How to digitize geological fractures and lineaments

Introduction

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

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.

Setting up your project

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 project directory (brittle_course_fracture_digitization_2026) for data. Make a data directory with 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_digitzation_2026.qgz
└── data
    ├── raster
    └── vector

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 TODO

The topological editing tool needs to be added to the toolbar for easy access. TODO

Adding data to the project

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 in QGIS go to TODO

Check that the added raster data uses the same CRS as the project.

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.

Create a new polygon vector layer for the target area by TODO

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.

Digitizing fracture and lineament traces in QGIS

1. 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 tolerance (e.g., TODO).

  • This helps ensure that line endpoints connect precisely, which is important for accurate network analysis, i.e., determining which trace abuts another and which crosscuts.

2. 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.

3. 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.

4. 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.

How to digitize fractures and lineaments using ArcGIS Pro

How to collect metadata for digitized fractures