/*! \page osr_tutorial OGR Projections Tutorial
\section osr_tutorial_intro Introduction
The OGRSpatialReference, and OGRCoordinateTransformation classes provide
services to represent coordinate systems (projections and datums) and to
transform between them. These services are loosely modeled 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 the Open Geospatial Consortium.
The GeoTIFF Projections Transform List
may also be of assistance in
understanding formulations of projections in WKT. The
EPSG
Geodesy web page is also a useful resource.
\section osr_tutorial_cs Defining a Geographic Coordinate System
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.
\code
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 );
\endcode
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().
\code
oSRS.SetWellKnownGeogCS( "WGS84" );
\endcode
Furthermore, any geographic coordinate system in the EPSG database can
be set by it's GCS code number if the EPSG database is available.
\code
oSRS.SetWellKnownGeogCS( "EPSG:4326" );
\endcode
For serialization, 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.
\code
char *pszWKT = NULL;
oSRS.SetWellKnownGeogCS( "WGS84" );
oSRS.exportToWkt( &pszWKT );
printf( "%s\n", pszWKT );
\endcode
gives something like:
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]]
or in more readable form:
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]]
The OGRSpatialReference::importFromWkt() method can be used to set an
OGRSpatialReference from a WKT coordinate system definition.
\section osr_tutorial_proj Defining a Projected Coordinate System
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 an underlying
geographic coordinate system (datum) of WGS84.
\code
OGRSpatialReference oSRS;
oSRS.SetProjCS( "UTM 17 (WGS84) in northern hemisphere." );
oSRS.SetWellKnownGeogCS( "WGS84" );
oSRS.SetUTM( 17, TRUE );
\endcode
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
be careful of the order of steps defining an OGRSpatialReference!
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.
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]]
There are methods for many projection methods including SetTM() (Transverse
Mercator), SetLCC() (Lambert Conformal Conic), and SetMercator().
\section osr_tutorial_query Querying Coordinate System
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.
\code
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) );
}
...
\endcode
\section osr_tutorial_transform Coordinate Transformation
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.
\code
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 );
\endcode
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 TRUE on success, or FALSE
if any of the points fail to transform. The point array is
left in an indeterminate state on error.
Though not shown above, the coordinate transformation service can
take 3D points, and will adjust elevations for elevation differences
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 geoid.
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.
\code
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 ) )
...
\endcode
\section osr_tutorial_apis Alternate Interfaces
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 C and Python bindings
are missing for some C++ methods.
C Bindings
\code
typedef void *OGRSpatialReferenceH;
typedef void *OGRCoordinateTransformationH;
OGRSpatialReferenceH OSRNewSpatialReference( const char * );
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);
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 ,
double dfPMOffset ,
const char * pszUnits,
double dfConvertToRadians );
double OSRGetSemiMajor( OGRSpatialReferenceH, OGRErr * );
double OSRGetSemiMinor( OGRSpatialReferenceH, OGRErr * );
double OSRGetInvFlattening( OGRSpatialReferenceH, OGRErr * );
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,
OGRErr * );
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 );
\endcode
Python Bindings
\code
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):
\endcode
\section osr_tutorial_impl Internal Implementation
The OGRCoordinateTransformation service is implemented on top of the
PROJ.4 library originally
written by Gerald Evenden of the USGS.
*/