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Building Spatial Databases – Theory / Maps and spatial functions

Learners guide

Summary

In this unit the principles of mapping will be introduced. It is essential in geoinformatics.

Requirement

High-level knowledge

Maps and spatial functions

Our requirements regarding maps can be summarized in some sentences. The map is a possible model of the reality. The mapping process is based on certain rules of imaging. If we apply the right model, it can help us to understand the circumstances, to navigate ourselves, to identify our position in the space or on the Earth surface. A right model supports us to overview large areas as well as describes every detail on a certain field, which is needed to users in a special context. Let the map be the guide for displaying a spatial process, a distribution of physical quantities in space, qualitative or quantitative approach of a spatial process. Maps can help decision makers to understand the problem, to outline the spatial distribution of a phenomenon, to compare different objects regarding their properties, spatial dependencies, interactions and relationships. Sometimes maps display enumeration of certain objects only, which can be a simple spatial register.

The functions mentioned above are valid for not only digital maps, but classic paper maps too. The differences between them are essential, because digital maps are dynamic, but paper maps are static. A special result of an analysis is an individual product, if paper maps are used. However, the same result in digital maps is changeable, flexible and it is easy to transport them to anywhere. If the question or the requirements are changed, the paper that contains the latest results can be produced very fast. Permanent result can be produced by printing only.

In the following, some basic spatial operations will be shown. They appeared often in geographical information systems. Although these functions turn up in different form and syntax in GIS software, their functionality is the same in every GIS system. The differences come from the different concepts of these systems.

Theme, thematics

Changing of theme or thematics is an essential part of the basic functionality of every GIS software. Theme is a set of objects with identical structure. The graphic properties of a theme can be changed individually or in groups. For example, rivers, cities, countries form themes, which are considered map layers, if they are displayed on a monitor or a printer device (Fig. 11). In this case, the legend and scale should be defined for them. More thematics can be imagined in certain maps, like in Fig. 11, where rivers can be seen over the country polygons.

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Spatial objects

As mentioned, a theme is a set of elements with identical structure. These elements are spatial objects. An object can be represented by point, line, polyline and polygon (Fig. 12). In certain cases the exact location or size of these objects are not known, such as marsh, bushy areas, scrublands. In some cases, only labels imply the existence of a land, such as Jászság or Zala Hills, where there is no exact boundaries of a territory, while everyone knows where it is.

You can open the (larger) image in new window.Fig.12. Many themes, many geometric representations can be12_abra_full.jpgFig.12. Many themes, many geometric representations can be

The spatial objects have two basic data groups. One of them is the attribute data (Fig. 12), right side table), which contains alphanumerical descriptive data of the objects, such as name, area, weight, population, category value and so on). The other basic data group is the spatial reference, which defines the position of an object (x, y, z coordinates). If the object is a line, this kind of data defines a set of points with coordinates, which are the components of a line. If an object is a polygon, the dataset consists of points, which create the polygon. In some cases, if the dataset is topological, the adjacency is also stored if it obvious, such as country boundaries, parcels, etc.

Regarding the complexity of the world, the objects that model the reality should be similarly complex. It is expedient to differentiate geometric primitives and complex spatial objects. Look at an abstract definition for theme:

theme = a set of spatial objects

spatial objects = (description, geometric primitives | (description, complex spatial objects)

Consequently, relational databases are obviously very suitable to describe and handle the theme consisting of elements having identical type and structure.

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A projekt az Európai Unió támogatásával, az Európai Szociális Alap társfinanszirozásával valósul meg.

A Társadalominformatika: moduláris tananyagok, interdiszciplináris tartalom- és tudásmenedzsment rendszerek fejlesztése az Európai Unió támogatásával, az Európai Szociális Alap társfinanszírozásával, az ELTE TÁMOP 4.1.2.A/1-11/1-2011-0056 projekt keretében valósult meg.
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