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Lol<br>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Tim Elrick <osm@elrick.de><br>
<b>Sent:</b> Sunday, March 22, 2020 6:49:48 PM<br>
<b>To:</b> Daniel @jfd553 <jfd553@hotmail.com><br>
<b>Cc:</b> talk-ca@openstreetmap.org <talk-ca@openstreetmap.org><br>
<b>Subject:</b> Re: [Talk-ca] Importing buildings in Canada</font>
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<div class="PlainText">Hi Daniel,<br>
<br>
I agree with you. I didn't pay attention to the fact that Squamish is <br>
located in a hilly area.<br>
<br>
Greetings from Quebec's flatlands,<br>
Tim<br>
<br>
On 2020-03-22 14:16, Daniel @jfd553 wrote:<br>
Hi all, sorry for this long Email.<br>
<br>
Thanks to Tim to have comment! He wrote: “I [...] found that you can <br>
either align the hospital with the underlying imagery or the houses to <br>
the right of the task, but not both at the same time. [...] If we <br>
assume that the aerial imagery data is the correctly projected [...], we <br>
would have to correct the position of all the buildings according to the <br>
underlying aerial imagery.”<br>
<br>
Well, you are right. Actually, I did not align most of the buildings to <br>
the image! Why? Because unless proven otherwise, ODB data should be more <br>
accurate (XY) than most images available, especially in hilly areas.<br>
Municipalities generally use aerial photos to create their maps (ODB <br>
data). Because these aerial photos provide multiple views of the same <br>
area, they can be used to compute digital elevation models (DEMs) <br>
showing even buildings’ height. Only once done, they can create accurate <br>
ortho-images (orthographic view [1]). Without an accurate DEM, objects <br>
location on an image is not accurate either, because we are in a <br>
perspective view [1].<br>
The DEMs used to create available OSM images generally do not have a <br>
sufficient accuracy in mountainous areas. This is the case of the <br>
Squamish area where the image shows many examples of perspective views <br>
[1]. In flat areas, this effect is minimal, which makes it possible to <br>
adjust an image over a large region with a great accuracy. The only <br>
visible effect is then related to buildings’ height.<br>
<br>
Regarding the hospital, it is located on a hill between two plateaus. <br>
The image can be adjusted with a good accuracy on the flat area near the <br>
river, or on the plateau on the top of the hill (potentially with <br>
another offset), but it is more difficult in between. I tried to adjust <br>
its geometry (details) from its original ODB location.<br>
<br>
I adjust the image to surrounding buildings when I need to map a new one <br>
or add details to an existing one. I may also look at available GPS <br>
tracks to confirm general ODB data location.<br>
<br>
Thanks again. Comments?<br>
Daniel<br>
<br>
[1] <a href="https://en.wikipedia.org/wiki/Orthophoto">https://en.wikipedia.org/wiki/Orthophoto</a><br>
<br>
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