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KEY CONCEPTS & TECHNIQUES IN GIS Part 6

Tất cả các bản đồ chức năng đại số làm việc một tế bào tại một thời điểm. Chức năng địa phương lấy được tên của họ từ thực tế rằng trong việc tính toán giá trị sản lượng chỉ có tế bào đầu vào chính xác cùng phối hợp được coi là (xem Hình 40). | MAP ALGEBRA 53 2 1 4 4 3 2 2 3 3 2 2 2 3 4 5 1 2 3 4 5 1 1 1 5 5 Figure 39 Zones of raster cells other typically out-of-range value. Upon encountering a NoData cell map algebra functions react in a well-defined way. Cell values can have two different purposes. They can represent a true quantitative value e.g. elevation or amount of precipitation or they can represent a class whose values are then described in an external table. In the latter case the cell value acts as a pointer to the correct record in the external table. 8.2 Local functions All map algebra functions work one cell at a time. Local functions derive their name from the fact that in the calculation of the output value only input cells with exactly the same coordinate are considered see Figure 40 . The somewhat tedious description of the procedure goes as follows. A local map algebra function reads the cell values of cell position 1 1 and applies a certain calculation on these input values. It then writes the result to cell 1 1 in the output layer and proceeds to the second cell in the row where the whole procedure is repeated one cell at a time until we get to the last cell in the last row. It is easy to see that this would be very slow if each reading writing step were to involve the hard disk. With the price of memory coming down most GIS nowadays are able to process one or two layers depending on their size virtually before writing the result to a file or table. The cumbersomeness of the process is mitigated by the fact that no complicated geometric calculations have to be performed as all cells 54 KEY CONCEPTS AND TECHNIQUES IN GIS Figure 40 Local function are of the same size and orientation and that the calculations themselves can often be executed within the processor itself which makes it extremely fast. So although a million cells may be processed the result of a local operation is often instantaneous. Compare this to the complexity of overlay operations in the vector world see Chapter 6 .