Simple operations with QGIS/SEXTANTE

1. Creating a simple buffer function comparison model

The aim of this simple model is to check if two different functions perform the buffering operation in the same way. The GRASS and QGIS geoalgorithms can be used for this comparison.

a) The first step is to define an input – in this case, it could be any vector layer.

b) The next step is to choose the first command – in this case GRASS v.buffer.distance, then define the buffer size to 50 map units (therefore it would be good to check the coordinate system and map unit beforehand) and nam the output layer as grass_buffer

GRASS_buffer

c) A second buffering operation is done with QGIS Fixed distance buffer tool.  Distance is set again as 50 map units and the output named qgis_buffer

QGIS_buffer

d) After that GRASS v.overlay is used to compare (as difference) these two buffers. The output is named  buffer_overlay_result. It is important to correctly set the parameter Operator to use as xor.

Buffer_overlay

e) How the model looks likeModel_buffer

2. Creating a simple Voronoi diagram comparison model

The same kind of exercise can be made using other geoprocessing tools, like GRASS v.voronoi and QGIS Voronoi polygons tools

voronoi_grass_png

voronoi_qgis_png

voronoi_model_png

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