FAQ & Troubleshooting¶

Frequently asked questions and solutions to common issues.

General Questions¶

What types of rasters can I use?¶

The plugin works with categorical (classified) rasters where each pixel value represents a land use/land cover class. Supported formats:

GeoTIFF (.tif, .tiff)
ERDAS Imagine (.img)

Data should be integer type. Float rasters will work but trigger a warning.

How many rasters can I analyze?¶

There's no hard limit, but practical considerations apply:

Minimum: 1 raster (area statistics only)
For change analysis: 2 or more rasters
Recommended: 2-10 rasters for most analyses
Large series: Processing time increases linearly

What's the maximum raster size?¶

The plugin uses block-based processing (256×256 chunks) so memory usage stays bounded regardless of raster size. Very large rasters simply take longer to process.

Do rasters need to be in the same location?¶

Yes. All input rasters must:

Have the same CRS
Have the same pixel size
Cover the same extent
Have aligned pixel grids

The validation step checks all of these requirements.

Input Issues¶

"CRS mismatch" error¶

Problem: Rasters have different coordinate reference systems.

Solution:

Reproject all rasters to a common CRS
Use Raster → Projections → Warp (Reproject)
Use Nearest Neighbor resampling for categorical data

"Pixel size mismatch" error¶

Problem: Rasters have different resolutions.

Solution:

Resample to a common resolution
Choose the coarsest resolution to avoid artificial precision
Use Nearest Neighbor resampling

"Extent mismatch" error¶

Problem: Rasters don't cover the same area.

Solution:

Clip all rasters to a common extent
Use Raster → Extraction → Clip Raster by Extent
Use the smallest common extent

"Grid alignment" error¶

Problem: Pixel grids are offset between rasters.

Solution:

Align rasters using Warp with a target extent
Ensure the target extent coordinates align with pixel boundaries
Consider recreating rasters from source data with consistent parameters

Years not detected correctly¶

Problem: The plugin didn't find the year in the filename.

Solution:

Manually edit the year in the table
Ensure filenames contain 4-digit years (1990-2099)
Examples that work: lulc_2010.tif, 2015_classified.tif

Validation Issues¶

Float data type warning¶

Problem: Raster uses float instead of integer data type.

Is this a problem?

If your data is truly categorical integers stored as float: No, it will work
If your data has decimal values: Yes, results may be unexpected

Solution (if needed):

Convert to integer using Raster → Conversion → Translate
Set output data type to Int16 or Int32

NoData not defined¶

Problem: Raster doesn't have a NoData value in its metadata.

Solution:

Use the Override option in the Inputs tab
Enter the value that represents NoData in your data (e.g., 0, 255, -9999)

Different NoData values¶

Problem: Rasters use different NoData values.

Solution:

Use the Override option to specify a single NoData value
This value will be applied to all input rasters

Analysis Issues¶

Analysis won't start¶

Possible causes:

Validation has FAIL items → Fix and re-validate
No output directory specified → Set in Inputs tab
No rasters added → Add at least one raster

Analysis is very slow¶

For large rasters:

Disable hotspot generation (most resource-intensive)
Disable chart generation
Use an AOI to limit the analysis area
Consider using lower-resolution data for initial exploration

For many rasters:

Processing time scales linearly with number of rasters
Consider analyzing in subsets

Analysis stopped unexpectedly¶

Check:

QGIS message bar for errors
Log panel in the plugin for messages
Available disk space in output directory
QGIS stability (try restarting QGIS)

Memory errors¶

Although rare due to block processing:

Close other applications
Restart QGIS
Disable hotspot generation
Process fewer rasters at once

Output Issues¶

Missing CSV files¶

Check:

Is that output type enabled in Options tab?
Is there an error in the log panel?
Is there disk space available?

Missing rasters in QGIS¶

Rasters not appearing in Layers panel:

Check if files exist in output directory
Manually add via Layer → Add Raster Layer
Check log for errors during raster creation

Charts not generated¶

Check:

Is "Generate charts" enabled in Options tab?
Does vendor/js/plotly.min.js exist in the plugin folder?
Check log for chart-related errors

If Plotly.js is missing:

Charts are disabled but other outputs proceed
Reinstall the plugin to restore Plotly.js

Charts won't open¶

Try:

Open in a different web browser
Check file size (should be several MB due to embedded Plotly.js)
Try a simple HTML test file to verify browser works

Incorrect values in outputs¶

Check:

Correct NoData value specified?
AOI covering expected area?
Class values match your expectations?
Output units configured correctly?

Performance Questions¶

How can I speed up analysis?¶

Reduce outputs: Disable charts, hotspots if not needed
Use AOI: Analyze only your study area
Fewer rasters: Process subset first
Lower resolution: Resample for exploratory analysis

How much disk space do I need?¶

Approximate output sizes:

Output	Size
CSV files	KB to MB (text)
Change frequency	Same as input raster
Hotspot rasters	Same as input raster per interval
Chart HTML files	~5MB each (includes Plotly.js)

Can I run multiple analyses simultaneously?¶

Not recommended. The plugin is designed for single-threaded operation. Run one analysis at a time for reliable results.

Interpretation Questions¶

What does negative net change mean?¶

The class is losing more area than it's gaining. This indicates:

Class contraction
Conversion to other classes
May indicate land cover conversion trends

What does high gross with low net mean?¶

The class has high turnover but stable total area. This indicates:

Active dynamics
Balanced gains and losses
The class is both expanding in some areas and contracting in others

How do I interpret change frequency?¶

Value	Interpretation
0	Stable/persistent area
1	Changed once during study period
High	Dynamic area, frequent change
Max possible	Changed every interval

What do hotspot values mean?¶

Low values: Sparse change
High values: Concentrated change
Values are relative within each raster
Not directly comparable across intervals without normalization

Getting Help¶

Where can I report bugs?¶

Report issues on the GitHub Issues page.

Include:

QGIS version
Plugin version
Steps to reproduce
Error messages from log panel
Sample data if possible (or description)

Where can I request features?¶

Use GitHub Issues with the "enhancement" label.

Is there a user community?¶

Currently, support is provided through GitHub Issues. Watch the repository for updates and discussions.