GEOPROCESSING

IN QGISBy

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

ADDING FILES TO QGIS

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 hull

Convex Hulls• Convex Hulls - creates the smallest possible convex polygon enclosing a

group of objects.

Working with Convex Hull Tool

Output Layer Of Convex Hull Tool

BUFFER

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

Working with Buffer Tool

Output Layer Of Buffer tool

UNION

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.

Working with Union Tool

Output Layer Of Union Tool

INTERSECT

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.

Working with Intersect Tool

Output Layer Of Intersect Tool

CLIP

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

Working with Clip Tool

Output Layer Of Clip Tool

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• 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

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

Working with Difference Tool

Output Layer Of Difference Tool

DISSOLVE

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

Working with Dissolve Tool

Output Layer Of Dissolve Tool

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

The end any queries?