Spatial data modelling Flashcards
Vad är en abstraktionsprocess och vad används det till?
To make decisions about how the data should be modelled in the GIS, you go through a series of stages of data abstraction. By applying this abstraction process you move from the position of observing the geographical complexities of the real world to one of simulating them in the computer.
For any GIS project this process involves:
1. Identifying the spatial features from the real world that are of interest in the context of an application and choosing how to represent them in a conceptual model. In the Happy Valley GIS, hotel features have been selected for inclusion and will be represented by point entities.
- Representing the conceptual model by an appropriate spatial data model. This involves choosing between one of two approaches: raster or vector. In many cases the GIS software used may dictate this choice.
- Selecting an appropriate spatial data structure to store the model within the computer. The spatial data structure is the physical way in which entities are coded for the purpose of storage and manipulation.
En GIS-modell kan delas in i två delar, vilka?
A GIS model can also be split into two parts: a model of spatial form and a model of spatial processes. The model of spatial form represents the structure and distribution of features in geographical space. In a model of spatial processes, the interaction between these features is considered.
Hur definierar man en entitet?
In order to define entities for inclusion in a computer model features of interest need to be identified, then a method for their representation in the computer must be established.
All geographical phenomena can be represented in two dimensions by three main entity types: points, lines and areas. Figure 3.2 also introduces two additional spatial entities: networks and surfaces.
Networks:
A network is a series of interconnecting lines along which there is a flow of data, objects or materials. There are several networks in Happy Valley. One is the road network, along which there is a flow of traffic to and from the ski areas. Another is the river, along which there is a flow of water. Others, not visible on the land surface, include the sewerage and telephone systems.
Surfaces:
A surface entity is used to represent continuous features or phenomena. For these features there is a measurement or value at every location, as is the case with elevation, temperature and population density. In Happy Valley, elevation and snow depth are represented by surfaces. Both eleva- tion and snow depth vary continuously over space – a value for each can be recorded at any location. This makes representation by a surface entity appropriate. The continuous nature of surface entities distinguishes them from other entity types (points, lines, areas and networks) which are discrete, that is either present or absent at a particular location.
There are a number of problems associated with simplifying the complexities of the real world into five basic, two-dimensional entity types. These include the dynamic nature of the real world, the identification of discrete and continuous features, and the scale at which a particular problem needs addressing.
