QGIS FAQ

Unlike the GIS FAQ, QGIS FAQ touches specifically on QGIS matters. It is my reference material based on personal experience as I stumble across things from queries which I embarrassingly could not answer due to plain ignorance or simply forgot or from complaints by others who blundered here and there while using QGIS but were too shy or have too much ego to admit it was them who did it (Yes I know, it's always the QGIS software, never them!). Just like the GIS FAQ, this article will be dynamic and updated from time to time.

How to move a node (point)?

Edit mode

A node or point is non-moveable until the active layer is set to edit mode. So firstly, select edit icon to set that layer to edit mode/click the preferred node until the red X changes into a red square/click the red square until it changes into a blue square/press cursor on blue square and move cursor to preferred location/select edit icon & save

Selectable mode

Moveable mode

How to re-activate the New Print Composer toolbar that went missing?

Place cursor on menu bar of New Print Composer/press the key in between 'Alt' and 'Ctrl' key/toolbar screen appears/tick all items on the toolbar to activate them 

How to print only a section of a map?

Zoom to a preferred section of the map only then activate the New Print Composer ('Add New Map' takes its image from image off the QGIS workscreen)

How to adjust the size of the map?

Firstly, resize the map by moving the cursor wheel. Alternatively, press Control key and move cursor wheel. Secondly, resize the frame of the map frame if necessary beyond the size of the paper. Thirdly, use 'Move item content' to readjust the location of the map. 

How to selectively delete an object on the digital paper?

Click in the region of a particular object frame until the 4 corners of its frame appear then press the delete button.

How to adjust the scale bar in the New Print Composer?

It is easiest to leave the default map units per bar unit to '1.00' then see the effect of changing the default segment size setting by typing '10', '100', '1000' or '10,000' or so then readjusting backwards if the scale bar exceeds the paper size. Some trial and error is required to find the correct fit against the preferred paper size.

How to save a layer to another format?

Alternative 1

If the intention is to save to a non-OGR format, activate a layer/under main menu/file/select save as image/select files of type/select preferred choice/ok

Alternative 2

If the intention is to save to an OGR format, activate a layer/ right-click mouse on layer in layer panel/select save as/select format/select preferred choice ok

What happens when I get a message like this from the Pyhton Plugin Installer?

That means the repository is down until a time the owner repairs it. Just click OK and carry on.

What happened to the 'Add 3rd party repositories' button?

It is only available in QGIS ver 1.7.5 and not in QGIS Ver. 1.8. This means the registeration of plugin repositories has to be manually done in QGIS Ver. 1.8.

How to re-activate Fetch Python Plugins?

Under menu bar/plugins/manage plugins/select 'Clear All'/OK

Under menu bar/plugins/manage plugins/select 'Select All'/OK (Fetch Python Plugins will re-appear)

How to  activate the North arrow or the scale bar?

In QGIS Ver. 1.7.5, both the North arrow and scale bar can be found under menu bar/plugins/decorations but in QGIS Ver. 1.8, it is found under menu bar/view/decorations.

By default, Enable Scale Bar is not ticked, thus, to activate it select 'Enable scale bar'.

How to make snapping options work?

The default size of the scale bar is 30metres/km. This scale does not show the snapping action well. Reduce that size to the minimum 1 meter/km and see the effect of the snapping action. If the snapping action is still minimal, adjust the tolerance unit proportionally to your map. This takes a bit of trial and error.

How to set the snapping options?

It is convenient and preferable in general to select the mode to 'vertex and segment' because there may be times when snapping action is required on both mode. The choice of tolerance value is subjective and relative to the scale the user decides. The user is required to do a few trial and errors between a fine setting such as 0.0001 and a coarse setting such as 0.1 only then will the user be able to compare and contrast which tolerance value is ideal.

It is preferable to select units in 'map units' rather than pixels because there is a natural tendency to make reference to snapping action base on what is seen on the canvas and not on the monitor.

How to change the default degree in the scale bar?

By default the scale bar is in degree, thus, to change it to meters or feet, selected the round icon at the bottom right corner of the QGIS workscreen/select General/select the preferred bullet e.g. meter or feet or degrees, minutes, seconds

How to make a plan that went missing to re-appear?

Under the Layer panel, right-click the layer map/select zoom to layer extent. (This causes the screen to 'jump' to the location where that plan has moved out of view.)

How to convert map projections?

Alternative 1:

Select menu bar/vector/data management tools/export to new projection/Decide between a predefined spatial reference system or import spatial reference from an existing layer/Create an output shapefile/OK

Alternative 2:

Under the Layer panel, right-click mouse on the preferred layer name/Decide between Layer CRS, Project CRS and Selected  CRS/Browse and decide between Geographic Coordinate Systems, Projected Coordinate Systems and  User Defined Coordinate System/Select 'Add saved file to map'/OK

Alternative 3:

Between 2 maps from the same Projected RS, the CRS of a properly projected map especially in the case where it refers to a User Defined System is copied and pasted onto the Parameter box of the map not properly aligned. Once copied, that map is saved to replace the original map. 'Zoom to layer' extent it to make it move to the location it ought to be.

Between 2 maps, one from a Geographic CR while the other from a Projected RS, I have yet to try this out as I do not have a sample with the former CRS so I can't answer this case... yet. 

How to get back the Layer panel?

Select menu bar/view/panels/ tick 'Layers'

How to link a database to QGIS?

Select menu bar/Layer/add vector/layer/select encoding e.g. system/ select database type e.g. PostgreSQL.

Select new/configure Connection information base on type e.g. PostgreSQL/Test Connect/OK/Open

Why an installed plugin does not get activated)?

After a 'not installed' plugin is installed, it does not necessarily mean that plugin was activated. To activate a particular plugin go to menu bar/plugins/manage plugins/tick the preferred plugin.

Why I cannot install QGIS to my Mac(kinstosh)?

It appears that when OSX Ver.1.6 upgraded to Ver 1.7, many engines that supported many games and applications including QGIS were taken out by Apple. Thus, those applications and games that worked well under Ver 1.6 now do not work . That seems to be the direction Apple has taken. (Commented by a friend who uses a Mac notebook and had QGIS successfully installed along with the plugins in manual mode in Ver 1.6 but QGIS cannot be installed when he upgraded to Ver.1.7)

What to do when a red line message appears when I try to activate Google Hybrid?

Quit QGIS/re-activate QGIS/re-activate Google Hybrid (It is suspected your cache is corrupted)

How to get rid of overlapping Globe menu?

For the time being, I think I have too many icons in the menu bar so when Globe was added, it overlap with others. Since I do not require it, Globe is unselect under 'Manage Plugins' and untick in the menu bar to clear it.

How to use a customize colour template?

Right-click a layer under the Layer panel/select properties/select load style/search & select your customized colour template e.g. plgcolour.qml/apply/ok

How to do I activate the missing "Join Attribute"?

Select menu bar/plugins/fetch python plugins/plugins/select ftools/select Downgrade plugin/close ('Join attibute' will appear under vector/data management tools)

How do I get rid of this message?

This screen will appear when QGIS is activated if the fTools was downgraded probably to take advantage of 'Join Attribute'. It will stay there so long as the installed fTools plugin is recognized by QGIS as technically obsolete because there is a newer version. If the installed fTools is upgraded, this message will be removed so the user must decide whether such features are important to oneself, no one else can decide that.

Why can't I find that .shp or .tab file?

Select menu bar/Layer/add vector layer/browse/select file type e.g. .shp or .tab /open (When browse screen was activated, if file type was '.tab' then only .tabs files will appear in the screen, vice versa) 

Which plugin should I download and install?

That depends on the user's requirement. Fundamentally, there are vector base plugins and raster base plugins for vector to suit a specific task. In QGIS Ver 1.8, when a raster base plugin is installed, it can be found under menu bar/raster whereas when a vector base plugin is installed, it can be found under menu bar/vector. Sometimes, there can be 2 vector headings under the menu bar, one for grouping vector base plugins and vector base tools so don't get confused.

How to re-activate the Georeferencer's GCP table toolbar that went missing?

Place cursor on menu bar of Georeferencer then right click mouse/tick items on the toolbar to activate them

How to re-do a georeference point that was badly done?

When a GeoReference point appears as a red line, it needs to be redone or else this will cause the image to be stretched when pasted later on the vector plan in accordance to the error as directed by the user.  To overcome this, place the cursor on the corresponding row in the GCP table under the on/off column, right-click the mouse/select 'remove' and that point disappears then repeat the georeferencing steps carried out earlier but this time with more accuracy and precision.

How to add diagrams?

Alternative 1

Right-click cursor on active layer under Layer Panel/properties/diagrams/select display diagram/select preferred diagram type/configure respective settings/ok

Alternative 2

Right-click cursor on active layer under Layer Panel/properties/overlay/select display diagram/select preferred diagram type/configure respective settings/ok

How to do away with on-the-fly projection?

On-the-fly projection interferes with a preferred map projection as it overrides it. To have control of over a preferred map projection, on-the-fly projection needs to be unselected. 

Alternative 1

Under menu bar/setting/options/CRS/unselect  'Enable on-the-fly reprojection by default'/OK

Alternative 2

Select the round icon at the bottom right corner of the QGIS workscreen/select Coordinate reference System (CRS)/ unselect Enable 'on-the-fly' CRS transformation/OK

How to do label features?

Alternative 1

Right-click cursor of active layer under Layer Panel/properties/labels/select 'Display labels'/Under label properties, select preferred 'field containing label'/configure respective label properties setting/OK

Alternative 2

Under menu bar, select labeling icon/select 'Label this layer with'/select preferred field/configure label settings/OK

How to ensure spelling correction works?

If the user jumps straight into the task of correcting spelling errors with the field calculator base on the appropriate procedure in the filed calculator be surprised because nothing happens. That is because QGIS was not taught which feature to correct and that requires a prior step to identify the required features using the SQL query after which all will turn out well.

How to calculate in hectares?

In the field calculator, use the 'area' operator where the expression is $area but add "/10000" to convert metre to hectare. The final result only appears after the file has been SAVED!

How to calculate the SUM of something?

'SUM' is a statistical function. Therefore, the total of a specific listing in an attribute table can be obtained by using the Basic Statistics module under Vector/Analysis Tools. Select appropriate layer/  select the appropriate column/Select OK.