GIS Data Models 1
Conceptual models of the real world The real world can be described using two conceptually different models: 1. As discrete objects, possible to represent as points, lines or polygons. 2. As a continuous surface with no discrete or distinct borders, like temperature and precipitation. To map houses and roads in an area, discrete objects are more suitable to use since these have a defined spatial extent, but to make a topographic map, a continuous surface should be used since topography has a continuous spatial variation.
Data models in GIS At the end of the lecture you are expected to know: -How GIS data is stored in: Vector data model Raster data model Examples of vector and raster data files 3
What is a Data Model? Data models in GIS A way of representing digital geographic data A set of constructs for describing and representing selected aspects of the real world in a computer (Longley, Goodchild, Maguire & Rhind (2005)) Two basic types of spatial data models have evolved for storing geographic data 1. Raster 2. Vector 4
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Source: Heywood et al ( 2006 )
Vector data model A Vector data model is an object based representation of the real world, Geographic objects are shown as discrete objects They are separated from each other by defined borderlines. 8
Homework The vector data models can be further sub-divided into spaghetti and topological models. Compare and contrast these 2 types of vector data models. 9
Raster data model 10
This lecture will give you an introduction about the raster data structure, which is particularly useful for handling continuous (data) surfaces, but often used also for other types of data. In a raster database, the data is stored in cells in a matrix and this is a very important difference from the vector data structure that has been discussed in the previous lecture. 11
Raster data model A raster model is a field based representation of the real world. It shows geographic data as continuous surfaces It shows gradual changes in topography with no distinct boarder lines for geographic objects. 12
Raster data model store data in cells in a matrix (Raster) Treats geographic space as populated by one or more spatial phenomenon, which vary continuously over space having no distinct boundaries. They are best used to represent geographic features that are continuous over large areas e.g. soil type and vegetation Each cell in a raster is defined by a coordinate location and an attribute that identifies the feature. 13
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Data models in GIS When representing geographic phenomena as raster, a raster/matrix/grid with a fixed cell size is placed over the area, and each raster cell is coded with a code representing the feature in that particular area. In this case, cells covering areas of the river are coded as R, cells covering pine forest are coded P, cells covering Spruce forest are coded S and the cell covering the house is coded H. 15
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In this example, a piece of land contains 3 classes (objects): lake, town and forest. To convert this landscape to a raster data structure a grid (matrix) is overlaid on the landscape and the classes are given a unique code (identifier), in this case lake=1, town=2 and forest=3. Each cell in the matrix represents a certain area in the real world, depending on the size of the cell. 19
One may ask what happens if there is more than one geometrical object found within the same cell?. One solution is to take to dominant area within a cell. In this example, the forest covers a bigger area than the other classes so the cell could be coded as forest. Another method is to code the cell with the class found at the center of the cell. In this example, the cell would now be coded as lake. As you can see in the example, the result may differ considerably depending on which coding method that is selected. 20
When creating a raster database, the first step is to decide the resolution of the grid (the size of the cells). Normally, the cells are squares with equal X and Y resolutions. All cells must be given a value (so if there is nothing to represent in the raster, the value zero can be given for example). Raster data therefore requires a lot of storing space in your computer because the raster structure does not allow for empty cells E.g the zero cells in the diagram contain no useful information, but still this has to be stored. 21
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A very important problem with the raster data structure is that it does not permit the user to know anything about what happens inside a cell. The cell is the smallest unit in the database and anything that is smaller than the cell will not show in the database. If point data is stored as raster structure data it is not possible to know exactly where the points were situated within the cells. To increase the precision, the cell size should be reduced (but more cells = more data = more storage space in your computer! There is always a tradeoff between resolution and memory. 25
Information about the exact location of linear objects is lost in a similar way when translating to raster data. In this example, both the red and the black line networks will be represented in exactly the same manor using the raster data structure despite the fact that they are very different to each other in reality. 26
The same problem occurs with polygons The longer the border a polygon has, the bigger will the difference between the area as measured in the raster data structure and the true area be. 27
4/9/2017 Source: Heywood 3. GIS Data et Models al ( 2006 ) 28
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Homework Give examples of geographic features that can be represented using: -Vector data model -Raster data model Compare the raster and vector data models 30