25 SPATIAL ANALYSIS (2): Buffer & Proximity Analysis

Dr Seema Mehra Parihar

    Learning objective

In this chapter you will learn about the following:

1. Concept of spatial analysis;
2. Understand various type of spatial analysis in Geographical world; and
3. Detail description of Proximity analysis, Buffer zonation, and Buffer analysis.

1. Introduction.

In the continuously progressing world, Geographical information system is playing a key role in understanding and accessing the information from real world. It is not only providing the image and visualisation of real world but also helps in planning the future prospects, glamorous, aesthetic and useful world. GIS as a tool allow users to perform many operations like interactive queries, edit data in maps, analyse spatial information and application. Spatial analysis in GIS is a rapidly changing and useful tool for analysing features and attribute of real world. Spatial analysis packages include variety of information like network analysis (road link, railway, stream network, chain of attribute etc.) proximity analysis (distance, neighbourhood functions), Buffering process (point, line and polygon), terrain analysis (slope, triangular irregular network model, visibility, aspect, surface curvature etc.), watershed analysis ( flow direction, basin delineation, stream order, drainage density etc.). Geographical analysis allows the study of real-world processes by techniques and applying models. These models illustrates various trends in the geographical data and thus make new information available and subsequently enhance our understanding of the real world.

2. Concept of spatial analysis

Spatial analysis is related with geographical analysis that explain patterns of human behaviour and its communication in geographical space in terms of performing locational analysis (mathematics and geometry and mapping). Nearest neighbour hood analysis and Theissen polygons are two readily used examples of this. There are many model grounded in geography and micro-economics that are useful and predict the spatial patterns e.g., the growth of networks and urban systems, movements of goods and services, interpolation of rainfall, a number of preconditions such as the isotropic plain, movement minimization, and profit maximization etc. It is based on the theory that man is responsible for the information about the development of the landscape and its attribute.

Figure 1: Spatial expression of different features near the industry.

Spatial analysis comprises variety of formal techniques that study spatial entities with reference of topological, geometric, or geographic properties. These technique applies in attribute or feature structures at the human scale, particularly in the analysis of geographic data. So the spatial analysis is used to produce the additional locational or geographical information in order to enhance the spatial expressions (Figure 1), structures and relationship between the real world representation and their attribute .

“There is a strong link between humans and their environment and spatial analysis techniques and methods help to incorporate spatial elements into studies, so that we develop a clearer picture of human/environment link, McGregor (1999)”.

Figure 2: Relationship between human-environment and spatial anlysis.

“GIS data describe answers the question “where?” GIS data analysis answers the question “why is it there? (Clarke, 1997)”. “GIS Analysis is the process of deriving information from one or more layers of spatial data, (DeMers, 1997)”. It can involve multiple steps and processes and it is perhaps the most important capability of a GIS.

3. Type of Spatial Analysis in Geographical world

In GIS world, there are variety of spatial analytical tools that enhances the information and understanding of real world by various analytical procedure. Volusia (1997) and Chou (1997) include two type of operations in spatial analysis.

1) Single layer operations are correspond to attribute queries, spatial queries, and alternations of data that operate on a single data layer.

2) Multiple-layer operations are useful for manipulation of spatial data on multiple data layers.

a. Spatial modelling involves the construction of explanatory and predictive models for statistical testing.

b. Point pattern analysis deals with the examination and evaluation of spatial patterns and the processes of point features.

c. Network analysis, designed specifically for line features organized in connected networks, typically applies to transportation problems and location analysis.

d. Surface analysis deals with the spatial distribution of surface information in terms of a three-dimensional structure.

e. Grid analysis involves the processing of spatial data in a spatial, regularly spaced form, spatial overlay, Boundary analysis, Proximity analysis, and Buffer analysis.

Fig 3: Thematic Layers

The spatial analysis models are based on both model (raster model and vector Model). Geographical information functions on these data model. The raster data model supports a wide variety of analysis methods including: Neighbourhood operations, Connectivity functions, Query, Classification and Measurement functions.

The vector-based analysis are little different from raster GIS. The operations largely deal with objects and measurements – as area has to be calculated from coordinates of objects instead of counting cells as in raster GIS. According to DeMers (1997), vector system most of the analyses are done by direct search in the database Compared with raster GIS, some of the vector GIS operations are more accurate (e.g. estimates of area based on polygons are more accurate than counts of pixels; and estimates of perimeter of polygon are more accurate than counting pixel boundaries on the edge of a zone), some are slower (e.g. overlaying layers and finding buffers) and some are faster (e.g. finding path through road network).

The idea of a vector representation is that we represent objects in the real world by points, lines and polygons in the GIS. By using a traditional kind of 2-D co-ordinate system, with a ‘continuous’ number system for the co-ordinate values, we can obtain a very high resolution very efficiently. Two numbers, x and y, and we have the location of a point. And we can store that position to zillionths of a millimetre in a database, if we wish.

4. Common GIS Analysis Functions

The list of common GIS analysis include :

• Measurements – Length, Perimeter and Areas
• Proximity Analysis
•  Reclassification
• Neighbourhood Functions
• Overlay Analysis
• Terrain Analysis –Statistical Surface / Spatial Interpolation
• Network Analysis
• And Combinations of above Analysis Functions and Many More…….

    Measurement Functions

GIS makes spatial measurements easy to perform. Spatial measurements can be the distance between two points, the area of a polygon or the length of a line or boundary. Calculations can be of a simple nature, such as measuring areas on one map, or more complex, such as measuring overlapping areas on two or more maps.

• How do we measure Euclidean or Straight-line distance between two points? Suppose we have to measure straight road that connects two village (A and B) so you can find out through below mentioned formula.

       Pythagoras theorem: AB2= BC2 + CB2

• How do we measure Area of a Right-angled Triangle? If we have to measure the perimeters of connecting node of the area. We can find the result by below mentioned formula.

      Area ABC = ½* AB* BC.

Vector GIS Measurements

   Raster GIS Measurements

Manhattan Distance: This is the distance along raster cell sides from one point to another.

Proximity Distance: Concentric Equidistant zones around the starting point.

Pythagorean Distance                 Manhattan Distance              Proximity Distance

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Figure 4: Vector Measurements

4: Proximity Analysis

The First Law of Geography as stated by Tobler is “everything is related to everything else but near things are more related than distant things”. Thus, it is very common in spatial process much analysis utilises information about the distance between objects or features in GIS. There are many way to assess the proximity between two locations. Spread function is another way of representation of Proximity Concept where the distance surface grows outwards from the feature of interest. e.g., Raster GIS model to show the spread of lava from Volcanic Vent, industrial expansion outside the city node etc.

Measures of accessibility, transportation distance, time and cost etc. are sometimes more suitable than ‘raw’ distance in geographical analysis. This type of analysis can be measure through Euclidean distance, a simple method or tool for measures of proximity. One of the very popular and widely used single layer operations in GIS includes “the creation of new information based on the measurement of Euclidean distance”. This operation is buffering analysis or zonation and it results in the creation of surrounding coverage of feature or around a geographical entity or set of entities.

Through this, we can find out the Village location nearer by road (figure 5). We can also analyse coverage of flooded area from stream by buffer zonation, distance of faulting landscape from plane (figure 6) and so on. This type of analysis can be done through the proximity distance. We can also analyse coverage of flooded area from stream by buffer zonation, distance of faulting landscape from plane etc.

Figure: 5.  Location of two village and Euclidean distance from the road.

Figure 6: Faulting landscape coverage in interpolated region.

Proximity analysis is one way of analysing locations of features by measuring the distance between them and other features in the area. The distance between point A and point B may be measured as a straight line or by following a networked path, such as a street network. As for instance, in a site selection scenario where a prospect is interested in building a manufacturing plant in the beach area, an important consideration might be distance from the interstates and the airport. A GIS user can simply click on the point locations representing the site and the interstate exit ramp or airport to obtain an approximate distance measure. Once the distances are determined, other pertinent information such as water and sewer availability, price per acre, and availability of labour can be analysed from the database.

5. What is Buffering?

“The process of creating areas of calculated distance from a point, line or area object, Michael DeMers (2005)”.

Buffer methods are very common used in desktop GIS. Buffering is a process of determining a distance from a separate object (a point, line or area). Buffer is the creation of a zone of a specific width around point, line and area (Figure 7). It is also referred as coverage feature. Buffering sometimes termed as a proximity process. For example you can create a buffer of circular zonation encompassing 1km of your house to find the nearest retailer store. Buffer is very helpful in determining potential impacts of floods or any type of hazard or disasters and its impact nearby. This analysis is also used to identify the neighbourhood of ethnic group. Buffering can be helpful in cluster households of ethnic group.

Figure 7: Buffer Zones

“The area outside the buffer zone is sometimes referred to as a skeleton zone, (Laurini and Thompson, 1991)”. Buffering is a local operator in that it operates on each object individually. In fact it operates on individual elements – its’ nodes and line segments of an object.

Types of Buffering

Figure 8: Buffer around point, line and polygon

Points: A point is the basic element of resolution in any GIS system. In the point buffering, data involves the creation of a circular polygon around point of interest (figure 9 and 10). The radius of circular polygon is termed as the buffer distance. Radius of that circular layer are fixed in all direction with specific unit. If multiple points in the same layer are being buffered, then buffer distances of each point has to be specified. If the value of circular polygon in multiple point are same then buffer zonation can be overlap to each other.

Figure 9: Point Buffers

Figure 10: BufferDistance from Taj Mahal

Lines: line buffering is different from the buffering point data. Because line joins segment from two points ant it can be made up of multiple segments. Line segments are handled independently of each other (figure 11 & 12). Generally, line buffering is very helpful in measure proximate distance from roads and rivers zones. It uses mostly for the utilities and transportation.

Figure 11: Line Buffer around national Highway 21, Himachal Paradesh

Figure 12: Buffer Distance around line Buffer

Polygons: Buffering of polygonal surfaces uses most of the similar concepts used for line buffering. The significant concern of the polygon surfaces is that it carries buffer only one side of the line that defines the polygon. In polygon buffering two options are available- the first one is outside polygon which surrounds or contains the polygonal surface under consideration and another one is inside polygon that is contained inside the polygonal surface under consideration (figure 13 & 14).

Figure 13: Outside Buffer in a Polygon Buffering

Figure 14 : Agra District Buffer Zone of 3 kms

Multiple buffer zones: A feature can also have more than one buffer zone. A nuclear power plant may be buffered with distances of 10, 15, 25 and 30 km, thus forming multiple rings around the plant as part of an evacuation plan. An example of multiple buffer zone can be seen in figure 15.

Figure 15: Multiple Buffer Zone

Buffering a point feature with distances of 10, 15, 25 and 30 km.

    Variation or Differences in Buffering

The value of buffer distance is vary as per the requirement of proximity. The value depends on the analysis of different feature or objects. For example, the value of stream buffer sizes may vary. It depends on the intensity of adjacent land use. A map feature may have more than one buffer zone termed as ring analysis. For example, the range of mobile network visibility within a distance of 5, 10 and 15 kilometres. Similarly, a nuclear power plant may be buffered with a distances of 5, 10, 15, and 20 kilometres, thus forming multiple rings around the plant.

Boundaries of buffer zones may remain unbroken so that each buffer zone is a separate polygon. These boundaries may be dissolved so that there are no overlapped areas between buffer zones. Data acquisition and manipulation techniques such as map digitization, photo interpretation, and map transformation often introduce errors in the data leading to problems in Buffering.

Application of Buffers

Let us now understand the usefulness of the concept of buffering. A buffer zone is very essential for planning or regulatory purposes, it often treated as a protection zone or the neutral zone. It is also very useful in resource allocation. “Anderson (1987) also made use of buffering to demonstrate the potential uses of the Swedish Land Data Bank. He used a series of buffers of expanding radii to examine the demographics of the population within a reach of planned bus stops, Nigel Trodd (1990)”.

Urban or city Planning: Urban expansion always occur on the bank of river or accessibility of the area. Road network and railway is key network of city for accessing the services. So buffer can be helpful in future planning of urban growth, connectivity and area can be included in urban expansion.

Buffering is also important for maintaining and managing rule and regulation in retail store and area planning. For example Government instructed that there will not be any liquor shops or smoking facility within 500 meters of temple and college campus. City planner can inquire or check through area buffering how many location inside the boundary of 500 meters.

Social Planning: A buffer zone may be treated as a neutral zone for maintain law and order in region. For example a mob is protesting against the system so police department can create buffer zone of 100 or 200 meters of area or building location for the security purposes or by ensuring that protesting mass do not turn in violence. Buffer zones can also be made for the clustering of settlement on the basis of caste, ethnicity and culture for creating different development clusters.

Site suitability analysis: Buffering of the sites may also play a key role in site suitability analysis because it can show inclusive sites required for various GIS applications. For example, an industrial park may specify that a potential site must be within 2 Kms of a national highway and therefore 2 kms of buffer zones of national highway become the inclusive zones.

As per the government instruction under environment protection act, wildlife protection act or other environmental law prohibited and different regulations, the industrial process, construction, mining etc. within the area of 5 or 10 kms of National park, forest, biosphere reserves etc. so functionality of site can be analysed by creation of buffer zone. “Areas within the 200 feet of streams as riparian zones, forest management plan, Nigel, 2012”. Under the plan, riparian zones are separate from non-riparian areas and are managed differently. “Oil and gas well drilling within 50 metres of roads or highways, within 500 metres from streams, lakes, ponds and reservoirs, national forest policy, Nigel, 2012” are some other examples.

Multiple rings buffering on the other hand ,can be useful in visualising stream network and thus suitable for the analysis of velocity of stream in a particular distance. For example stream networks in 10 meter increases to a distance of 300 meters one may be the composition and pattern of woody vegetation as a function of distance from the stream network. Local buildings mainly houses, roads, agricultural fields, and orchards require buffering for ideal location from the point of view of accessibility are pertinent examples of this.

Environment Planning: Town and urban planning department may set aside land along the edges of streams to reduce the effects of nutrient, sediment, and pesticide runoff, to maintain shade to prevent the rise of stream temperature and to provide shelter for wildlife and aquatic life.

Problems or Limitation of Buffers

    Buffering is very important in planning and policy making not only for the development of region but also for protecting the environment. Sometimes buffering can be barrier due to physicality of earth surface and unit used in buffer creation.

• The unit and value of buffer creation will always be dependent of objective, case and terms.
• Each and every location or site should be used wisely for the buffer creation, because in the buffering the value will differ from case to case otherwise it can create overlaps in mapping.
• During the area buffering it is largely the outside zonation that can be created by most of the existing tools in the GIS environment. And shape of the buffering used as per the requirement.
• The accuracy is sometimes compromised while delivering the output buffering because of the multiple reasons and largely it is dependent on the quality of the availability of the spatial data.

Summary

Spatial analysis in GIS are becoming emerging tool to understand the world and helping in development of nation and future prospect of planned world. Proximity analysis is a very important tool for measuring the Euclidean distance in vector and raster data model and its application in buffering operations in GIS. Buffering on the other hand, is very important as it helps in multipurpose analysis like planning, security purpose, protecting forest and other resources from the exploitation, determining the potential impact of floods and other hazards or disasters and most important accessibility. These days even institutional intelligence uses buffer operations very often. One of the interesting fields of research is predictive and adaptive buffering techniques. Since any improvement in these techniques can have a drastic impact on spatial query response times, much effort has to be put in this field in the coming future and more research need to be incorporated in this field..

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