[Qgis-user] Working with DEMs
Fernando M. Roxo da Motta
petro at roxo.org
Tue Dec 17 09:05:20 PST 2019
On Mon, 16 Dec 2019 13:36:26 -0900, Falk Huettmann
<fhuettmann at alaska.edu> wrote:
Hi all,
Just a bunch of lose thought, or kinda brain storm, or just a lot of
BS. You judge it.
Ok, I think that whatever the measurement system (analogue or digital)
whenever you convert it to a sampled discrete measurement it start
having all sort of characteristic of a discrete data, that is band
limitation (spacial and/or temporal), possible alias with
misrepresentation of the measured object and so far.
In a DEM we have that each pixel represents (possibly) an average of
the measured quantity, that is the elevation in this case. The
average seems to be a good high cut filter, at least good enough to
avoid aliasing. Perhaps it can show up in certain border case, like a
big cliff or very hilly areas, but I assume it could be god enough in
almost all cases.
When one extract the contour lines, [s]he is in reality subsampling
an already sampled data, that is making a coarser sampling, with bigger
sampling interval than the original data. In order to provide a good
quality the contour lines must be a representation of a even smoother
DEM, otherwise it would be aliased.
Provided that all sampling process meet the requirements to avoid
aliasing, that is a misrepresentation of the measurements, the
reconstruction of DEM from the contour lines, according to Nyquist
Theorem, should be possible within the limits of the maximum frequency
(or minimum wavelength) to the sampling rate.
Based on the above loose thoughts, I would say that the obtained DEM
from the contour lines should have a lower spacial frequency, that is
details, than the original DEM. Of course the difference may be
dependent os the sampling rate of the original DEM. For example, a DEM
sampled at 100m sampling rate should contain a smaller frequency band
(detail) than one sampled at 30m.
Hope not be too clueless about the subject and be of some help in
understand the challenge on the subject matter.
Cheers.
> Hi there,
>
> thanks,
> just to elaborate, for those who are interested:
>
> (I would like to see the study that has compared those DEM products;
> it matters a lot for climate models, for instance; happy to read the
> citations).
>
> Getting precise DEMs remains a massive struggle, regardless of what
> people, govs and products claim.
> What is needed is a 'Quality Standard', as well as metadata (FGDC,
> ISO) for transparency.
> It's as simple as that.
> While in QGIS it's not our maintastk, it matters a lot and for
> added value. So the more we can push for it, the better, and
> include such tools.
>
> Why are DEMs from contour lines 'good' ?
> Because they come from an era where people worked more precise and
> used analoge methods,
> e.g. digitizing drums.
>
> For your own quality control, compute manually slope2 (=2nd
> derivative) from a DEM and see what you get.
> If you see irregularities, lines and tiles something is rather odd,
> caused by the DEM production process.
>
> Very best
> Falk Huettmann
>
>
>
>
>
>
>
>
>
> On Mon, Dec 16, 2019 at 9:44 AM Nicolas Cadieux <
> nicolas.cadieux at archeotec.ca> wrote:
>
> > Hi,
> >
> > I agree Falk that DEMs made from contour lines have their problems
> > (stepped slopes, flat areas, elevation drops in lakes...).
> > Depending on the input data, they can still be a very accurate
> > model for elevation. I tested the SRTM, NASADEMs, Alos, ASTER and
> > the CDED (Canadian Digital Elevation Data) in Canada with LiDAR in
> > various environments. Surprisingly, The CDED, made from 50k contour
> > line are still the most accurate elevation wise.
> >
> > Nicolas Cadieux
> >
> > On 2019-12-12 5:29 p.m., Falk Huettmann wrote:
> > > Hi there,
> > > my comment to that DEM/contour profile question is:
> > > if you have a contour map, it's usually a pretty poor DEM and
> > representation
> > > of reality, e.g. nature has no lines...
> > > If you then create a raster (back again) from the contour, it
> > > gets
> > circular
> > > and even worse = pretty odd. A double whammy.
> > >
> > > So I would never ever recommend doing this, in case the DEM or
> > > Contour
> > data
> > > are to show any meaningful ground information, or are to be used
> > > for
> > such.
> > >
> > > That's my view.
> > > Very best
> > > Falk Huettmann
> > >
> > > On 12/12/19, Håvard Tveite <havard.tveite at nmbu.no> wrote:
> > >> On 11.12.2019 15:14, kirk wrote:
> > >>> Hi Bernd. You will have to create a raster dem from your
> > >>> contour data. First extract nodes from lines and ensure the
> > >>> elevation field is extracted.
> > >>> Once you have the point data, you can generate a raster using a
> > >>> tin or spline interpretation.
> > >> You can also create the raster DEM directly from the contour data
> > >> (for instance with the "TIN interpolation" algorithm, using your
> > >> contour layer as "Structured lines").
> > >>
> > >> Håvard
> > >>
> > >>> The resulting raster should work with the plugin.
> > >>>
> > >>>
> > >>>
> > >>> Sent from my Samsung Galaxy smartphone.
> > >>>
> > >>> -------- Original message --------
> > >>> From: Dave Gardiner <2dave.gardiner at gmail.com>
> > >>> Date: 2019-12-10 11:43 PM (GMT-05:00)
> > >>> To: qgis-user at lists.osgeo.org
> > >>> Subject: [Qgis-user] Working with DEMs
> > >>>
> > >>> I am trying to create a geological cross section using the
> > >>> qProf
> > plugin.
> > >>> This
> > >>> asks for the source of the DEM on which to base the elevations.
> > >>> My
> > problem
> > >>> is my source for the elevation data is vector layer of
> > >>> contours
> > imported
> > >>> from a csv file. How do I create a DEM from this please?
> > >>>
> > >>>
> > >>>
> > >>> --
> > >>> Sent from:
> > >>> http://osgeo-org.1560.x6.nabble.com/QGIS-User-f4125267.html
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Roxo
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Fernando M. Roxo da Motta <petro at roxo.org> | Editor?
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