[Qgis-user] Working with DEMs

Mon Dec 16 15:32:55 PST 2019

Hi

On 2019-12-16 5:36 p.m., Falk Huettmann wrote:
> 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).
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.
>
> 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.
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.
>
> 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 <mailto: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
>     <mailto: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
>     <mailto:2dave.gardiner at gmail.com>>
>     >>> Date: 2019-12-10 11:43 PM (GMT-05:00)
>     >>> To: qgis-user at lists.osgeo.org <mailto: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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