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투영 지원

좌표계

Coordinate reference systems (CRS) are encapsulated by QgsCoordinateReferenceSystem class. Instances of this class can be created by several different ways:

  • 좌표계의 ID로 CRS 설정

    # PostGIS SRID 4326 is allocated for WGS84
crs = QgsCoordinateReferenceSystem(4326, QgsCoordinateReferenceSystem.PostgisCrsId)

    QGIS는 모든 좌표계를 3가지 형태의 ID로 식별할 수 있습니다.

    • PostgisCrsId — PostGIS 데이터베이스에서 사용되는 ID

    • InternalCrsId — QGIS 데이터베이스에서 내부적으로 사용되는 ID

    • EpsgCrsId — EPSG 기구에서 부여한 ID

    앞의 예시에서 두 번째 파라미터를 지정하지 않으면, 기본값인 PostGIS SRID가 사용됩니다.

  • WKT(well-known text)로 CRS 설정

    wkt = 'GEOGCS["WGS84", DATUM["WGS84", SPHEROID["WGS84", 6378137.0, 298.257223563]],'
       PRIMEM["Greenwich", 0.0], UNIT["degree",0.017453292519943295],'
       AXIS["Longitude",EAST], AXIS["Latitude",NORTH]]'
crs = QgsCoordinateReferenceSystem(wkt)

  • create invalid CRS and then use one of the create*() functions to initialize it. In following example we use Proj4 string to initialize the projection

    crs = QgsCoordinateReferenceSystem()
crs.createFromProj4("+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs")

It’s wise to check whether creation (i.e. lookup in the database) of the CRS has been successful: isValid() must return True.

Note that for initialization of spatial reference systems QGIS needs to look up appropriate values in its internal database srs.db. Thus in case you create an independent application you need to set paths correctly with QgsApplication.setPrefixPath() otherwise it will fail to find the database. If you are running the commands from QGIS python console or developing a plugin you do not care: everything is already set up for you.

Accessing spatial reference system information

print "QGIS CRS ID:", crs.srsid()
print "PostGIS SRID:", crs.srid()
print "EPSG ID:", crs.epsg()
print "Description:", crs.description()
print "Projection Acronym:", crs.projectionAcronym()
print "Ellipsoid Acronym:", crs.ellipsoidAcronym()
print "Proj4 String:", crs.toProj4()
# check whether it's geographic or projected coordinate system
print "Is geographic:", crs.geographicFlag()
# check type of map units in this CRS (values defined in QGis::units enum)
print "Map units:", crs.mapUnits()

Projections

You can do transformation between different spatial reference systems by using QgsCoordinateTransform class. The easiest way to use it is to create source and destination CRS and construct QgsCoordinateTransform instance with them. Then just repeatedly call transform() function to do the transformation. By default it does forward transformation, but it is capable to do also inverse transformation

crsSrc = QgsCoordinateReferenceSystem(4326)    # WGS 84
crsDest = QgsCoordinateReferenceSystem(32633)  # WGS 84 / UTM zone 33N
xform = QgsCoordinateTransform(crsSrc, crsDest)

# forward transformation: src -> dest
pt1 = xform.transform(QgsPoint(18,5))
print "Transformed point:", pt1

# inverse transformation: dest -> src
pt2 = xform.transform(pt1, QgsCoordinateTransform.ReverseTransform)
print "Transformed back:", pt2