[Imports] San Francisco building heights import

[Christoph Hormann]

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 
non-building structures.

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 
that regard.

-- 
Christoph Hormann
http://www.imagico.de/