QGIS - Importing Georeferenced Images

This article relates to importing images from Pozi into QGIS.  

In the tools menu of Pozi it is possible to save the displayed map as an image:

When selected, the option to "Save map as image" will save the map as a PNG, and will expose the option to save a corresponding world file:

Select "Save world file".  A world file contains georeference information about the image, which is saved as a separate file.

The image and the world file will have the same name but different extensions. 

So if the image file is "Bendigo Community Compass 20190724160209.png", the world file created will be called "Bendigo Community Compass 20190724160209.pngw".

QGIS Settings in relation to CRSs (Coordinate Reference Systems)

QGIS can be set up is several ways regarding the CRS being used and how new layers are loaded:

1. When QGIS is started and a new project is created, there will be a default CRS applied (predefined in Settings->Options)

2. When new layers are created, or when a layer is loaded which has no CRS, then the setting can be predefined to:

2.1 Prompt for the CRS to use; or

2.2 Use the Project CRS; or

2.3 Use a predefined default CRS (which can be different from the default CRS for new projects)

(again, see Settings->Options for these options)

Note that, for the purposes of this exercise it is recommended to choose "Prompt for the CRS to use", but it is not critical.

Adding Georeferenced Image to QGIS

Start QGIS.

Select Layer -> Add Layer -> Add Raster Layer to open the following dialog:

Navigate to the PNG file (usually in the \Downloads folder) and then click the add button (not shown in the above image).

You will probably see a warning message that the CRS for the image is undefined, which will result in one of two possibilities depending on the settings->options discussed above:

1. If the settings->options are NOT set to "Prompt for the CRS to use" for new layers, then the image will load with a predefined CRS, and not require further action; however

2. If the  settings->options ARE set to "Prompt for the CRS to use" for new layers, then you will be presented with the following dialog:

It is suggested to enter 3857 in the Filter field, which will help identify the preferred CRS (WGS 84 / Pseudo-Mercator), then select that option before selecting OK.

Note that the image will display using other CRSs, but may become slightly distorted (ever so slightly!).

At this point you may wish to add other layers to the map (eg. Property boundaries or Road networks), and perhaps use the  QGIS Layout Manager to add annotations and create a refined output.

Final Note Regarding the Project CRS

At the bottom right of QGIS you will see the project CRS. If you click on this you will be presented with the option to change the project CRS.

If you click on this you will be presented with the option to change the project CRS:

You may prefer to be using a particular CRS, however be prepared for your image to become adjusted, should your chosen CRS be noticeably different from WGS 84 / Pseudo-Mercator.