投影法概要

Overview of Projection Support

QGIS has support for approximately 2,700 known CRS. Definitions for each of these CRS are stored in a SQLite database that is installed with QGIS. Normally you do not need to manipulate the database directly. In fact, doing so may cause projection support to fail. Custom CRS are stored in a user database. See Section <A HREF="node5.html#sec:customprojections">8.4</A> for information on managing your custom coordinate reference systems. <P> The CRS available in QGIS are based on those defined by EPSG<A NAME="7"></A> and are largely abstracted from the spatial_references table in PostGIS<A NAME="8"></A> version 1.x. The EPSG identifiers are present in the database and can be used to specify a CRS in QGIS. <P> In order to use OTF projection, your data must contain information about its coordinate reference system or you have to define a global, layer or project-wide CRS. For PostGIS layers QGIS uses the spatial reference identifier that was specified when the layer was created. For data supported by OGR, QGIS relies on the presence of a format specific means of specifying the CRS. In the case of shapefiles, this means a file containing the Well Known Text (WKT)<A NAME="9"></A> specification of the CRS. The projection file has the same base name as the shapefile and a prj extension. For example, a shapefile named alaska.shp would have a corresponding projection file named alaska.prj. <P>