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# Correct building area position (roof or base)?

 2 Hi there. Sorry if this is answered elsewhere (it probably is), but I was searching for this question and I didn't find anything about it. When drawing a building area, for example, should I guide myself with the roof or with the base of the building? I've been doing building areas from the base of the building and inferring the correct size of the area, but I see many people drawing the areas over the roof. What's the best practice here? Regards. asked 12 Nov '17, 17:35 AntMadeira 176●9●9●17 accept rate: 0% aseerel4c26 ♦ 32.6k●18●248●554

 3 There is a comprehensive page on the wiki addressing this issue, although it's title makes it seem more specific: https://wiki.openstreetmap.org/wiki/Roof_modelling. It includes lots of useful tips. answered 12 Nov '17, 21:43 SK53 ♦ 28.1k●48●268●433 accept rate: 22% That was exactly what I was looking for and couldn't find because of the correct title. Great tips in there. I'll surely pay more attention to the wiki from now on. Thank you very much, SK53! (12 Nov '17, 22:23) AntMadeira
 6 Hi @AntMadeira. Of course the best parctice is trying to trace the base of the building, not the roof, especially when your imagery is inclined and there's an offset between the roof and the actual base. But sometimes it might be hard for you to guess the shape of the base (it can be hidden) and it might help drawing from the roof then moving the polygon so that it gets located at the base of the building. You might also want to switch between several imageries to try to see how the building really looks like as inclination may vary from an imagery to another. answered 12 Nov '17, 18:41 Privatemajory 1.1k●14●22●33 accept rate: 23% 3 Yes, it's better to "guide yourself with the base", but it's usually easier to trace the roof and then move the polygon to the base. Note though that aerial imagery is rarely super accurately aligned, so you'll often see buildings etc appear "shifted", when using newer or other imagery. That's why I often trace the roof and leave it at that (particularly with relatively low buildings). I don't feel like keep moving buildings around to make them fit current imagery. What's important is to get the shape of the building roughly right. (12 Nov '17, 19:50) Hjart Ok, thank you both for your answers. I believe that the middle way is always the best way, i.e., drawing the roof and move the polygon to fit the true position of the building as much as possible and leave the shape of the roof when the true shape is not obvious. (12 Nov '17, 20:02) AntMadeira

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question asked: 12 Nov '17, 17:35

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last updated: 12 Nov '17, 22:23

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