Introduction• Quantum GIS (often abbreviated QGIS) is a cross-platformfree and open source desktop geographic information systems(GIS) application that provides data viewing, editing, andanalysis capabilities.• Geoprocessing is a GIS operation used to manipulate spatialdata. A typical geoprocessing operation takes an input dataset,performs an operation on that dataset, and returns the result ofthe operation as an output dataset
• Convex Hulls - creates the smallest possible convex polygon enclosing a groupof 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 andattributes of both• Symmetrical Difference - creates new layer based on areas of two layers thatdo 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 inthe attribute table
BUFFERConstruction of proximity boundary forming polygons at a distancespecified 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 withoutany features from the input theme• In case of point or line feature buffer generation, if the distancebetween two features is less than twice the search radius, outputpolygons will overlap• In buffer generation search radius can be based on featureattributes which is called weighted buffer
Polygon Polygon-BufferLine Line-BufferPoint Point-BufferBuffer for differentfeatures
UNIONIt is a geometric combination of two or more coverages. Union operationcreates 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 inputcoverages are selected for union operation• The attribute table of the combined coverage will have the attributesof both the input coverages• The number of polygons in the combined coverage is more than inthe input and analysis coverage.
INTERSECTPerforms the intersect operation of two coverages of which one iscalled the input coverage and the coverage used for intersecting iscalled the analysis coverage and the coverage obtained after analysisoperation 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 ispreserved.
CLIP• This overlay function is performed to extract features inwhich the area of extraction is decided from the clipcover and the features to be extracted are from the inputcoverage.• Features:• Input coverage can be points, lines or polygons• Clip cover is always polygon coverage• The attributes of the resultant coverage is mainlyfrom input and not from analysis coverage.– e.g. Extraction of datasets from various coveragespertaining to an administrative boundary
Diff Between Clip And IntersectThe main difference will be in the attributes of the results.When using Clip only the input feature’s attributes will be inthe output (none from the clip feature), where if you usedIntersect the attributes form all features used will be in theoutput.
SYMMETRICAL DIFFERENCE• Symmetrical Difference creates newlayer based on areas of two layers thatdo not overlap.• The Symmetrical Difference tool isused to perform overlay analysis onfeature classes. This tool creates afeature class from those features orportions of features that are notcommon to any of the other inputs.
DIFFERENCE• creates a new feature based on the area of the inputlayer that isn’t overlapped by the clipping layer.
Difference Between "Difference" And"Symmetrical Difference"• Symmetrical difference of two sets contains the unionof the two sets except the intersection of the two sets.• The difference of two sets contains the elements ofthe first set except those belonging to the second settoo.DifferenceSymmetrical difference
DISSOLVE• Creates a new set of features from a given set of features based on theattribute specified for the purpose.• Input theme is a spatially combined polygon theme and decision is basedon an attribute value.• Neighboring features having the same attribute are searched and if foundthe polygon boundary dropped.• New spatial geometry is formed.• The number of polygons formed are reduced
Conclusion• Geoprocessing tools are mainly helpful inmanaging data.• Solving queries.• Obtaining additional information.• Modifying data and managing according tosituation.
Reference• https://www.google.co.in/• Quantum GIS Desktop - How do I do that inQGIS - QGIS Issue Tracking• QGIS Workshop – Analyze• Quantum GIS (QGIS) Tutorials: Tutorial:Performing Spatial Queries in QGIS• Part 3 - Geographic Analysis