A lot of of GIS work involves editing polygons and polylines as well as calculating the size of any overlapping features. For example when I provided GIS support to a local authority’s planning team, I was regularly asked to calculate what proportion of proposed developments overlapped constraints such as flood zones.
In this tutorial, I first look at the re-shaping features on the Advanced Digitising Toolbar. Then I will go through geo-processing tools such as buffer, union, intersect etc.
Advanced Digitising Tools
From left to right, the tools are:
- Simplify feature
- Add ring
- Add part
- Delete ring
- Delete part
- Reshape features
- Merge selected features
- Merge attributes of selected features
To use any of the tools,
- Begin an edit session, by right click on the layer’s name in the Layer Panel and select toggle editing.
- Select the feature that you wish to edit
- Select the tool
This is the polygon at the start of the edit session, I will show a screen shot of each result:
is used to subtract a new internal region from the existing polygon. Select the tool and then draw the desired internal polygon:-
deletes the selected internal region. Select the tool, and then click on the internal region’s nodes.
adds a new external region to the existing region. Select the tool, then draw the new region, ensure it snaps to the existing region:-
deletes the selected region. Select the tool, and then click on the region’s nodes.
draws a temporary region which is then subtracted from the existing region:
draws a line through the region. The region is then split along the line
Merge Features – merges two or more selected features. Note Ctrl+click is used to select multiple features
A dialogue box allows you to choose which attributes to keep, discard. It is also possible to carry out mathematical operations such as sum, maximum on numeric fields and to concatenate text fields:-