[Talk-ca] Importing buildings in Canada

[Tim Elrick]

Hi Daniel,

I agree with you. I didn't pay attention to the fact that Squamish is 
located in a hilly area.

Greetings from Quebec's flatlands,
Tim

On 2020-03-22 14:16, Daniel @jfd553 wrote:
Hi all, sorry for this long Email.

Thanks to Tim to have comment! He wrote: “I [...] found that you can 
either align the hospital with the underlying imagery or the houses to 
the right of the task, but not both at the same time. [...]  If we 
assume that the aerial imagery data is the correctly projected [...], we 
would have to correct the position of all the buildings according to the 
underlying aerial imagery.”

Well, you are right. Actually, I did not align most of the buildings to 
the image! Why? Because unless proven otherwise, ODB data should be more 
accurate (XY) than most images available, especially in hilly areas.
Municipalities generally use aerial photos to create their maps (ODB 
data). Because these aerial photos provide multiple views of the same 
area, they can be used to compute digital elevation models (DEMs) 
showing even buildings’ height. Only once done, they can create accurate 
ortho-images (orthographic view [1]). Without an accurate DEM, objects 
location on an image is not accurate either, because we are in a 
perspective view [1].
The DEMs used to create available OSM images generally do not have a 
sufficient accuracy in mountainous areas. This is the case of the 
Squamish area where the image shows many examples of perspective views 
[1]. In flat areas, this effect is minimal, which makes it possible to 
adjust an image over a large region with a great accuracy. The only 
visible effect is then related to buildings’ height.

Regarding the hospital, it is located on a hill between two plateaus. 
The image can be adjusted with a good accuracy on the flat area near the 
river, or on the plateau on the top of the hill (potentially with 
another offset), but it is more difficult in between. I tried to adjust 
its geometry (details) from its original ODB location.

I adjust the image to surrounding buildings when I need to map a new one 
or add details to an existing one. I may also look at available GPS 
tracks to confirm general ODB data location.

Thanks again. Comments?
Daniel

[1] https://en.wikipedia.org/wiki/Orthophoto