Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)...

Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)

Spatial Data Analysis

WMO/FAO Training Workshop on GIS and Remote Sensing Applications in Agricultural

Meteorology for the SADC

Thelma A. CincoThelma A. CincoSenior Weather Specialist ,PAGASA Senior Weather Specialist ,PAGASA

Resource Person Resource PersonPhilippinesPhilippines


Spatial analysis

Spatial analysis is the vital part of GIS. Spatial analysis in GIS involves three types of operations• attribute query (also known as non-spatial), • spatial query• and generation of new data sets from the

original databases.


Spatial Data Analysis

• Representation of reality

• Purpose is to understand, describe, predict the real world scenarios

• Gives a simplified , manageable view of the real world


Attribute Query

ArcView’s Query Builder([State name] = “California” or “New York”)([City name] = “San*”)New SetAdd to SetSelect from Set

Command line Query (Arc/Info)find in states where state_name = ‘California’<1 record in result>calculate in states population_density = population / area<50 records in result>restrict in states where population_density > 1000<20 records selected in result>>


Attribute Query Using Boolean Logic– Data retrieval is done by applying the rules of

Boolean logic to operate on the attributes.– Boolean algebra uses the operators AND, OR,

XOR and NOT to see whether a particular condition is true or not.

– Ex. TYPE = ‘ASPHALT’ AND LENGTH = 4000 AND LANES = 4

– Simple Boolean logic is often portrayed visually in the form of Venn diagrams


A B A

A A

A A

B

B B

B B

C C

A AND B A NOT B

A OR B A XOR B

(A AND B) OR C A AND (B OR C)

VENN DIAGRAM

A AND B = ResultT T TT F FF T FF F F

A OR B = ResultT T TT F TF T TF F F

A XOR B = ResultT T FT F TF T TF F F


Spatial Search/Query

• Overlay is a spatial retrieval operation that is equivalent to an attribute join.

• Buffering is a spatial retrieval around points, lines, or areas based on distance.


Find all houses within a certain area that have tiled roofs and five bedrooms, then list their characteristics.


Buffering

• can be constructed around a point, line or area. • Buffering algorithm creates a new area enclosing

the buffered object. • The applications of this buffering operations

include, for example, identifying protected zone around lakes and streams, zone of noise pollution around highways, service zone around bus route, or groundwater pollution zone around waste site.



Spatial Overlay

• An operation that merges the features of two coverage layers into a new layer and relationally joins their feature attribute table.

• When overlay occurs, spatial relationships between objects are updated for the new, combined map.

• In some circumstances, the result may be information about relationships (new attributes) for the old maps rather than the creation of new objects.


GIS usage in Spatial AnalysisGIS operational procedure and analytical task that are particularly

useful for spatial analysis• Single layer operations• Multi layer operations/ Topological overlay• Spatial modeling• Geometric modeling

• Calculating the distance between geographic features• Calculating area, length and perimeter• Geometric buffers.

• Point pattern analysis• Network analysis• Surface analysis• Raster/Grid analysis• Fuzzy Spatial Analysis• Geostatistical Tools for Spatial Analysis


Point pattern analysis

• It deals with the examination and evaluation of spatial patterns and the processes of point features. Distribution of an

endangered species examined in a point pattern analysis


Vector Based Spatial Data Analysis

• There are multi layer operations, which allow combining features from different layers to form a new map and give new information and features that were not present in the individual maps.

• Topological overlays: Selective overlay of polygons, lines and points enables the users to generate a map containing features and attributes of interest, extracted from different themes or layers.


Topological overlays

• Point-in-polygon algorithm overlays point objects on areas and compute "is contained in" relationship.

• The result is a new attribute for each point specifying the polygon it belongs to. Map overlay - point in polygon

Point-in-polygon overlay


Line-on-Polygon Overlay• Line-on-polygon algorithm

overlays line objects on area objects and compute "is contained in" relationship.

• Lines are broken at each area object boundary to form new line segments and new attributes created for each output line specifying the area it belongs to.


Polygon-on-Polygon Overlay• Polygon-on-polygon

algorithm overlay two layers of area objects.

• Boundaries of polygons are broken at each intersection and new areas are created.

• During polygon overlay, many new and smaller polygons may be created, some of which may not represent true spatial variations.


Sliver or spurious polygons


Overlay operation


Network analysis:

• Designed specifically for line features organized in connected networks, typically applies to transportation problems and location analysis such as school bus routing, passenger plotting, walking distance, bus stop optimization, optimum path finding etc.


Surface analysis

• Deals with the spatial distribution of surface information in terms of a three-dimensional structure.

• The distribution of any spatial phenomenon can be displayed in a three dimensional perspective diagram for visual examination.


Grid analysis

• Involves the processing of spatial data in a special, regularly spaced form.

The following illustration shows a grid-based model of fire progression. The darkest cells in the grid represent the area where a fire is currently underway.


Geostatistical Tools For Spatial Analysis

Geostatistics studies spatial variability of regionalized variables: Variables that have an attribute value and a location in a two or three-dimensional space.

• Tools to characterize the spatial variability are:

– Spatial Autocorrelation Function and

– Variogram.


Point interpolation

• Interpolation to a Grid

• Assumption– The influence of one known point over an

unknown point increases as distance between them decreases.


Accuracy of Interpolation

• Depends on accuracy, number and distribution of the known points used in the calculation

• Depends on how accurate the mathematical function used correctly models the phenomenon. As the assumptions of the model are more severely violated, the interpolation results become less accurate.

• No matter which interpolator is selected, the more input points and the greater their distribution, the more reliable the results.


Interpolation Using Thiessen Method

• graphical technique which defines the individual 'regions of influence' around each of a set of points.

• Thiessen polygon boundaries are the perpendicular bisectors of straight lines drawn between two neighboring points


Thiessen Polygon


Interpolation Using Neighborhood Model

• Inverse-Distance theory dictates that:– the value of X > 58

– the value of X < 97

– the value of X is closer to 58 than 97.


Types of “Neighborhood” with IDW

• Nearest n Neighbors– in this example, n = 3

– this method is not effective when there are clusters of points

• Fixed Radius– a radius is selected

– points are selected only if they lie within that fixed radius


Using IDW in Spatial Analyst


Interpolation Using the Spline Method

• The Spline interpolator fits a minimum-curvature surface through input points– “Rubber sheet fit”

• Uses a piecewise polynomial to provide a series of patches resulting in a surface that has continuous first and second derivatives



• output data structure is points on a raster– note that maxima and minima do not necessarily occur at

the data points

• is a local interpolator– can be exact or used to smooth surfaces

– computing load is moderate

• best for very smooth surfaces– poor for surfaces which show marked fluctuations, this

can cause wild oscillations in the spline


Using the Spline Method in Spatial Analyst



• Two options:– Regularized: usually produces smoother

surfaces; typical values are 0, 0.001, 0.01, 0.1, and 0.5.

– Tension: higher values entered for the weight parameter results in somewhat coarser surfaces, but surfaces that closely conform the control points; typical values are: 0, 1, 5, and 10.


Interpolation Using Kriging

• Based on regionalized variable theory that assumes that the same pattern of variation can be observed at all locations on the surface.– the basis of this technique is the rate at which the

variance between points changes over space– this is expressed in the variogram which shows how

the average difference between values at points changes with distance between points

• This method produces a statistically optimal surface, but it is very computationally intensive.


Raster Based Spatial Data Analysis

• In raster analysis, geographic units are regularly spaced, and the location of each unit is referenced by row and column positions. Because geographic units are of equal size and identical shape, area adjustment of geographic units is unnecessary and spatial properties of geographic entities are relatively easy to trace.


Advantages of using the Raster Format in Spatial Analysis

• Efficient processing: Because geographic units are regularly spaced with identical spatial properties, multiple layer operations can be processed very efficiently.

• Numerous existing sources: Grids are the common format for numerous sources of spatial information including satellite imagery, scanned aerial photos, and digital elevation models, among others.

• Different feature types organized in the same layer: For instance, the same grid may consist of point features, line features, and area features, as long as different features are assigned different values


Grid Format Disadvantages

• Data redundancy: When data elements are organized in a regularly spaced system, there is a data point at the location of every grid cell, regardless of whether the data element is needed or not.

• Resolution confusion: Gridded data give an unnatural look and unrealistic presentation unless the resolution is sufficiently high. Conversely , spatial resolution dictates spatial properties. For instance, some spatial statistics derived from a distribution may be different, if spatial resolution varies, which is the result of the well-known scale problem.

• Cell value assignment difficulties: Different methods of cell value assignment may result in quite different spatial patterns.


Grid Operations used in Map Algebra

Map Algebra performs following four basic operations:• Local functions: that work on every single cell,• Focal functions: that process the data of each cell

based on the information of a specified neighborhood,

• Zonal functions: that provide operations that work on each group of cells of identical values, and

• Global functions: that work on a cell based on the data of the entire


Reclassification

• Reclassification is to reassign new thematic values or codes to units of spatial feature, which will result in merging polygons.

• A set of "reclassify attributes", "dissolve the boundaries" and "merge the polygons" are used frequently in aggregating area objects


Grid operations

Local Function Focal Function

Zonal Function Global Function


Raster Overlay

Replace all 0’s in B

with data from A

A B


Raster Data Buffering


Some important raster analysis operations

• Renumbering areas in a grid file• Characterizing Terrain Feature• Performing a Cost surface analysis• Performing on Optimal Path

analysis• Performing proximity Search• Creating Variable-Width Buffers


• Classification A-B : agriculture soil C-E : non agriculture soil

Soil map Agricultural soil

map

-Altering attribute values without changing geometry.-to see new pattern and connection


Characterizing Terrain Feature

• Identifying Convex and Concave features by deviation from the trend of the terrain.

Source:Lecture notes from the Asian Institute of Technology

Figures from Asia Asian Institute of Technology


Characterizing Terrain Feature

• 2-D, 3-D and draped displays of terrain slope




Routing and Optimal Paths

The steepest downhill path from the Substation over the Accumulated Cost surface identifies the Most referred Route minimizing visual exposure to houses.

Source:Lecture Notes from Asian Institute of Technology



Routing and Optimal Paths

Alternate routes are generated by evaluating the model using weights derived from different group perspectives




References

Spatial Data Analysis by P.L. Raju

Intro to Interpolation Engr. Bobby Crisostomo, NAMRIA

Lecture from Asian Institute of Technology


Date post:	29-Mar-2015
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Training workshop on GIS and Remote Sensing Applications in Agricultural Meteorology for the (SADC)...

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