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<p>Hi<br>
</p>
<div class="moz-cite-prefix">On 2019-12-16 5:36 p.m., Falk Huettmann
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CA+BKNPB9HhBPOnJ3Wtvi2YJWRzZrb7jb7d+2UgeC5AOfvjqJfw@mail.gmail.com">
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<div dir="ltr">Hi there,
<div><br>
</div>
<div>thanks,</div>
<div>just to elaborate, for those who are interested:</div>
<div><br>
</div>
<div>(I would like to see the study that has compared those DEM
products;</div>
<div>it matters a lot for climate models, for instance; happy to
read the citations).</div>
</div>
</blockquote>
You will find many on web of science or google scholar. My own
research has been presented at the 39th Canadian Symposium on Remote
Sensing, Saskatoon, SK and chapter will be published soon for my
Ph.D. <span style="color: rgb(87, 89, 91); font-family: Lato,
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<blockquote type="cite"
cite="mid:CA+BKNPB9HhBPOnJ3Wtvi2YJWRzZrb7jb7d+2UgeC5AOfvjqJfw@mail.gmail.com">
<div dir="ltr">
<div><br>
</div>
<div>Getting precise DEMs remains a massive struggle, regardless
of what people, govs and products claim.</div>
<div>What is needed is a 'Quality Standard', as well as metadata
(FGDC, ISO) for transparency. </div>
<div>It's as simple as that.</div>
<div>While in QGIS it's not our maintastk, it matters a lot and
for</div>
<div>added value. So the more we can push for it, the better,
and include such tools.</div>
<div><br>
</div>
<div>Why are DEMs from contour lines 'good' ?<br>
Because they come from an era where people worked more precise
and used analoge methods,</div>
<div>e.g. digitizing drums. <br>
</div>
</div>
</blockquote>
You know what they say.... B.S in = B.S out. If the input data is
very precise, then the output will be good. DEMs from contour lines
are far from perfect but they are much better than expected if the
input data is reliable. Building a slope model from that will
definitely expose the problems with the DEM, but the elevations can
be very good. <br>
<blockquote type="cite"
cite="mid:CA+BKNPB9HhBPOnJ3Wtvi2YJWRzZrb7jb7d+2UgeC5AOfvjqJfw@mail.gmail.com">
<div dir="ltr">
<div><br>
</div>
<div>For your own quality control, compute manually slope2 (=2nd
derivative) from a DEM and see what you get.</div>
<div>If you see irregularities, lines and tiles something is
rather odd, caused by the DEM production process.</div>
<div><br>
</div>
<div>Very best</div>
<div> Falk Huettmann</div>
<div><br>
</div>
<div><br>
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<div><br>
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<div><br>
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<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Mon, Dec 16, 2019 at 9:44
AM Nicolas Cadieux <<a
href="mailto:nicolas.cadieux@archeotec.ca"
moz-do-not-send="true">nicolas.cadieux@archeotec.ca</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi,<br>
<br>
I agree Falk that DEMs made from contour lines have their
problems <br>
(stepped slopes, flat areas, elevation drops in lakes...).
Depending on <br>
the input data, they can still be a very accurate model for
elevation. <br>
I tested the SRTM, NASADEMs, Alos, ASTER and the CDED
(Canadian Digital <br>
Elevation Data) in Canada with LiDAR in various environments.
<br>
Surprisingly, The CDED, made from 50k contour line are still
the most <br>
accurate elevation wise.<br>
<br>
Nicolas Cadieux<br>
<br>
On 2019-12-12 5:29 p.m., Falk Huettmann wrote:<br>
> Hi there,<br>
> my comment to that DEM/contour profile question is:<br>
> if you have a contour map, it's usually a pretty poor DEM
and representation<br>
> of reality, e.g. nature has no lines...<br>
> If you then create a raster (back again) from the
contour, it gets circular<br>
> and even worse = pretty odd. A double whammy.<br>
><br>
> So I would never ever recommend doing this, in case the
DEM or Contour data<br>
> are to show any meaningful ground information, or are to
be used for such.<br>
><br>
> That's my view.<br>
> Very best<br>
> Falk Huettmann<br>
><br>
> On 12/12/19, Håvard Tveite <<a
href="mailto:havard.tveite@nmbu.no" target="_blank"
moz-do-not-send="true">havard.tveite@nmbu.no</a>> wrote:<br>
>> On 11.12.2019 15:14, kirk wrote:<br>
>>> Hi Bernd. You will have to create a raster dem
from your contour data.<br>
>>> First extract nodes from lines and ensure the
elevation field is<br>
>>> extracted.<br>
>>> Once you have the point data, you can generate a
raster using a tin or<br>
>>> spline interpretation.<br>
>> You can also create the raster DEM directly from the
contour data<br>
>> (for instance with the "TIN interpolation" algorithm,
using your<br>
>> contour layer as "Structured lines").<br>
>><br>
>> Håvard<br>
>><br>
>>> The resulting raster should work with the plugin.<br>
>>><br>
>>><br>
>>><br>
>>> Sent from my Samsung Galaxy smartphone.<br>
>>><br>
>>> -------- Original message --------<br>
>>> From: Dave Gardiner <<a
href="mailto:2dave.gardiner@gmail.com" target="_blank"
moz-do-not-send="true">2dave.gardiner@gmail.com</a>><br>
>>> Date: 2019-12-10 11:43 PM (GMT-05:00)<br>
>>> To: <a href="mailto:qgis-user@lists.osgeo.org"
target="_blank" moz-do-not-send="true">qgis-user@lists.osgeo.org</a><br>
>>> Subject: [Qgis-user] Working with DEMs<br>
>>><br>
>>> I am trying to create a geological cross section
using the qProf plugin.<br>
>>> This<br>
>>> asks for the source of the DEM on which to base
the elevations. My problem<br>
>>> is my source for the elevation data is vector
layer of contours imported<br>
>>> from a csv file. How do I create a DEM from this
please?<br>
>>><br>
>>><br>
>>><br>
>>> --<br>
>>> Sent from: <a
href="http://osgeo-org.1560.x6.nabble.com/QGIS-User-f4125267.html"
rel="noreferrer" target="_blank" moz-do-not-send="true">http://osgeo-org.1560.x6.nabble.com/QGIS-User-f4125267.html</a><br>
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