overlay and buffer analysis

8/13/2019 Overlay and Buffer Analysis

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BUFFER ANALYSIS & OVERLAYANALYSIS

By,

ABHILASH RAJENDRA


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Buffer - a zone - specified width - point, line, polygon

The resulting buffer is anew polygon, which can beused in queries to

determine which entitiesoccur either within oroutside the defined bufferzone.


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Two types of buffers:

1. Constant width buffers, and2. Variable width buffers.

Both types can be generated for a set ofcoverage features based on each features

attribute values.

Buffer primitives are classified as point,

line, and polygon buffering operations.


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Buffering points:

A point is the basic unit of resolution in anyGIS system.

Buffering point data involves the creation of acircular polygon about the point of interest.

The radius of this circular polygon is called thebuffer distance.


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If one is buffering multiple points in thesame layer, then the buffering algorithmscheck for overlaps in each points buffer and

remove the overlapping sections.

Figure 1. Buffering Multiple Points


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If multiple point buffers intersect or overlap, asillustrated in Figure 1, then the system takes allthe overlapping polygons and combines them

into one or more polygons that represent alayer.

Figure 2. Removal of Overlaps


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Buffering Lines:

Buffering lines is a little more complicatedthan buffering point data.

This is mainly due to the fact that lines can bemade up of multiple segments. Line segments

are handled independently of each other.

Figure 3. Line Buffering


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Buffering Polygons:

Buffering of polygonal surfaces uses most of

the same concepts used for line buffering. The

only significant change is that the polygon

buffer is created on only one side of the line

that defines the polygon.

In polygon buffering two options are

available, namely an outside polygon which

surrounds or contains the polygonal surfaceunder consideration or an inside polygon that

is contained inside the polygonal surface

under consideration.


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Figure 4. Polygon Buffering


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Overlay Analysis:

Overlay analysis is an technique of deriving new

information from two or more layers of data covering thesame area.


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For instance, if we have a layer of land use and alayer of soil fertility, we can derive from the twolayers the percentage of agricultural land

developed over fertile soil. In the land use layer of our example, we are

interested in the agricultural lands. So we groupall other land use types into non-agricultural

lands. Similarly, we identify fertile and non-fertilesoils in the soil fertility layer.


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After we have reduced each layer into twoclassifications, we can perform intersection,union, subtraction, and other logicaloperations on them.

Intersection Union Subtraction


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Vector Overlay:

In vector overlay, the input layers and the

output composite layer are topologicallystructured. Hence an overlay operationgenerates new nodes, arcs, and polygons.

Overlay analysis using vector data involvesquite a few calculations like, cut arcs from thetwo layers to form new ones, form new

polygons out of the new arcs, and so on.


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Vector overlay can be performed ondifferent types of map features: viz.,

1. Polygon-on-polygon overlay

2. Line-in-polygon overlay3. Point-on-polygon overlay

The process of overlay will depend uponthe modeling approach the user needs.


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Raster Overlay: It is a relatively straightforward operation

and often many data sets can be combinedand displayed at once.

This involves no geometrical calculationsand no creation of new polygons.


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Overlay Methods "Piercing Needle" Approach:

Point by point overlay analysis is referred

to as the"piercing needle"approach. Whenmultiple themes of data -- such as forests,water, soils, elevation, etc. -- are in relativeposition with one another, inserting a "digitalpin" through the stack of overlaid data, allows

questions to be answered concerningeverythingoccurring at a particular location.


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However, the point by point view of the world

limits focus to a particular point which precludes

wide-area landscape analysis.


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Region Wide: "Cookie Cutter Approach"

The region wide, or "cookie cutter,"approach to overlay analysis allows naturalfeatures, such as forest stand boundaries orsoil polygons, to become the spatial area(s)which will be analyzed on another map.


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CONCLUSION:

Overlaying does not involve only areas. Wesometimes overlay areas upon lines to findout, for instance, how many countries ariver flows through , areas upon points for

example would be to determine how manytrees there are within a park

A buffer is an area defined by the boundingregion determined by a set of points at aspecified maximum distance from all nodesalong segments of an object.


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Thank You!!!

overlay and buffer analysis

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