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Hey, i want to calculate the area of trees in a city (Zürich, Switzerland), but when i do this (Python or QGIS) the result is 0. When you take a look on the map, there are many trees found with "natural=tree". How does the calculation of Nodes work? And what is the area of the tree that is calculated - only the trunk? And maybe it's just a really small number that the output is 0? Thanks for your help. |
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Nodes are points, so have no area. For trees the wiki mentions optional tags of circumference, diameter, and diameter_crown which you could use to calculate an approximate area. Overpass suggests these are little used in Zurich (diameter_crown is on one tree note and three tree row ways for example). There might also be larger areas tagged with landuse=forest or natural=wood. 3
Perhaps you could count the trees and multiple by an average circumference, add to the areas of separately mapped wood and forest, abd then try and see if there's any overlap between the two (individual trees mapped in an area of wood or forest)?
(30 May '22, 17:31)
SomeoneElse ♦
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For completeness, in addition to the nodes and areas already mentioned, there is also a line: natural=tree_row. I don't know about Zürich but in my city this is used quite a lot on tree-lined avenues.
(30 May '22, 17:43)
alan_gr
Thank you very much! @SomeoneElse: is there any technique to check if there's an overlap or do i just check some samples in the map?
(31 May '22, 09:15)
milo22948392
Sampling might be the easiest way, because I suspect that most single trees won't be in areas mapped as woodland, but it might be possible to do something with Overpass turbo (perhaps see https://wiki.openstreetmap.org/wiki/Overpass_API/Overpass_API_by_Example - there are some "X within Y" examples there) or using a geographically aware database (an example of this sort of question is https://gis.stackexchange.com/questions/290072/locating-geometries-within-a-polygon-area - have a look at that and links from there).
(31 May '22, 11:52)
SomeoneElse ♦
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