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Viewing Data With Quantum GIS

I downloaded QGIS 0.11 for Windows.  When the download completes, double-click on the file to start the install.  Unless you know what you are doing, I suggest you accept the defaults during this process.  When complete, you'll have a new item in your Programs list (Start > All Programs > Quantum GIS> Quantum GIS).  Fire it up, and lets go...

I happen to have GIS data of Georgia's conservation lands (downloaded from the Georgia GIS Data Clearinghouse) that I'll refer to as the "conserv_lands" dataset.   

From left to right:

  • New Project
  • Save Project
  • Save Project w/ New Name
  • Open Project
  • Print
  • Add Vector Layer (shapefiles, etc)
  • Add Raster Layer (images, grids, etc)
  • Add Database Layer
  • Add WMS Theme (access data from internet sites, pretty cool)

In QGIS, select "Add a Vector Layer" (use icon, also under the Layer menu).  Navigate to the folder in which your data is stored, and select the files you want to display.  My data is in shapefile format, so I click the "conserv_lands.shp" file and hit "Open".  Like magic, I now see polygons showing the conservation lands in Georgia.

 

Show/Select/Query attribute information:

From left to right:

  • Pan
  • Zoom In
  • Zoom Out
  • Zoom to Full Extent
  • Zoom to Layer
  • Zoom to Last Extent
  • Redraw Map
  • Feature Information
  • Feature Select
  • Open Attribute Table

Use the Feature Informationtool to display attribute information for a polygon,  Activate the button (click it) and click on a polygon.  A window appears displaying all of that polygon's attributes.

The Feature Selectbutton lets you select one or many polygons.  Activate it, and then draw a box on the polygon or around a group of polygons you want to select.  It will probably help if you zoom close to the polygon(s) before you select.  Hold the CTRL button while drawing the select box to add to your selection.

If you want to select polygons that satisfy specific criteria, then you should query the the data.

  1.  Right-click on the layer shown in the legend (left side of the screen) and select "Open attribute table" (or click on the Open Attribute Table button)
  2. Click on the Advanced... button to open the query dialog
  3. Click on the field that contains the information you're querying against
  4. Click on the All button under the Values window to list all of the different entries for that field
  5. Put your cursor in the SQL where clause window and do the following:
  • double-click on the field
  • single-click on the operator
  • double-click on the value
  • hit OK

Your query should look something like this:

For example, if I wanted to select all of the polygons associated with the Allatoona WMA, my SQL where clausewould look like:

     NAME = 'Allatoona WMA'

If I wanted to select the Allatoona WMA and the Altamaha WMA, the clause would look like:

     NAME = 'Allatoona WMA' OR NAME = 'Altamaha WMA'

When you hit the OK button, an information window will appear displaying the number of polygons that were selected.

Save the selected polygons as a new shapefile:

  • Right-click on the layer shown in the legend (left side of the screen) and select "Save selection as shapefile..."
  • Navigate to the folder in which you want to save the new file, and give it a new name, and hit Save.

This creates 4 new files in the folder you pointed to.  If you want to send this to somebody, you must send all 4 of them.

Need more help on QGIS, here are some tutorial links.

http://gis.coaps.fsu.edu/FOSS_GIS/Introduction_to_Quantum_GIS_0_8_0.pdf

http://blog.qgis.org/taxonomy/term/3