TRD 05: Data Presentation techniques (Mapping) Quantum GIS ...

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7. Working with Projections QGIS supports on-the-fly (OTF) projection of vector layers. This feature allows you to display layers with different coordinate systems and have them overlay properly. 7.1. Overview of Projection Support QGIS has support for approximately 2,700 known projections. Projections 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 projections are stored in a user database. See Section 7.3 for information on managing your custom projections. The projections available in QGIS are based on those defined by EPSG and are largely abstracted from the spatial_references table in PostGIS version 1.x. Note that the identifiers used in QGIS do not correspond to the EPSG or PostGIS spatial reference identifiers. The EPSG and PostGIS identifiers are present in the database and can be used to specify a projection in QGIS. In order to use OTF projection, your data must contain information about its coordinate system. 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 coordinate system. In the case of shapefiles, this means a file containing the Well Known Text (WKT) specification of the the coordinate system. The projection file has the same base name as the shapefile and a prj extension. For example, a shapefile named lakes.shp would have a corresponding projection file named lakes.prj. 7.2. Getting Started At startup, QGIS does not have OTF projection enabled. To use OTF projection, you must open the Project Properties dialog, select a projection for the map, and enable projections. There are two ways to open the Project Properties dialog: 1. Select Project Properties from the Settings menu 2. Click on the projector icon in the lower right-hand corner of the statusbar Tip 25 PROJECT PROPERTIES DIALOG If you open the Project Properties dialog from the Settings menu, you must click on the Projection tab to view the projection settings. Opening the dialog from the projector icon will automatically bring the Projection tab to the front. QGIS 0.8 User Guide 49
7.2 Getting Started The Projection dialog contains four important components as numbered in Figure 12 and described below. Figure 12: Projection Dialog (GNU/Linux) 1. Enable projections - this checkbox is used to enable or disable OTF projection. When off, no projection takes place and each layer is drawn using the coordinates as read from the data source. When on, the coordinates in each layer are projected to the coordinate system of the map canvas. 2. Projections - this is a list of all projection supported by QGIS, including Geographic, Projected, and Custom coordinate systems. To use a coordinate system, select it from the list by QGIS 0.8 User Guide 50
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THE REGIONAL TRAINING WORKSHOP ON L
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Contents Contents Title Table of Co
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Contents 6. Working with OGC Data 4
Page 7 and 8: Contents E. Dependencies 95 E.1. Pr
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Page 17 and 18: 3.4 QGIS GUI 3.4. QGIS GUI When QGI
Page 19 and 20: 3.4 QGIS GUI - Add a PostGIS Layer
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Page 23 and 24: 3.6 Measuring features are read fro
Page 25 and 26: 3.9 Spatial Bookmarks $HOME/.config
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Page 29 and 30: 4.1 ESRI Shapefiles Figure 4: QGIS
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Page 33 and 34: 4.2 PostGIS Layers SPIT Plugin QGIS
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Page 37 and 38: 4.5 Editing 4.4.2. Using Actions Ac
Page 39 and 40: 4.5 Editing Tip 12 CONCURRENT EDITS
Page 41 and 42: 4.5 Editing Deleting Vertices of a
Page 43 and 44: 4.6 Query Builder To complete the c
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Page 67 and 68: 8.6 GRASS Toolbox When clicking on
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Page 71 and 72: 9. Map Composer The map composer is
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Page 93 and 94: A.2 GDAL Raster Formats A.2. GDAL R
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Page 101 and 102: C. Installation The described proce
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Page 105 and 106: E.2 GEOS E.2. GEOS Note: As of vers
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E.7 GRASS 5. Build and install GDAL
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#define QT_VERSION_STR "4.1.0" In t
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F.2 Configuring QGIS the right way
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Literature Web-References [1] GEOS.
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Index measure, 14 measure:areas, 14
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Index limits, 47 coordinate referen
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