Coating survey of underground pipelines is very important to prevention of corrosion. According to NACE SP0502 standard, there are 3 famous methods for coating survey: DCVG, CIPS and C-SCAN. Other methods (ACVG, PEARSON, …) aren't conventional. All methods have some advantages and disadvantages individually that limit their application. Combined method of DCVG and CIPS has developed to distinguish coating defect locations and cathodic protection level. In the beginning , results of the first project (180 km of 30” gas pipeline) show the disability of one coating survey method and concluded that disadvantage of method could effect on distinguish of coating defects. There are sever coating damage that only could be found by C-SCAN or some defects that were found by DCVG or CIPS. To find the reasons of this shortage, we did some tests on high pressure transmition gas pipeline for 4 months as well as negotiation with researchers and manufactures. Finally, we concluded that survey by only one method has some mistakes to distinguish coating defect locations because of their original recognize procedure differences. This means that some defects are hidden by magnetic method (c-scan) or potential method (DCVG & CIPS). Then, the best way is that combination of these 3 methods. These methods were combined together according to Fuzzy logic and a function ("Severity Function”) was defined to regard each point by a number. Categorization and rehabilitation of coating damage and covering of all coating defects without any error are some of excellent advantages of this function. Survey of 350 km of gas , oil and water pipelines and their results show reliability of this method. Full report has many examples of sever coating damage with corrosion signs.