Draw a Circle Around a Point Feature Gis

Vector Spatial Analysis (Buffers)¶

	Objectives:	Understanding the use of buffering in vector spatial analysis.
	Keywords:	Vector, buffer zone, spatial analysis, buffer distance, dissolve boundary, outward and inward buffer, multiple buffer

Overview¶

Spatial analysis uses spatial information to extract new and additional meaning from GIS information. Unremarkably spatial analysis is carried out using a GIS Awarding. GIS Applications ordinarily have spatial analysis tools for feature statistics (e.g. how many vertices brand up this polyline?) or geoprocessing such every bit feature buffering. The types of spatial assay that are used vary according to subject areas. People working in h2o management and enquiry (hydrology) volition about likely be interested in analysing terrain and modelling water as it moves beyond it. In wildlife direction users are interested in analytical functions that deal with wildlife point locations and their human relationship to the environment. In this topic we will hash out buffering equally an instance of a useful spatial analysis that tin can be carried out with vector data.

Buffering in item¶

Buffering usually creates two areas: one area that is within a specified distance to selected real earth features and the other area that is beyond. The area that is within the specified altitude is called the buffer zone.

A buffer zone is any surface area that serves the purpose of keeping real globe features distant from 1 another. Buffer zones are often ready to protect the environs, protect residential and commercial zones from industrial accidents or natural disasters, or to forbid violence. Common types of buffer zones may be greenbelts between residential and commercial areas, border zones between countries (see figure_buffer_zone), noise protection zones around airports, or pollution protection zones forth rivers.

Figure Buffer Zone 1:

The edge between the United states of america and Mexico is separated past a buffer zone. (Photograph taken by SGT Jim Greenhill 2006).

In a GIS Application, buffer zones are always represented as vector polygons enclosing other polygon, line or point features (see figure_point_buffer, figure_line_buffer, ).

Figure Point Buffer one:

A buffer zone around vector points.

Figure Line Buffer i:

A buffer zone around vector polylines.

Figure Polygon Buffer 1:

A buffer zone effectually vector polylines.

Variations in buffering¶

In that location are several variations in buffering. The buffer distance or buffer size can vary co-ordinate to numerical values provided in the vector layer attribute table for each characteristic. The numerical values have to be defined in map units according to the Coordinate Reference System (CRS) used with the information. For instance, the width of a buffer zone along the banks of a river tin vary depending on the intensity of the adjacent land use. For intensive tillage the buffer altitude may be bigger than for organic farming (run across Figure figure_variable_buffer and Table table_buffer_attributes).

Figure Variable Buffer i:

Buffering rivers with different buffer distances.

River	Adjacent land use	Buffer altitude (meters)
Breede River	Intensive vegetable tillage	100
Komati	Intensive cotton fiber cultivation	150
Oranje	Organic farming	50
Telle river	Organic farming	50

Table Buffer Attributes 1: Attribute table with different buffer distances to rivers based on information about the side by side land use.

Buffers effectually polyline features, such as rivers or roads, do not have to be on both sides of the lines. They can be on either the left side or the right side of the line feature. In these cases the left or right side is determined by the direction from the starting point to the stop point of line during digitising.

Multiple buffer zones¶

A feature tin can also have more one buffer zone. A nuclear power plant may be buffered with distances of 10, 15, 25 and xxx km, thus forming multiple rings around the plant every bit office of an evacuation program (see figure_multiple_buffers).

Figure Multiple Buffers i:

Buffering a betoken feature with distances of x, 15, 25 and 30 km.

Buffering with intact or dissolved boundaries¶

Buffer zones oftentimes accept dissolved boundaries and so that there are no overlapping areas between the buffer zones. In some cases though, it may also be useful for boundaries of buffer zones to remain intact, so that each buffer zone is a divide polygon and you tin can identify the overlapping areas (see Effigy figure_buffer_dissolve).

Figure Dissolve Buffers 1:

Buffer zones with dissolved (left) and with intact boundaries (right) showing overlapping areas.

Buffering outward and inward¶

Buffer zones around polygon features are usually extended outward from a polygon boundary but it is also possible to create a buffer zone inward from a polygon boundary. Say, for example, the Section of Tourism wants to programme a new road around Robben Island and ecology laws require that the road is at least 200 meters inwards from the coast line. They could use an inwards buffer to find the 200 yard line inland and so plan their route not to go across that line.

Common problems / things to exist enlightened of¶

Most GIS Applications offering buffer creation as an analysis tool, but the options for creating buffers can vary. For example, not all GIS Applications allow y'all to buffer on either the left side or the correct side of a line characteristic, to dissolve the boundaries of buffer zones or to buffer inward from a polygon boundary.

A buffer altitude always has to be defined equally a whole number (integer) or a decimal number (floating point value). This value is defined in map units (meters, feet, decimal degrees) according to the Coordinate Reference System (CRS) of the vector layer.

What take we learned?¶

Permit'south wrap up what nosotros covered in this worksheet:

• Buffer zones draw areas around existent world features.
• Buffer zones are ever vector polygons.
• A characteristic tin can have multiple buffer zones.
• The size of a buffer zone is defined by a buffer distance.
• A buffer distance has to be an integer or floating bespeak value.
• A buffer altitude tin can be different for each feature within a vector layer.
• Polygons can be buffered in or outward from the polygon boundary.
• Buffer zones can exist created with intact or dissolved boundaries.
• Also buffering, a GIS usually provides a diversity of vector analysis tools to solve spatial tasks.

Now you endeavour!¶

Here are some ideas for you to try with your learners:

• Because of dramatic traffic increment, the town planners want to widen the main route and add a second lane. Create a buffer effectually the route to find properties that fall within the buffer zone (see figure_buffer_road).
• For controlling protesting groups, the police want to establish a neutral zone to keep protesters at to the lowest degree 100 meters from a building. Create a buffer around a building and colour it and so that event planners tin can see where the buffer area is.
• A truck manufactory plans to expand. The siting criteria stipulate that a potential site must be within i km of a heavy-duty road. Create a buffer along a chief road so that you can see where potential sites are.
• Imagine that the city wants to introduce a police force stipulating that no canteen stores may be within a chiliad meter buffer zone of a schoolhouse or a church. Create a 1 km buffer around your school and and then go and meet if at that place would be whatsoever bottle stores too close to your school.

Figure Buffer Road ane:

Buffer zone (green) around a roads map (brown). You tin can see which houses autumn within the buffer zone, so now you could contact the owner and talk to him well-nigh the state of affairs.

Something to think about¶

If you don't have a calculator available, y'all can utilise a toposheet and a compass to create buffer zones around buildings. Make minor pencil marks at equal altitude all along your characteristic using the compass, then connect the marks using a ruler!

Further reading¶

Books:

• Galati, Stephen R. (2006). Geographic Information Systems Demystified. Artech Business firm Inc. ISBN: 158053533X
• Chang, Kang-Tsung (2006). Introduction to Geographic Information Systems. 3rd Edition. McGraw Hill. ISBN: 0070658986
• DeMers, Michael Due north. (2005). Fundamentals of Geographic Information Systems. 3rd Edition. Wiley. ISBN: 9814126195

Websites:

• http://www.manifold.net/doc/transform_border_buffers.htm

The QGIS User Guide besides has more detailed information on analysing vector information in QGIS.

What's side by side?¶

In the section that follows we will take a closer look at interpolation equally an example of spatial analysis yous tin do with raster data.

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Source: https://docs.qgis.org/2.8/en/docs/gentle_gis_introduction/vector_spatial_analysis_buffers.html

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