GEOPROCESSING IN QGIS

GEOPROCESSING
IN QGIS
By
Swetha A

Introduction
• Quantum GIS (often abbreviated QGIS) is a cross-platform
free and open source desktop geographic information systems
(GIS) application that provides data viewing, editing, and
analysis capabilities.
• Geoprocessing is a GIS operation used to manipulate spatial
data. A typical geoprocessing operation takes an input dataset,
performs an operation on that dataset, and returns the result of
the operation as an output dataset

Geoprocessing Tools In QGIS
• Convex Hull
• Buffer
• Insert
• Union
• Symmetrical Difference
• Clip
• Difference
• Dissolve

• Convex Hulls - creates the smallest possible convex polygon enclosing a group
of objects
• Buffers - creates an equal zone around specific features at a specified distance
• Intersect - creates new layer based on the area of overlap of two layers
• Union - melds two layers together into one while preserving features and
attributes of both
• Symmetrical Difference - creates new layer based on areas of two layers that
do not overlap
• Clip - cuts a layer based on the boundaries of another layer
• Difference - subtracts areas of one layer based on the overlap of another layer
• Dissolve - merges features within a single layer based on common attributes in
the attribute table

Convex Hulls
• Convex Hulls - creates the smallest possible convex polygon enclosing a
group of objects.

Output Layer Of Convex Hull Tool

BUFFER
Construction of proximity boundary forming polygons at a distance
specified by the user called as buffer zones
 Features:
• Distance is used as an input measure for constructing buffers
• Buffers can be generated for any of the spatial features (point,
line or polygon)
• The output coverage after buffering is always a polygon without
any features from the input theme
• In case of point or line feature buffer generation, if the distance
between two features is less than twice the search radius, output
polygons will overlap
• In buffer generation search radius can be based on feature
attributes which is called weighted buffer

Polygon Polygon-Buffer
Line Line-Buffer
Point Point-Buffer
Buffer for different
features

UNION
It is a geometric combination of two or more coverages. Union operation
creates a new coverage by overlaying two polygon coverages.
 Features:
• Union takes place between two polygon coverages
• The features falling in the combined region of both the two input
coverages are selected for union operation
• The attribute table of the combined coverage will have the attributes
of both the input coverages
• The number of polygons in the combined coverage is more than in
the input and analysis coverage.

INTERSECT
Performs the intersect operation of two coverages of which one is
called the input coverage and the coverage used for intersecting is
called the analysis coverage and the coverage obtained after analysis
operation is called the resultant/combined coverage.
 Features:
• The input coverage can have any kind of features.
• The analysis coverage should have polygon topology
• The features within the common area of both coverages is
preserved.

Output Layer Of
Intersect Tool

CLIP
• This overlay function is performed to extract features in
which the area of extraction is decided from the clip
cover and the features to be extracted are from the input
coverage.
• Features:
• Input coverage can be points, lines or polygons
• Clip cover is always polygon coverage
• The attributes of the resultant coverage is mainly
from input and not from analysis coverage.
– e.g. Extraction of datasets from various coverages
pertaining to an administrative boundary

Diff Between Clip And Intersect
The main difference will be in the attributes of the results.
When using Clip only the input feature’s attributes will be in
the output (none from the clip feature), where if you used
Intersect the attributes form all features used will be in the
output.

SYMMETRICAL DIFFERENCE
• Symmetrical Difference creates new
layer based on areas of two layers that
do not overlap.
• The Symmetrical Difference tool is
used to perform overlay analysis on
feature classes. This tool creates a
feature class from those features or
portions of features that are not
common to any of the other inputs.

Working with
Symmetrical Difference
Tool

Output Layer Of
Symmetrical
Difference Tool

DIFFERENCE
• creates a new feature based on the area of the input
layer that isn’t overlapped by the clipping layer.

Difference Between "Difference" And
"Symmetrical Difference"
• Symmetrical difference of two sets contains the union
of the two sets except the intersection of the two sets.
• The difference of two sets contains the elements of
the first set except those belonging to the second set
too.
DifferenceSymmetrical difference

Output Layer Of
Difference Tool

DISSOLVE
• Creates a new set of features from a given set of features based on the
attribute specified for the purpose.
• Input theme is a spatially combined polygon theme and decision is based
on an attribute value.
• Neighboring features having the same attribute are searched and if found
the polygon boundary dropped.
• New spatial geometry is formed.
• The number of polygons formed are reduced

Conclusion
• Geoprocessing tools are mainly helpful in
managing data.
• Solving queries.
• Obtaining additional information.
• Modifying data and managing according to
situation.

Reference
• https://www.google.co.in/
• Quantum GIS Desktop - How do I do that in
QGIS - QGIS Issue Tracking
• QGIS Workshop – Analyze
• Quantum GIS (QGIS) Tutorials: Tutorial:
Performing Spatial Queries in QGIS
• Part 3 - Geographic Analysis

GEOPROCESSING IN QGIS

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