[Imports] San Francisco building heights import
chris_hormann at gmx.de
Thu Nov 17 14:41:09 UTC 2016
First of all thanks for doing a more elaborate preparation than back in
May. The whole process is now much clearer.
To verify my understanding: the height values you assign are the median
height values of the city footprint data set when there is a matching
footprint within the area percentage cutoff chosen.
This median height value is the median height in the 0.5m gridded data
set within the city dataset footprint that has been calculated as the
difference between a gridded first reflector data set and a gridded
ground level data set, both derived from the raw LIDAR data.
I am sure this often leads to fairly reasonable results, in particular
with flat top buildings and flat ground with no significant structures
except the buildings but it does not appear to be a really good
approach in principle.
Sources of error here are not only the systematic error with non-flat
roofs, the footprint mismatch and obstructions of the roof. Likewise
important are the inaccuracies introduced by the grid sampling step and
ambiguities in the ground level definition like plants, cars and
These technical things aside i am not sure it is a good idea to enter
this kind of data in OSM. This likely won't encourage community
mapping and it will be of little gain for data users. Producing a
separate point data set with the height values that can be easily
matched with the OSM geometries would IMO be better. You could then
also replace the fixed geometry matching cutoff with a reliability
attribute and data users could decide how strict they want to be in
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