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Lucrul cu Proiecții

QGIS permite utilizatorilor să definească un CRS (sistem de coordonate de referință) global și la nivel de proiect, pentru straturi fără un CRS predefinit. De asemenea, permite utilizatorului să definească sisteme de coordonate de referință personalizate și acceptă proiecția din-zbor (OTF) a straturilor vectoriale și raster. Toate aceste entități permit utilizatorului să afișeze straturi cu CRS-uri diferite și să le suprapună în mod corespunzător.

Privire de ansamblu asupra Suportului Proiecției

QGIS has support for approximately 2,700 known CRSs. Definitions for each 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 CRSs are stored in a user database. See section Sistem Personalizat de Coordonate de Referință for information on managing your custom coordinate reference systems.

The CRSs available in QGIS are based on those defined by the European Petroleum Search Group (EPSG) and the Institut Geographique National de France (IGNF) and are largely abstracted from the spatial reference tables used in GDAL. EPSG identifiers are present in the database and can be used to specify a CRS in QGIS.

In order to use OTF projection, either your data must contain information about its coordinate reference system or you will need 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 recognized means of specifying the CRS. In the case of shapefiles, this means a file containing the well-known text (WKT) specification of the CRS. This 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.

Whenever you select a new CRS, the layer units will automatically be changed in the General tab of the options Project Properties dialog under the Project (Gnome, OS X) or Settings (KDE, Windows) menu.

Specificații Globale de Proiecție

QGIS starts each new project using the global default projection. The global default CRS is EPSG:4326 - WGS 84 (proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs), and it comes predefined in QGIS. This default can be changed via the [Select...] button in the first section, which is used to define the default coordinate reference system for new projects, as shown in figure_projection_1. This choice will be saved for use in subsequent QGIS sessions.

Figure Projection 1:


Fila CRS din Dialogul Opțiunilor QGIS

When you use layers that do not have a CRS, you need to define how QGIS responds to these layers. This can be done globally or project-wide in the CRS tab under Settings ‣ options Options.

Opțiunile prezentate în figure_projection_1 sunt:

  • radioButtonOn Se cere CRS-ul

  • radioButtonOff Utilizează CRS-ul proiectului

  • radioButtonOff Utilizează CRS-ul implicit, afișat mai jos

If you want to define the coordinate reference system for a certain layer without CRS information, you can also do that in the General tab of the raster and vector properties dialog (see Meniu General for rasters and Meniu General for vectors). If your layer already has a CRS defined, it will be displayed as shown in Meniul general din fereastra de proprietăți a stratului vectorial.


CRS-ul din Legenda Hărții

Right-clicking on a layer in the Map Legend (section Panoul Straturilor) provides two CRS shortcuts. Set layer CRS takes you directly to the Coordinate Reference System Selector dialog (see figure_projection_2). Set project CRS from Layer redefines the project CRS using the layer’s CRS.

Definirea Din Zbor (OTF) a Reproiectării

QGIS supports OTF reprojection for both raster and vector data. However, OTF is not activated by default. To use OTF projection, you must activate the checkbox Enable on the fly CRS transformation checkbox in the CRS tab of the projectProperties Project Properties dialog.

Există trei căi de a face asta:

  1. Selectați options Proprietățile Proiectului din Proiect (Gnome, OSX) sau din meniul Setări (KDE, Windows).

  2. Clic pe pictograma geographic CRS status, din colțul din dreapta-jos al barei de stare.

  3. Turn OTF on by default in the CRS tab of the Options dialog by selecting checkbox Enable ‘on the fly’ reprojection by default or Automatically enable ‘on the fly’ reprojection if layers have different CRS.

If you have already loaded a layer and you want to enable OTF projection, the best practice is to open the CRS tab of the Project Properties dialog, select a CRS, and activate the checkbox Enable ‘on the fly’ CRS transformation checkbox. The geographic CRS status icon will no longer be greyed out, and all layers will be OTF projected to the CRS shown next to the icon.

Figure Projection 2:


Dialogul Proprietăților de Proiecție

The CRS tab of the Project Properties dialog contains five important components, as shown in Figure_projection_2 and described below:

  1. Enable ‘on the fly’ CRS transformation — This checkbox is used to enable or disable OTF projection. When off, each layer is drawn using the coordinates as read from the data source, and the components described below are inactive. When on, the coordinates in each layer are projected to the coordinate reference system defined for the map canvas.
  2. Filter — If you know the EPSG code, the identifier, or the name for a coordinate reference system, you can use the search feature to find it. Enter the EPSG code, the identifier or the name.
  3. Recently used coordinate reference systems — If you have certain CRSs that you frequently use in your everyday GIS work, these will be displayed in this list. Click on one of these items to select the associated CRS.
  4. Sisteme de coordonate de referință ale lumii — Aceasta este o listă a tuturor CRS-urilor acceptate de QGIS, inclusiv sistemele de coordonate de referință Geografice, Proiectate și Personalizate. Pentru a defini un CRS, alegându-l din listă, prin extinderea nodului corespunzător, și prin selectarea CRS-ului. CRS-ul activ este preselectat.

  5. PROJ.4 text — This is the CRS string used by the PROJ.4 projection engine. This text is read-only and provided for informational purposes.


Dialogul cu Proprietăţile Proiectului

If you open the Project Properties dialog from the Project menu, you must click on the CRS tab to view the CRS settings.

Deschiderea ferestrei de dialog, apăsând pe pictograma geographic CRS status va aduce automat fila CRS în prim-plan.

Sistem Personalizat de Coordonate de Referință

If QGIS does not provide the coordinate reference system you need, you can define a custom CRS. To define a CRS, select customProjection Custom CRS... from the Settings menu. Custom CRSs are stored in your QGIS user database. In addition to your custom CRSs, this database also contains your spatial bookmarks and other custom data.

Figure Projection 3:


Dialogul CRS-urilor Personalizate

Defining a custom CRS in QGIS requires a good understanding of the PROJ.4 projection library. To begin, refer to “Cartographic Projection Procedures for the UNIX Environment - A User’s Manual” by Gerald I. Evenden, U.S. Geological Survey Open-File Report 90-284, 1990 (available at ftp://ftp.remotesensing.org/proj/OF90-284.pdf).

Acest manual descrie utilizarea proj.4 și a utilitarelor aferente, pentru linia de comandă. Parametrii cartografici utilizați cu proj.4 sunt descriși în manualul de utilizare, fiind identici cu cei utilizați de QGIS.

P entru a defini un utilizator CRS, dialogul de Definire a Sistemului de Coordonate de Referință necesită doar doi parametri:

  1. Un nume descriptiv

  2. Parametrii cartografici în format PROJ.4

Pentru a crea un nou CRS, faceți clic pe butonul signPlus Adăugare CRS nou, apoi introduceți un nume descriptiv și parametrii CRS.

Notați că Parametrii trebuie să înceapă cu un bloc +proj=, pentru a reprezenta noul sistem de coordonate.

You can test your CRS parameters to see if they give sane results. To do this, enter known WGS 84 latitude and longitude values in North and East fields, respectively. Click on [Calculate], and compare the results with the known values in your coordinate reference system.

Transformări de datum implicite

OTF depends on being able to transform data into a ‘default CRS’, and QGIS uses WGS84. For some CRS there are a number of transforms available. QGIS allows you to define the transformation used otherwise QGIS uses a default transformation.

În fila CRS de sub Setări ‣ options Opțiuni puteți:

  • set QGIS to ask you when it needs define a transformation using radioButtonOn Ask for datum transformation when no default is defined
  • editarea unei liste de transformări de datum implicite.

QGIS asks which transformation to use by opening a dialogue box displaying PROJ.4 text describing the source and destination transforms. Further information may be found by hovering over a transform. User defaults can be saved by selecting radioButtonOn Remember selection.