[HOT] [info-hotosm] Reference Project #1030 Nepal Earthquake

Steve Bower sbower at gmavt.net
Wed May 6 18:59:46 UTC 2015


Ross - If you haven't already, see the recent threads on "data alignment to
satellite imagery" and "imagery alignment", in the archives for May:
https://lists.openstreetmap.org/pipermail/hot/2015-May/thread.html

Note some links pointed out there by althio:
  http://wiki.openstreetmap.org/wiki/Using_Imagery
  http://learnosm.org/en/editing/correcting-imagery-offset/

Because the DigitalGlobe 2015-05-03 (DG) images have had minimal
georectification (needed mainly for elevation distortion), they may be
offset by 100m or more. On one tile (5.5km wide) I saw offsets relative to
Bing of 125m to the west and, elsewhere, 85m to the east. The offsets may
vary considerable even in nearby areas, especially in steep terrain.

You should align your work with Bing imagery. Thus to digitize from the DG
imagery you should first adjust the DG imagery to the Bing imagery, and
re-adjust it as you move from place to place. As you noted, adjusting in
one area makes it worse in others, so you have to keep re-adjusting as you
go. You should be able to compare the Bing and DG imagery to confirm where
a feature visible on DG is located on the Bing imagery (if Bing is clear
enough). I try to adjust based on buildings, or road intersections/curves
(keeping in mind that roads are sometimes relocated), or even less
permanent features (rivers generally are not good, they move around to
much). It's a time-consuming process, but needed to correctly locate
features.

It's not essential that everything be within a few meters of its true
location, but features should be mapped correctly relative to one-another.

The links above provide guidance on how to align imagery to correct
locations. It's easy in JOSM with the Imagery Offset tool (on the toolbar).

Steve

On Wed, May 6, 2015 at 1:53 PM, Steve Bower <sbower at gmavt.net> wrote:

> I don't think Chad's IDP guidance document (though very helpful) addresses
> the issue of spatial accuracy of the DG imagery, raised by Ross. I'm going
> to post that as a separate issue with more detail.
>
> On Wed, May 6, 2015 at 4:35 AM, Heather Leson <heather.leson at hotosm.org>
> wrote:
>
>> HI Ross, sorry for my delayed response. It is best if you ask your
>> questions on the main Hot at openstreetmap.org mailing list.
>>
>> Chad provided this guidance document on IDPs
>> http://hotosm.github.io/tracing-guides/guide/Nepal.html#IDP%20Collection%20Guidance
>>
>> Hope this helps
>>
>> Heather
>>
>> On Tue, May 5, 2015 at 12:40 AM, Ross Taylor <ross at byrdtechnology.com>
>> wrote:
>>
>>> Hi, I am seeing many more IDP sites using DigitlaGlobe imagery vs Bing.
>>> I can toggle between the two image sets, but they are significantly
>>> nonaligned. I created a landuse=brownfield tagged area which aligns with
>>> Bing, but if I mark and tag the individual IDP sites showing up in
>>> DigitalGlobe imagery, the brownfield and idp are not going to be aligned.
>>> I want to help out as much as possible and would like the data to be
>>> correct. Please advise, thanks!
>>>
>>> Note: I tried to adjust alignment but it fixes one area and creates more
>>> offset in other areas.
>>>
>>> -Ross
>>>
>>> Sent from mobile
>>
>>
>>
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>>
>>
>
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