1. How to digitize geological fractures and lineaments

1.1. Introduction

Though drawing lines on a map might not seem complex, there are still rules to follow to make the data you produce analyzable without inconsistencies.

Digitizing lineaments and fractures follows the same process. Usually the scale of observation and underlying raster data are different but actual process of digitization is the same. Consequently, any specific references to “fractures” or “lineaments” are mostly interchangeable, unless otherwise specified.

The purpose of digitizing is usually creating digital data about bedrock discontinuities, i.e. fractures and faults. By drawing along a fracture on a georeferenced picture of an outcrop, you are documenting the length and orientation of a bedrock feature. Furthermore, by accurately digitizing relationships between fractures, you are producing topological information about how fractures interact and form a network.

1.2. How to digitize fractures and lineaments using QGIS

This guide expects some basic knowledge of geographic information systems (GIS) and their nomenclature. However, the guide is meant to be as detailed as necessary for a beginner of QGIS to follow.

This guide has been written for QGIS version 4.0. Some deviation in terms of different option names and locations is therefore possible if you are using a different version.

1.2.1. Setting up your project

1.2.1.1. Project directory

Before opening QGIS, set up an appropriate project directory for digitization. Create a project directory, with a suitable name, e.g. brittle_course_fracture_digitization_2026 in a suitable location under your user directory. E.g. in C:\Users\<your-username>\projects\ or any other locations you have used for projects. Set up a suitable directory structure within that created project directory for data by creating a data directory with two subdirectories: raster and vector.

In the following text, I will refer to the project directory with the name brittle_course_fracture_digitization_2026 and yours may differ.

Tree-view of the project folder:

brittle_course_fracture_digitization_2026
└── data
    ├── raster
    └── vector

Now start up QGIS. Make a new project and save the project file in brittle_course_fracture_digitization_2026. I would recommend using the same name for the project file as you have for the project directory. After saving the project file in the directory, you should have a new brittle_course_fracture_digitization_2026.qgz file there.

brittle_course_fracture_digitization_2026
├── brittle_course_fracture_digitization_2026.qgz
└── data
    ├── raster
    └── vector

1.2.1.2. Project settings

The project coordinate reference system (CRS) should be set to a metric coordinate system to avoid confusing results from analysis of the finished digitized traces. If you are in Finland, the recommended CRS is EPSG:3067 (EUREF-FIN / TM35FIN(E,N) - Finland).

To check and set it, go to Project -> Properties -> CRS. Use Filter to search for EPSG:3065 and select it. Then click OK to save the setting.

The topological editing tool needs to be added to the toolbar for easy access. Go to View -> Toolbars and toggle Snapping toolbar on.

1.2.2. Adding data to the project

1.2.2.1. Raster data

Moving the raster data you are going to use to the data/rasters/ folder of the project directory is recommended as the data will then be easily accessible in QGIS.

Note

If you use the same raster data in multiple projects, it might be better to store it in a central location rather than copying it to each individual project folder, as the raster data itself is never edited during digitization.

To add raster data into QGIS, go to Layer -> Add Layer -> Add Raster Layer and select your raster file, which usually has a .tif or .tiff extension, and click Add to add it to the project.

Check that the added raster data uses the same CRS as the project. Right click on the layer in the Layers tab, click Properties. In the Layer Properties window, go to Source and check the coordinate system and change it to the project coordinate system, if necessary.

1.2.2.2. Vector data

The rasters can cover large areas and digitizing the whole extent might not be needed. Consequently, it is a good idea to create a preliminary target area for digitizing at this point.

If you do not have an existing target area, create a new polygon vector layer for the target area by going to Layer -> Create layer -> New GeoPackage Layer.

Create a new LineString layer for the traces to be digitized. At its simplest, the trace layer can only consist of the trace geometries without any attribute information. When creating your trace layer, select LineString as the geometry type.

Note

Avoid using MultiLineString geometry type. If your lines are accidentally stored as MultiLineStrings, use QGIS’s “Explode Lines” tool (Processing Toolbox > Vector geometry > Explode lines) to convert them to individual LineStrings.

1.2.3. Digitizing fracture and lineament traces in QGIS

1.2.3.1. Layer organization

QGIS determines what gets shown on top of what layer in the ordering of layers in the Layers tab. Make sure the raster layer is at the bottom, target area layer is on top of it and the layer with the traces you are digitizing is at the top.

1.2.3.2. Raster layer styling

Orthomosaics, or pictures of outcrops in general, are RGB images where additional styling is usually not required. However, when digitizing lineaments using, e.g., digital elevation model (DEM) data, you might need to configure styling.

1.2.3.3. Vector layer styling

The target area is defined by polygon(s). By default, QGIS styles them with a single color fill, i.e., they mask the layers beneath them. You can change the styling in the Symbology section, accessed by right clicking the layer in the Layers tab and selecting Properties.

To make digitizing easier, you can try adjusting the trace styles. Changing the color is particularly helpful for ensuring your work stands out against the underlying raster layer.

1.2.3.4. Enable and configure snapping

Click the magnet icon in the QGIS toolbar or go to Project > Snapping Options. Set snapping to “Vertex” and “Segment” for your trace layer, and choose a small snapping tolerance (e.g., 12 px). Snapping helps to ensure that traces precisely abut, i.e. endpoint of one trace is exactly along a segment or on top of a vertex of another trace, to other traces, which is important for accurate topological network analysis.

1.2.3.5. Digitizing new traces

Before starting to digitize traces, you set disable the pop-up that appears for inputting attribute data after each feature is digitized. Go to Settings -> Options -> Map Tools -> Digitizing. Check Suppress attribute form ... to disable the pop-up.

To start digitizing new features, make sure you have selected the trace layer in the Layers tab. Next, put Toggle Editing toggle on. To create a new feature, click on Add Line Feature button. Click on the map to create a vertex, then click again to connect the vertices, and continue digitizing from one end of the lineament or fracture to the other end. If either end of the fracture seems to abut another fracture

1.2.3.6. Modifying existing traces

It is often necessary to modify already digitized features due to reinterpretation and to match them with other digitized fractures. Start editing similar to when creating new traces. Instead of clicking Add Line Feature, click on Vertex Tool. Now when moving your cursor above traces, you should see their vertices highlighted. To modify a single vertex, click on it. You can then move it and the trace will be modified to fit the new vertex. To add a new vertex between two existing vertices, click along the trace somewhere where there is no vertex between the two vertices. To continue a trace, click on the plus-symbol at either end of the trace to start appending vertices.

Note

When editing traces, you might accidentally cause another trace to no longer abut the modified trace. You can avoid this by adding a vertice along the modified trace at the endpoint of fracture abutting the modified trace.

1.3. How to digitize fractures and lineaments using ArcGIS Pro

1.4. Things to keep in mind while digitizing

Avoid unintended intersections
- Do not let more than two lines intersect at a single point. If multiple lines cross at one spot, edit them so only two intersect.
- When two lines are meant to connect, make sure their endpoints are snapped together. Use the “Vertex Tool” to adjust endpoints as needed.
Prevent self-intersections and duplicate lines
- Make sure each line does not cross itself. Use the “Check Geometry Validity” tool (Processing Toolbox > Vector geometry > Check validity) to identify and fix self-intersections.
- Avoid drawing duplicate lines directly on top of each other. If you find duplicates, delete them.
Trace length and target area
- If you have created a target area to control where you are going to digitize, make sure you do not stop your traces at the boundary. Rather, continue them outside the boundary as far as they can be interpreted to continue. Otherwise trace lengths might be improperly samples. Furthermore, the target area you currently have might be extended in the future.

For more detailed instructions regarding the different digitization errors, go to the Validation errors page.

1.5. How to collect metadata for digitized fractures

1.6. Next steps

If you want to validate your data using fractopo, you can do so using the command-line interface (See fractopo), using Python code in a script or a notebook (See Notebook - Fractopo – KB11 Trace Data Validation) or using the validation web interface if you have it available (See Validation).

Similarly, to analyze data using fractopo, you can use the command-line interface, follow a notebook (Notebook - Fractopo – KB11 Fracture Network Analysis) or use the web interface (See Network analysis).