tailieunhanh - KEY CONCEPTS & TECHNIQUES IN GIS Part 5

Bạn có thể nhớ lại rằng ba nhà khai thác cơ bản Boolean NOT, AND và OR. Trong Chương 4, chúng tôi sử dụng chúng để tạo thành chuỗi truy vấn để lấy hồ sơ từ các bảng thuộc tính. Các nhà khai thác tương tự cũng áp dụng đối với sự kết hợp của hình học. | 40 KEY CONCEPTS AND TECHNIQUES IN GIS Spatial Boolean logic In Chapter 4 we looked briefly at Boolean logic as the foundation for general computing. You may recall that the three basic Boolean operators were NOT AND and OR. In Chapter 4 we used them to form query strings to retrieve records from attribute tables. The same operators are also applicable to the combination of geometries and in the same way that the use of these operators resulted in very different outputs the application of NOT AND and OR has completely different effects on the combination or overlay of layer geometries. Figure 26 illustrates the effect of the different operands in a single overlay operation. This is why we referred to overlay as a group of functions. Figure 26 is possibly the most important in this book. It is not entirely easy to digest the information provided here and the reader is invited to spend some time studying each of the situations depicted. Again for pedagogical reasons there are only two layers with only one feature each. In reality the calculations are repeated thousands of times when we overlay two geographic datasets. What is depicted here is the resulting geometry only. As in the example of Figure 23 above all the attributes from all the input layers are passed on to the output layer. Depending on whether we use one or two Boolean operators and how we relate them to the operands we get six very different outcomes. Clearly one overlay is not the same as the other. At the risk of sounding overbearing this really is a very important figure to study. GIS analysis is dependent on the user understanding what is A Intersect A and B Coincidence A and B Not intersect A or B but not both Union A or B Any part A or B All but A and B Everything not A or B Union levels Separate identities for each segment A not B Any A that does not include B Figure 26 Spatial Boolean logic COMBINING SPATIAL DATA 41 happening here and being able to instruct whatever system is employed to .