3. How to represent spatial features?
Geographic information systems work with two fundamentally different types of geographic models: the "vector" model and the "raster" model.
3.1 Vector model
In the vector model, information about points, lines, and polygons is encoded and stored as a collection of x,y coordinates. The location of a point feature, such as an ocean floor sampling site, can be described by a single x,y coordinate. Linear features in terms of arcs, such as ship tracks, contour lines or frontier systems can be stored as a collection of point coordinates. Areal features, such as ocean basins and biological provinces, can be stored as closed polygons of coordinates. The vector model stores the boundaries of objects, and uses a labelling scheme to keep track of their attributes. This scheme involves the notion of topological attributes. These spatial attributes define adjacency and containment of relationships between spatial objects. Topological and multiple descriptive attributes of each feature are stored in Data Base Management Systems (DBMS).
Points can represent marine stations of field observations, that are directly linked via attribute tables to numerical, categorical or descriptive data like abundance of certain species, chemical parameters like salinity, temperature, wind fields, nutrient concentration, etc.
Arcs may represent bathymetric contours, coastlines, frontier systems, navigation/camera/submarine/seismic tracks, migration or ice frontiers, etc. Each arc can be linked with numerical values like waterdepth or descriptive attributes like names, cruise-number or date to allow analytical or statistical retrieval.
Polygons can represent ocean basins, marine biogeographical provinces, water mass systems, etc. with according attributes.
It is not necessary to store numerical and descriptive attributes with the physical data inside a GIS. External database systems can be linked over ID-No, coordinates or other key attributes to any kind of physical feature that is stored inside an internal GIS database.
3.2 Topology
A GIS can recognize and analyse the spatial relationships among mapped phenomena independent of their exact position. In digital data, topological relationships such as connectivity and adjacency (what is next to what), containment (what is enclosed by what), proximity (how close something is to something else) and relative position are usually expressed as relationships between nodes, links and polygons. For example, the topology of a line includes its from- and to-nodes, and its left and right polygons. Topology is useful in GIS because many spatial modelling operations don't require coordinates, only topological information. For example, to find an optimal path between two points requires a list of the lines or arcs that connect to each other and the cost to traverse each line in each direction. Coordinates are only needed for drawing the path after it is calculated.
3.3 Raster Model
The raster data model is more like a photograph than a map. If you look at a photograph through a strong magnifying glass, you will see that it is made up of a series of dots of different colours or shades of grey. The raster data model works in a similar way; it is a regular grid of dots (called cells, or pixels) filled with values. In fact, when a picture is stored in a computer, the raster data model is used.
Each grid represents a spatial variable. While vector models are stored as a series of x,y coordinates and topological relationships, grid cells are stored as rows and columns.
In the raster data model, each location is represented as a cell. The matrix of cells, organized into rows and columns, is called a grid. Each row contains a group of cells with values representing a geographic phenomenon. Cell values are numbers, which represent nominal data such as marine ecosystem classifications, measures of light intensity or relative measures. Because the raster data model is a regular grid, spatial relationships are implicit. Therefore, explicitly storing spatial relationships is not required as it is for the vector data model in terms of topology.
Advantages of a raster model
Cell-based systems can be used for continuous surface-data modelling. One type of phenomenon that the grid-cell data structure is best suited to represent is continuous spatial data. These are phenomenon that produce a continuous surface where each location on the surface is based on the inherent characteristics of a location relative to a known fixed point or from an emanating source, like an oil spill. They include elevation (the fixed point being sea level) and aspect (with the fixed point being a directional system: north, east, south and west) e.g. bathymetric charts and decreasing chlorophyll concentrations at the sea surface as you move away from the coast.
Because the grid-based systems foundation uses uniform grids, the mathematics are very simple and very fast when completing analysis between grids. Once registered, computing or deriving a value for an output cell from two or more input cells is a matter of direct value computation. No geometric detection, topology building and error checking is necessary.
3.4 Data base model
Data storage and database management concerns the way in which data about the location, linkages (topology), and attributes of geographical elements (points, lines, areas, raster, and more complex entities representing objects on the earth's surface) are structured and organized, both with respect to the way they must be handled in the computer and how they are perceived by the user. The computer program used to organize the database is known as a Database management system (DBMS).
Attribute data are those properties of a spatial entity that need to be handled in the GIS, but which are not themselves spatial. For example, descriptive attributes for the spatial occurrence of marine sediment types like clay, silt or sand. Although these attribute values and associated identifiers may be attached to graphic entities directly on input, it is not efficient to enter large numbers of complex attributes interactively. The data are therefore either stored separately from the spatial information in the GIS, or are kept along with spatial description in an external relational database. Where relational databases are used an identifier is included in the attribute record to link the spatial and attribute data together.