[HOT] How bad is mapping from offset imagery?

Florian Niel florian.niel at gmail.com
Tue Apr 4 16:07:02 UTC 2017

I often see that other mappers don't align the background imagery. I am 
wondering how a big deal this is.

Aligning the imagery in tasks like the malaria elimination projects is 
difficult, if the square does not contain any data (and no gps traces). 
I do it (in JOSM) by clicking some points of a road or path or something 
other visible in both Bing and the recommended imagery, and check if 
there is an offset and then align it to Bing. Then I delete the path I 
just mapped and start mapping the buildings.

With this method I need more time for the alignment than for mapping the 
two or three buildings to finish the task.

So I have questions:
1) Is there a faster way to align the imagery on empty squares?
2) Is it really necessary to align it for just a few buildings and a lot 
of nothing (mapped) around? As I dont know what the data is used for so 
I cannot tell. The size and the shape of buildings are correct anyways 
and my intention would say it is no big deal if it is a bit offset.
3) Is it possible to align the imagery in advance before opening the 
task for HOT so mappers do not have to care about it?
4) Is Bing really always better aligned or are we doing just a shift 
from one unknown offset to another?

Another comment on alignment: I just looked at the video from Blake 
(https://www.youtube.com/watch?v=1Pcj_wFTHeE) about how to align imagery 
in iD. I think we need a better one. One thing is that it is not 
comparable to empty squares (like in many malaria-project squares) and 
the other thing is that he comes to the conclusion that he should not 
align the imagery at all (because it is a messed up square with objects 
mapped from different imageries).


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