[HOT] [info-hotosm] Reference Project #1030 Nepal Earthquake
sbower at gmavt.net
Thu May 7 16:48:50 UTC 2015
You raise important points, and are not "raining on a parade". The
resulting data will not be suitable for all purposes, but it can be very
useful for this crisis response.
I do think there is significant risk that some mappers will map directly
from un-rectified imagery, and introduce problematic location errors. That
needs to be minimized, e.g., through clear instructions and good
validation. I think there's room for improvement on the instructions, e.g.,
it would be good to have a wiki page on mapping from un-rectified imagery
in combination with rectified imagery, for crisis response.
On Thu, May 7, 2015 at 5:14 AM, Milo van der Linden <milo at dogodigi.net>
> Hello Springfield Harrison,
> As a 20 year GIS veteran I understand what you say. I do agree that in
> communication with first responders it is important to have them clearly
> understand that the accuracy of features can be off ~100m. But for them
> having maps that give a good indication is way better then having no maps
> at all. In the end, and that is what I hope for, it can save lives.
> I have a long running discussion with y'olde GIS community on "how can a
> map created by amateurs be better then what we professionals do?". It is my
> opinion that it can be. I believe that "the many are smarter than the few"
> (quote by James Surowiecki). And the HOT tasks have all the ingredients to
> 1. There is diversity of opinion
> 2. People involved in the mapping process have opinions not influenced by
> those around them
> 3. People operate decentralized
> The only thing that might need more attention (and this is where
> geospatial experts can take their role) is that HOT and openstreetmap as a
> whole could use more mechanisms to turn all these little "private
> judgements" into collective quality. This process could involve analysing
> quantity and different representations of the same feature through time. In
> that way, you could see the mapping activity (in dense area's) as GPS.
> There are faults, influenced by methodology, opinion and conditions. And as
> a GPS professional, you know that it is _knowing the error_ that
> automagically creates accuracy. I would love the GIS/GPS community to think
> about how to know the error in community mapping.
> I love this new way of mapping. It creates new opportunities. It involves
> new ways of thinking. It is not influenced by what GIS people say GIS
> should be like.
> Kind regards, with respect,
> 2015-05-07 10:21 GMT+02:00 Springfield Harrison <stellargps at gmail.com>:
>> Hello Steve,
>> Sorry to rain on the parade yet again but I find this matter of
>> image alignment to be puzzling and concerning.
>> One of the first things I learned when embarking upon GIS/GPS
>> mapping was that accurate georeferencing of all layers, but especially the
>> base layers (imagery in this case) was sacrosanct. If things are not in
>> their correct point in space, what use is that to the end user? Especially
>> in rugged terrain, with difficult access and rapidly changing stream flows,
>> it is important to know where a trail or road really is. Why try to cross
>> a raging torrent when you don't need to?
>> Having untrained users realign the imagery willy-nilly is
>> amazing to me. What faith can anyone have in the new tracings if the earth
>> is literally moving every time a new user opens up the file? Accurate map
>> datums and projections were created for a reason.
>> How is it that, "...the DigitalGlobe 2015-05-03 (DG) images have
>> had minimal georectification.." This is bizarre, this is not GIS, this is
>> merely sketching. Why is such imagery being offered and accepted? I know
>> that this is a major emergency but then all the more need for quality data.
>> However, I am newly arrived, and it seems that most people are
>> content with a world that can be up to 200 m out of whack. I'm not sure if
>> I can contribute much under the circumstances other than this gloomy
>> criticism. Sorry, will try not to dampen the enthusiasm further.
>> Thanks for your patience, Cheers . . . . . . . . Spring
>> At 06-05-2015 11:59 Wednesday, Steve Bower wrote:
>> Ross - If you haven't already, see the recent threads on "data alignment
>> to satellite imagery" and "imagery alignment", in the archives for May:
>> Note some links pointed out there by althio:
>> Â http://wiki.openstreetmap.org/wiki/Using_ImageryÂ
>> Â http://learnosm.org/en/editing/correcting-imagery-offset/Â
>> 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.Â
>> 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
>> 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).
>> On Wed, May 6, 2015 at 1:53 PM, Steve Bower <sbower at gmavt.net> wrote:
>> I don't thinkÂ Chad's IDPÂ 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 IDPsÂ
>> Hope this helps
>> On Tue, May 5, 2015 at 12:40 AM, Ross Taylor <ross at byrdtechnology.com >
>> 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.Â
>> 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.
>> Sent from mobile
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> [image: http://www.dogodigi.net] <http://www.dogodigi.net>
> *Milo van der Linden*
> web: dogodigi <http://www.dogodigi.net>
> tel: +31-6-16598808
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