Digitisation

Digitisation#

Digitization is the process of converting geographic data from maps or images into digital form commonly represented as vector data. During this procedure, spatial information from maps or images is traced, forming points, polylines, or polygons. Proficiency in digitisation stands as a cornerstone for GIS specialists. It empowers them to convert spatial information into a digital format, facilitating more efficient data manipulation compared to traditional methods of interpreting images or paper maps.

../../_images/Digitizsation_concept.drawio.svg — Fig. 53 Digitisation with GIS Concept. Source:#

Digitising in QGIS#

The digitisation of geodata is a useful method to collect and modify your own data. The following subchapter will explain step by step, how to digitize data within QGIS.

Digitisation toolbars#

First, you need to check if the ‘Digitizing Toolbox` is activated. To do that

Click on the View tab in the menu bar and click Toolbars. Check if the Digitizing and Advanced Digitizing toolbox is activated.

Activate Digitizing Toolbox

First, you need to check if the Digitizing Toolbox is activated. To do that

Click on the view tab in the menu bar and click Toolbars. Check if the digitizing and Advanced digitizing toolbox is activated.
A tool box like this should appear on top of the QGIS interface

How to:

The normal digitisation toolbar offers all the necessary functions . However, for everything that goes above just creating new features and deleting features, the advanced digitalization toolbar is needed. The advanced digitalization toolbar allows you to move features, delete parts of features and much more. All functions are listed in the two tables below.

Digitalization Toolbar

Tool	Purpose	Tool	Purpose
	Access to save, rollback or cancel changes in all or selected layers simultaneously		Turn on or off edit status of selected layer(s) based on the active layer status
	Save edits
	Digitize using straight segments		Digitize using curve lines
	Enable freehand digitizing		Digitize polygon of regular shape
	Add new record		Add Feature: Capture Point
	Add Feature: Capture Line		Add Feature: Capture Polygon
	Vertex Tool (All Layers)		Vertex Tool (Current Layer)
	Set whether the vertex editor panel should auto-open		Modify the attributes of all selected features simultaneously
	Delete Selected features from the active layer		Cut Features from the active layer
	Copy selected Features from the active layer		Paste Features into the active layer
	Undo changes in the active layer		Redo changes in active layer

Advanced digitizing Toolbar

Tool	Purpose	Tool	Purpose
	Enable Advanced Digitizing Tools
	Move Feature(s)		Copy and Move Feature(s)
	Rotate Feature(s)		Simplify Feature
	Scale Feature
	Add Ring		Add Part
	Fill Ring		Swap direction
	Delete Ring		Delete Part
	Offset Curve		Reshape Features
	Split Parts		Split Features
	Merge Attributes of Selected Features		Merge Selected Features
	Rotate Point Symbols		Offset Point Symbols
	Trim or Extend Feature

Data Creation#

To digitize data for a new dataset you always have to start with creating the dataset before filling it with digitised data.

Layer –> Create Layer -> New GeoPackage Layer or New Shapefile Layer
Click on and navigate to the folder where you want to save the dataset.
File encoding: Make sure this is set to UTF-8
Geometry type: Select the type of feature you want to digitise e.g. points or lines.
Under Additional dimension you should always make sure that you check None. Except if there is the possibility to collect the Z-values (Elevation) as well. But this is mostly not the case.
CRS dropdown: Select the EPSG/CRS you want to set for the new layer. By default, the QGIS selects the project CRS. If you want to change the CRS click on .
Under New Field you can add columns to the new layer.
- Typ: Select the data type the column will have e.g. Text, Whole number, Decimal Number, Date.
Note

The number of options depends on the data format the layer will have. GeoPackage for example offers far more options than the Shapefileformat.
- Click on to add the new column to the Fields List.
Click OK

Attention

An important concept to understand before starting to add data to datasets is, that, whenever you make changes to a dataset other than styling, you have to put it in editing mode. This is done by selecting the layer and clicking on Toggle Editing. Now, the buttons for many functions of the digitisation toolbar are clickable. After you are done manipulating the layer click on Save Layer Edits to save your edits.

Creation of point data#

To digitise points, you need an existing point layer or you need to create a new point layer (see Digital Data Creation above).

Select the point layer you want to add data to in the Layer panel.
Go to the digitisation toolbar and click on Toggle Editing. No the layer is in the editing mode.
Click on .
Left-click on the feature you want to digitise.
Once you click, a window will appear [Your Layer Name]- Feature Attribute. Here you can add the information about this feature to the different columns, based on the attribute table of the layer.
Once you are done with digitalization to save your edits.
Click again on to end the editing mode.

../../_images/point_creation.png — Fig. 56 Point creation.#

Creation of line data#

Creating line data works in the same way as creating point data (see above). First, you have to created a new line layer or use an existing one.

Attention

If you create a new line layer, remember to change the geometry type into lines.

Select the line layer you want to add data to in the Layer panel
Go to the digitisation toolbar and click on. Now, the layer is in the editing mode.
Click on .
To digitalize line features, click along the line. When you are done, right-click on the last point of the line to finish the feature.
A window will appear [Your Layer Name]- Feature Attribute. Here you can add the information about this feature to the different columns, based on the attribute table of the layer.
Once you are done with digitisation, click on Save Layer Edits to save your edits.
Click on Toggle Editing again to end the editing mode.

Creation of polygon data#

The creation of polygon layers works in the same way as for point and line data.

Select the polygon layer you want to add data to in the Layer panel.
Go to the digitisation toolbar and click on. Now, the layer is in the editing mode.
Click on .
To digitise polygon features, left-click around the area you want do digitize. When you are done, right-click on the last point of the area to finish the feature.
A window will appear [Your Layer Name]- Feature Attribute. Here you can add the information about this feature to the different columns, based on the attribute table of the layer.
Once you are done with digitisation to save your edits.
Click on Toggle Editing again to end the editing mode.

Data Editing#

In some cases, you might want to modify or correct vector datasets because of inaccuracies. Editing vector data is done with the same toolbar as for the creation of vector data. This chapter will cover the change of geometries of data, not covering changing data in the attribute table.

For all cases…

You have to select the layer you want to edit.
Click on to activate editing mode.
Do your changes.
Save your edit by clicking on .
Click again on to end the editing mode.

Tip

You can use the & buttons to reverse changes easily.

Note that this is only possible before you save the changes.

Deleting features#

Select the line layer you want to add data to in the Layer panel.
Go to the digitisation toolbar and click on Toggle Editing.
Click on and select the feature you want to delete (see wiki).
Once you have selected the features, click on to delete the feature.
Once you are done with editing, click on to save your edits.
Click again on to end the editing mode.

Moving features#

There are multiple methods to move features. Here we show the method that works for point, line and polygon features the same. For that you need the Advanced Digitisation Toolbox.

Select the line layer you want to add data to in the Layer panel.
Go to the digitisation toolbar and click on.
Click on and click on the feature you want to move. Then, click on the location where you want to move the feature.
Once you are done with editing, click on to save your edits.
Click on again to end the editing mode.

Modifying geometries#

Select the line layer you want to add data to in the Layer panel.
Navigate to the digitisation toolbar and click on.
Click on .
Now you can now move every vertex (corner) of a feature. Click on the vertex/corner you want to move and then click on the location where you want to move the vertex to.
Once you are done with editing click on to save your edits.
Click on again to end the editing mode.

Adding a ring to a polygon feature#

A ring in QGIS is a part inside a polygon that is not part of the polygon. Image a polygon representing a lake. The ring is an island in the lake. For a better understanding, watch the video below.

Select the line layer you want to add data to in the Layer panel.
Go to the digitisation toolbar and click on.
Click on .
Create a ring by clicking the area you want to exclude. Right-click to close the ring.
Once you are done with editing, click on to save your edits.
Click on again to end the editing mode.

Digitisation Errors in QGIS#

Positional errors are inevitable when data are manually digitised. The most common examples include undershooting and overshooting. When your coordinates do not connect as they should, and overshooting, when the lines go past where they should. Often these errors are not visible unless you zoom in quite a bit on the coordinates. Setting a fuzzy tolerance (snapping tolerance) is used to reduce undershoots and overshoots. The snapping tolerance is the minimum tolerated distance between nodes, lines and/or vertices.

../../_images/Digitization_Errors.PNG — Fig. 57 Digitising Errors in QGIS. Source: SpatialPost.#