Possible issues with QGIS

A user can encounter numerous issues while working with QGIS (or any other software). This section provides a guide to a QGIS user on how to resolve some issues, or a workaround for the issues. These approaches will not always solve your problem.

QGIS crashes when opening a project

The likely cause of this that the project were saved in a different version of QGIS. Do the following to resolve such cases: - Install an LTR version of QGIS (e.g. 3.16), and use it to open the project - Attempt to find out what version the project is stored in - If the user knows the version, install that version

It might be easier to do this in Windows, as it's much easier to install older version in Windows than Linux

• If you are opening a project in a Postgres DB, be sure the DB is stored in the correct postgres version
• Be sure that the project has access to the data it needs to load
• Check if the data is not corrupted (e.g. try loading the data in another project)

Missing layers

The user might encounter a missing layer when opening a QGIS project. This usually happens if the layer source name or directory has changed. A quick solution will be to just remove the layer and readd it, but in some instances a user would like to keep the symbologies and other properties set for the layer. A better solution is therefore required for such cases.

• Right-click on the faulty layer, and click on Repair Data Source
• The Repair Data Source dialog will open

• Browse and select the layer which should be used

• Click OK
• The layer will now be repaired.

Loading rasters

Rasters can sometimes take forever to load, or cause QGIS to crash while a user waits for it to finish. This can happen for several reasons, most likely that the raster has an extremely high spatial resolution and covers a large area. A computer with poor performance will also have more problems. Here are two possible approaches at remeding the issue.

Generate pyrymid layers

Pyrymid layers will drastically improve raster load times in spatial software. If a raster takes too long to load, the best will be to generate pyrimid layers if it has none.

Be aware that for large raster the pyrimid layers can take up a lot of space

• Open the QGIS Processing toolbox

• Type "pyramid" in the Search
• Open the Build overviews (pyramids) tool;
• Select the raster layer for which pyramids should be generated;
• Advanced Parameters can be changed as disired, but default should suffice for most cases;
• Click Run.

Depending on the raster size, this may run a while. Once done, loading the raster will be much faster in QGIS.

Raster resolution

If pyramid layers did not solve the user's loading problems, reducing the spatial resolution can also help.

Do not use this approach if you want to keep the accuracy/quality of the data, or if you need to perform analysis on the data.

• Right-click on the raster, go to Export and click on Save As

• Select the output Format
• Set the output File name
• The CRS is best to be set to that of the raster
• Change the extent as desired
• Change the horizontal and vertical values to the desired spatial resolution
• Click OK

Reducing the spatial resolution, especially if the higher resolution is not required, will allow a user to spend less time waiting for the QGIS canvas to refresh, or needing to restart QGIS because it crashed.

Here is an example of the number of pixels of a raster at a high spatial resolution:

Here is the same raster at a much lower spatial resolution: