Research Of The Urban Security Evaluation Model Based On Rough Set

City is the center of politics, economy, culture and technology in a country's specified region, and it is the place that has a large population, abundant material, and where information technology is the most concentrated and active. The process of urbanization provides a strong driving force for a sustained high growth in the future economy, and also makes the urban population highly concentrated and urban buildings relatively dense. But the urban infrastructure is relatively weak, coupled with damage to the environment and lack of urban safety awareness, which inevitably produces a series of social problems. A very important aspect of these problems is the city's security.City safety is an important component of national security, and has a bearing on all aspects of our society, and it also affects every city resident's self-interest. And some incidents that jeopardize the urban safety have occurred in recent years, so concerns about urban safety have been put on the agenda in every city manager's work, and how to evaluate the security of a city becomes a priority.City safety is a kind of special and indefinite problems and the factors affecting city safety involve political, economy, society, culture and other fields, and some factors are indefinite or difficult to quantify the characteristics, but also the data affecting the factor is not complete. The rough set theory is a mathematical tool used to solve the question of being incomplete and indefinite, which can not only effectively analyze the inaccurate, inconsistent, incomplete, imperfect information, etc., but also analyze the data and come to a reasonable conclusion, thus discovers the hidden knowledge and reveals the potential law. The most significant difference between rough set theory and other theories dealing with uncertain and imprecise questions is that rough set theory has a set of strict mathematical methods for classification, it does not need any prior information other than the data sets, and it can objectively describe and handle the indefiniteness of a problem. This article applies the rough set theory to the research of the urban security evaluation and based on this theory, builds an urban security index evaluation model.Suppose S'=(U',A',V',f') is the urban security index evaluation model, where U'represents all the cities to evaluated, A' refers to the set of all the urban security evaluation indexes, V'=(?)., the set of evaluation index value,V'a.is the domain of index a', and f'(x,a'), the value of the cities to evaluate under index a', of which, (?)x∈U', (?)a'∈A'.Typically, when we evaluate the city's security as a whole or from one aspect, we often select certain indicators and make corresponding weigh according to the size of the index value. However, the research of urban security evaluation involves so many evaluation indexes, how do we choose the most decisive one? This paper uses "discernibility matrix" in rough set theory to solve the problem. And it is not objective to determine the city's security in some area only on the basis of the size of a certain index value, or to evaluate the security of many cities according to the different values of the same indicator. And this paper solves the problem by calculating the comprehensive weigh of the index value, that is, by comparison of two different indicators to establish the subjective weigh from the matrix, by using the importance of the property of the rough set theory to obtain the objectives weigh, then combines these two weighs to get the comprehensive weigh from experience, and uses the LaGrange function to show that it is the best solution, finally on this basis, this article defines the security index evaluation model, which has the comprehensive weigh, for the evaluation between different cities.Suppose that M is the security index evaluation model which has index comprehensive weigh, that is, M=(U, A, V, f, wi), of which U={u1, u2,…, um} refers to the set of all the objects to be evaluated, A={a1,a2,…,an} is the index set;V=(?), the domain of index a; and f:U×A→V is an information function, each object of U with an information value that a about V, that is,(?)ai∈A,(?)uj∈U, f(uj,ai)∈Va, and i=(1,2,…,n), j=(1,2,…,m); and wi refers to the comprehensive weigh of the ith index, the sum of the comprehensive weigh of the object's all indexes can be defined as: m n Wi=∑(∑waiVaii) j=1 i=1This paper uses the security evaluation of air quality in a number of cities as an example, by calculating the comprehensive weigh of every evaluation index and the sum of them, and calculates the degree to which the index affects the city's air quality and compares the air quality between different cities, and also to show that they tally with the facts. This indicates that the urban security evaluation model based on rough set has certain rationality and feasibility in practice, this study also expects that this model can provide the decision-makers who want to evaluate city security as a whole or from one aspect with corresponding basis.