投影法サポート¶

空間参照系¶

空間参照系(CRS)は QgsCoordinateReferenceSystem クラスによってカプセル化されています。このクラスのインスタンスの作成方法はいくつかあります:

• specify CRS by its ID

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

  QGISは参照系ごとに3種類のIDを使います。

  • PostgisCrsId — IDs used within PostGIS databases.
  • InternalCrsId — IDs internally used in QGIS database.
  • EpsgCrsId — IDs assigned by the EPSG organization

  2番目のパラメータが指定されなければ、PostGIS SRID がデフォルトで使用されます。

• specify CRS by its well-known text (WKT)

  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")
  

CRSの作成(例:データベース内のルックアップ)が成功したかどうかをチェックするのは賢明です: isValid() は True を返さなければなりません。

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.proj4String()
# 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()

投影法¶

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