[Qgis-user] Working with DEMs

[Falk Huettmann]

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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