[HOT] How bad is mapping from offset imagery?

Mike Thompson miketho16 at gmail.com
Tue Apr 4 18:20:36 UTC 2017


I don't have answers to your specific questions, but here are some general
observations regarding the topic:
1) Project managers need to specify in the instructions which source of
imagery is to be considered positional truth, or if imagery should be
aligned to some other control (existing features, GPS traces, Strava, etc).

2) If imagery is not to be aligned with existing features, project managers
need to specify if the existing features should be re-aligned.
3) Generally one cannot simply align the imagery once, or even once per
task/tile.  The offset will vary based upon location within the square.
This is especially true where there is high relief (i.e. hilly,
mountainous), and the satellite look angles (e.g. off nadir angles) differ
4) Specific to your question - There is no reason why Bing would be any
better than some other sources.


On Tue, Apr 4, 2017 at 10:07 AM, Florian Niel <florian.niel at gmail.com>

> 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).
> Best,
> Florian
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