What is "Geoprocessing"?
Have a look at A Swetha's Geoprocessing in QGIS slides.
In QGIS "geoprocessing" refers to a set of standard spatial operations that involve combining features in different layers in different ways.
| Tool | Objects | Description | Data | Notes | Example Application |
|---|---|---|---|---|---|
| Convex Hull | Points, lines or polygon layer | Create smallest bounding convex polygon around points, lines, or polygon features | Use SEXTANTE Plugin | Convert elevation points to contour lines; convert animal sitings or tracks to range/territory | |
| Buffer | Points, lines or polygon layer | Polygon containing all points within specified distance of a feature | Fields from buffered feature. Data preserved if individual buffers. Lost if dissolved buffers. | Distance units are those of CRS | Identify land within X miles of a river; homes within X meters of a homicide. |
| Intersection | input A, intersect B | New polygon where other polygons overlap | Fields from A and B | Overlapping habitats/ranges or catchment areas; | |
| Symmetric Difference | input A, difference B | Polygons from A and B with overlap removed | Fields from A and B | ||
| Union | input A, union B | ||||
| Clip | input A, clip layer B | Keep part of features in A that are overlapped by B | A | Keep only inland features in a water layer | |
| Difference | input layer A and difference layer B | Polygons from A with overlaps removed | A | Keep only coastal waters in a water layer | |
| Dissolve | A B | Merging census tracts into districts based on a "district name" field | |||
[http://www.gistutor.com/concepts/9-beginner-concept-tutorials/44-gis-convex-hull.html ]here
Buffers
Unions, Intersections, and Differences
Suppose we have two polygon layers. One has rectangular features and one has elliptical features. A small section of the map is shown below.
Intersection produces a new feature with area 1. Data columns come in from both layers.
Union produces a new feature containing the whole area covered by 1, 2, 3
Symmetric difference produces a feature that contains areas 2 and 3.
References
- Harvard. Buffer Workshop
- OSGeo. Buffer with QGIS
- Nicolaides, L. Geo-Processing in QGIS
- MappingAcrossBorders. How To: Create a Buffer (QGIS 1.6) (Video 4:18)
- Swetha A. Geoprocessing in QGIS
- GeoInformation Blog. Geoprocessing in QGIS
Convex Hull
A convex hull is the smallest polygon that bounds a set of features such that a line between any two interior points will always be completely within the polygon.
We might, for example, collect point data on wildlife sitings and want to construct polygons that represent the range of the animal.
In version 1.8 (and perhaps others) the Convex Hull command under Vector » Geoprocessing > Convex Hull(s) does not function correctly. Instead, use the SEXTANTE plug-in. Under QGIS geoalgorithms » Vector geometry tools you will find Convex Hull.
Data Considerations
Ideally, if we create separate polygons by creating a convex hull for each value of a data field (as we did with animal type above), the field on which multiple convex hulls were created would be included in the data. At this writing, the code does not do that. Thus, you might want to go into the attribute table of the convex hull layer and add a column.
Take Geoprocessing I Quiz
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