Dear Jose: Here are some reason why data must be topologically clean data: 1. if the data is not clean, errors will begin to creep into your analysis. Especially when working with areas and perimeters of polygons. For example, I need to know the areas of the various polygons that are contain a rare plant species. If the data is not clean, how will I know that the area reported by the GIS is accurate or not. 2. as I begin to do overlay, union and intersect type analysis between several coverages, I may get incorrect results. For example, I am trying to find areas which are prone to landslide. Using the GIS I overlay a slope map with a terrain stability map, and a soils map. If my data is not clean on all 3 maps, I may get areas in the new map which are incorrectly classified, have the wrong attributes, etc. I have have to use that map to guide my road layout I may be locating the road in the wrong place. 3. data sharing amongst different departments, companies. Other users of your data will not have any confidence in your information. 4. rebuilding datasets sometime in the future. By using unclean data, you will at sometime be faced with the challenge/task of having to rebuild your project. This is very costly and time-consuming. Not to mention what becomes of any analysis you have done using that data. 5. Confidence in the accuracy of the analysis done on the data. The analogy I use when asked about why data must be clean is one of building a house. Your data forms the foundation of the house upon which you will be adding rooms, painting the walls, adding wallpaper and so forth. If the foundation is not solid and reliable, it does not matter how good the paint job is, the house will fall down. The only question is when. Then you have start all over again. So I guess the question is how strong do I wish to make the house. Hope this helps you to convince the powers that be that your data should/must be topologically clean. Bob Plummer Smart Map Services 649 East Keith Road North Vancouver, B.C. Canada V7L 1W4 e-mail: [EMAIL PROTECTED] ph: 604-980-3252
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