Generalization Abstraction And Method Chapter 7 Generalization Abstraction And Method

Overview :- Introduction Generalization Basics Method of Generalization Measuring the Degree of Generalization Metadata

Introduction *A geographic database cannot contain a perfect description –instead , its contents must be carefully selected to fit within the limited capacity of computer storage devices. *Efficient compressing is made possible because many definitions are already shared, and do not need to be communicated. *Metadata are formal descriptions of data

Generalization Basic GIS utilize six ways of representing a field:- A. Capturing the value of the field variable at each of a set of irregularly spaced sample point. B. Capturing the value of the variable at each of a grid of regularly spaced sample point. C. Capturing a single value of the variable for a regularly shaped cell

Continue …. D. Capturing a single value of the variable over an irregularly shaped area E. Capturing the liner variation of the field variable over an irregularly shaped triangle F. Capturing the isolines of a surface, as digitized lines

The six approximate representations of a field used in GIS (B) (C) (D) (E) (F) The six approximate representations of a field used in GIS

Method of Generalization 1. Simplification, for example by weeding out points in the outline of a polygon to create a simpler shape. 2. Smoothing, or the replacement of shape and complex forms by smother ones 3.Collapse, or the replacement of an area object by a combination of point and line object. 4.Aggregation, or the replacement of several area object by smaller number of a new symbols 5. Amalgamation, or the replacement of several area objects by a single area object. 6. Merging, or the replacement of a several line objects by a smaller number of line object.

Continue …. 7. Refinement, or the replacement of a complex pattern of objects by a selection that preserve that patterns general form 8.Exaggeration, or the relative enlargement of an object to preserve its characteristics when these would be lost if the object were shown to scale. 9. Enhancement, through the alteration of the physical sizes and shapes of symbols. 10.Displacement, or the moving of objects from their true position to preserve their visibility and distinctiveness

Measuring the Degree of Generalization It is often important to be able to measure the degree of generalization present in a geographic dataset. Such measures allows us to describe generalization very simply, using standard methods that are understands by others; to compare datasets with different degrees of generalization; and to decide whether datasets meet the requirement of specific applications. This section describe the various measures that area= in widespread use.

Representative fraction * Knows as a scale * It is a akin to the size relationship between a toy car and a real one. * Associate with standard map series, such as 1:24,000 in the USA * The Representative fraction is the preferred measure of the degree of details in a maps representation. * Although it is not well-define for digital data, numerous convention allow it to be estimated.

Minimum mapping unit The MMU describe earlier applies to a specific set of the data models, those that represent either discrete object or fields as collections of primitive areas. The MMU is the preferred measure for the level of details in a map of classes of land use, soils, or vegetation.

Spatial Resolution Define as the minimum distance over which change is recorded. The spatial resolution of rectangular cells is defined by the length of the longer cell side.

Metadata It must serve many different purpose, from description of contents to instructions for handing. Light or stripped-down metadata, provide a short but useful description of a dataset that is cheaper to create.

Some prototype geolibraries www.alexandria.ucsb.edu

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