atof( papszArgv[i+4] );
            
        if( poCT == NULL || !poCT->Transform( 1, &x, &y ) )
            printf( "Transformation failed.\n" );
        else
            printf( "(%f,%f) -> (%f,%f)\n", 
                    atof( papszArgv[i+3] ),
                    atof( papszArgv[i+4] ),
                    x, y );
</pre>

There are a couple of points at which transformations can
fail.  First, OGRCreateCoordinateTransformation() may fail, 
generally because the internals recognise that no transformation
between the indicated systems can be established.  This might
be due to use of a projection not supported by the internal 
PROJ.4 library, differing datums for which no relationship
is known, or one of the coordinate systems being inadequately
defined.  If OGRCreateCoordinateTransformation() fails it will
return a NULL. 

The OGRCoordinateTransformation::Transform() method itself can
also fail.  This may be as a delayed result of one of the above
problems, or as a result of an operation being numerically 
undefined for one or more of the passed in points.  The 
Transform() function will return FALSE if any of the points fail
to transform, and the point array is left in an indeterminate 
state.  

Though not shown above, the coordinate transformation service can
take 3D points, and will adjust elevations for elevation differents
in spheroids, and datums.  At some point in the future shifts 
between different vertical datums may also be applied.  If no Z is
passed, it is assume that the point is on the geoide. 

The following example shows how to conveniently create a lat/long coordinate
system using the same geographic coordinate system as a projected coordinate
system, and using that to transform between projected coordinates and
lat/long. 

<pre>
    OGRSpatialReference    oUTM, *poLatLong;
    OGRCoordinateTransformation *poTransform;

    oUTM.SetProjCS("UTM 17 / WGS84");
    oUTM.SetWellKnownGeogCS( "WGS84" );
    oUTM.SetUTM( 17 );

    poLatLong = oUTM.CloneGeogCS();
    
    poTransform = OGRCreateCoordinateTransformation( &oUTM, poLatLong );
    if( poTransform == NULL )
    {
        ...
    }
    
    ...

    if( !poTransform->Transform( nPoints, x, y, z ) )
    ...
</pre>

<h2>Alternate Interfaces</h2>

A C interface to the coordinate system services is defined in 
ogr_srs_api.h, and Python bindings are available via the osr.py module. 
Methods are close analogs of the C++ methods but in C and Python bindings
are missing for some C++ methods. 

<h3>C Bindings</h3>

<pre>
typedef void *OGRSpatialReferenceH;                               
typedef void *OGRCoordinateTransformationH;

OGRSpatialReferenceH OSRNewSpatialReference( const char * /* = NULL */);
void    OSRDestroySpatialReference( OGRSpatialReferenceH );

int     OSRReference( OGRSpatialReferenceH );
int     OSRDereference( OGRSpatialReferenceH );

OGRErr  OSRImportFromEPSG( OGRSpatialReferenceH, int );
OGRErr  OSRImportFromWkt( OGRSpatialReferenceH, char ** );
OGRErr  OSRExportToWkt( OGRSpatialReferenceH, char ** );

OGRErr  OSRSetAttrValue( OGRSpatialReferenceH hSRS, const char * pszNodePath,
                         const char * pszNewNodeValue );
const char *OSRGetAttrValue( OGRSpatialReferenceH hSRS,
                             const char * pszName, int iChild /* = 0 */ );

OGRErr  OSRSetLinearUnits( OGRSpatialReferenceH, const char *, double );
double  OSRGetLinearUnits( OGRSpatialReferenceH, char ** );

int     OSRIsGeographic( OGRSpatialReferenceH );
int     OSRIsProjected( OGRSpatialReferenceH );
int     OSRIsSameGeogCS( OGRSpatialReferenceH, OGRSpatialReferenceH );
int     OSRIsSame( OGRSpatialReferenceH, OGRSpatialReferenceH );

OGRErr  OSRSetProjCS( OGRSpatialReferenceH hSRS, const char * pszName );
OGRErr  OSRSetWellKnownGeogCS( OGRSpatialReferenceH hSRS,
                               const char * pszName );

OGRErr  OSRSetGeogCS( OGRSpatialReferenceH hSRS,
                      const char * pszGeogName,
                      const char * pszDatumName,
                      const char * pszEllipsoidName,
                      double dfSemiMajor, double dfInvFlattening,
                      const char * pszPMName /* = NULL */,
                      double dfPMOffset /* = 0.0 */,
                      const char * pszUnits /* = NULL */,
                      double dfConvertToRadians /* = 0.0 */ );

double  OSRGetSemiMajor( OGRSpatialReferenceH, OGRErr * /* = NULL */ );
double  OSRGetSemiMinor( OGRSpatialReferenceH, OGRErr * /* = NULL */ );
double  OSRGetInvFlattening( OGRSpatialReferenceH, OGRErr * /* = NULL */ );

OGRErr  OSRSetAuthority( OGRSpatialReferenceH hSRS,
                         const char * pszTargetKey,
                         const char * pszAuthority,
                         int nCode );
OGRErr  OSRSetProjParm( OGRSpatialReferenceH, const char *, double );
double  OSRGetProjParm( OGRSpatialReferenceH hSRS,
                        const char * pszParmName, 
                        double dfDefault /* = 0.0 */,
                        OGRErr * /* = NULL */ );

OGRErr  OSRSetUTM( OGRSpatialReferenceH hSRS, int nZone, int bNorth );
int     OSRGetUTMZone( OGRSpatialReferenceH hSRS, int *pbNorth );

OGRCoordinateTransformationH
OCTNewCoordinateTransformation( OGRSpatialReferenceH hSourceSRS,
                                OGRSpatialReferenceH hTargetSRS );
void OCTDestroyCoordinateTransformation( OGRCoordinateTransformationH );

int OCTTransform( OGRCoordinateTransformationH hCT,
                  int nCount, double *x, double *y, double *z );
</pre>

<h3>Python Bindings</h3>

<pre>
class osr.SpatialReference
    def __init__(self,obj=None):
    def ImportFromWkt( self, wkt ):
    def ExportToWkt(self):
    def ImportFromEPSG(self,code):
    def IsGeographic(self):
    def IsProjected(self):
    def GetAttrValue(self, name, child = 0):
    def SetAttrValue(self, name, value):
    def SetWellKnownGeogCS(self, name):
    def SetProjCS(self, name = "unnamed" ):
    def IsSameGeogCS(self, other):
    def IsSame(self, other):
    def SetLinearUnits(self, units_name, to_meters ):
    def SetUTM(self, zone, is_north = 1):

class CoordinateTransformation:
    def __init__(self,source,target):
    def TransformPoint(self, x, y, z = 0):
    def TransformPoints(self, points):
</pre>

<h2>Internal Implementation</h2>

The OGRCoordinateTransformation service is implemented on top of the
<a href="http://www.remotesensing.org/proj">PROJ.4</a> library originally
written by Gerald Evenden of the USGS.  Datum shifting is currently implemented
using 7 coefficient bursa-wolf equations based on the TOWGS84[] values
provided with the datum if available.   

In the future it is planned to incorporation optional use of NADCON for
datum conversion between NAD27 and NAD83, and possibly lookup of datum
shifting parameters from the EPSG database if no TOWGS84 clause is provided.

*/
