Manipulating DEM

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Manipulating DEM

Ulrich Kriechbaum
Dear all!
 
I investigate a Glacial-Lake Outburst Flood and try to back-calculate the event using the simulation tool r.avaflow. During the event there was a big trench eroded. I have a post-event DEM with a clearly visible eroded trench. What i want to do now is to "flatten" out the trench so that i get a pre-event DEM to start my simulation. What i tried to do is to create the contour lines of the post-event DEM, change them so that they would not show the trench and then create a pre-event DEM out of this contour lines. I already asked the question, if anyone knows how to do that, i already got some nice answers but unfortunately none of the inputs really work for me....
 
So now to my actual question: Does anyone know another method how to "flatten" out the trench in the DEM? I would be really glad if someone would have an answer, as i have ben struggling a while now with these problem..
 
Best regards,
 
Ulli

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Re: Manipulating DEM

berndv.

Hi,

I do not know any alternative approach, but I would like to know what exactly did not work out with your "faked" DEM so far.


Cheers,

Bernd


Am 08.11.18 um 14:49 schrieb Ulrich Kriechbaum:
Dear all!
 
I investigate a Glacial-Lake Outburst Flood and try to back-calculate the event using the simulation tool r.avaflow. During the event there was a big trench eroded. I have a post-event DEM with a clearly visible eroded trench. What i want to do now is to "flatten" out the trench so that i get a pre-event DEM to start my simulation. What i tried to do is to create the contour lines of the post-event DEM, change them so that they would not show the trench and then create a pre-event DEM out of this contour lines. I already asked the question, if anyone knows how to do that, i already got some nice answers but unfortunately none of the inputs really work for me....
 
So now to my actual question: Does anyone know another method how to "flatten" out the trench in the DEM? I would be really glad if someone would have an answer, as i have ben struggling a while now with these problem..
 
Best regards,
 
Ulli

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Re: Manipulating DEM

Kirk Schmidt
In reply to this post by Ulrich Kriechbaum

Hi Ulli:

If I understand your project correctly, you only have the post DEM and you want to remove the new trench so that you can perform some additional analysis.  SO, off the top of my head, I would digitize a polygon whoch defines the extent of the trench.  I would use this polygon to erase the points derived from the contour lines and then create your DEM from the resulting pointg layer without elevations inside the trench.  You can try different techniques for creating the dem using the saga griding functions within the processing toolbox.  By using a TIN approach, the result may be a bit blocky.  If yopu try a B-spline interpretation, the results will be smoother, however individual elevations are interpreted along a spline and therefore may over or underestimate the actual elevations.

Hope this helps


Kirk

On 11/8/2018 9:49 AM, Ulrich Kriechbaum wrote:
Dear all!
 
I investigate a Glacial-Lake Outburst Flood and try to back-calculate the event using the simulation tool r.avaflow. During the event there was a big trench eroded. I have a post-event DEM with a clearly visible eroded trench. What i want to do now is to "flatten" out the trench so that i get a pre-event DEM to start my simulation. What i tried to do is to create the contour lines of the post-event DEM, change them so that they would not show the trench and then create a pre-event DEM out of this contour lines. I already asked the question, if anyone knows how to do that, i already got some nice answers but unfortunately none of the inputs really work for me....
 
So now to my actual question: Does anyone know another method how to "flatten" out the trench in the DEM? I would be really glad if someone would have an answer, as i have ben struggling a while now with these problem..
 
Best regards,
 
Ulli

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-- 
Kirk Schmidt, MScF, BScF, RPF
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Nortek Resource Solutions Inc.
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B0K 1W0
Tel (902) 922.3607
Email: [hidden email]
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Re: Manipulating DEM

Nicolas Cadieux
In reply to this post by Ulrich Kriechbaum

Hi,

This is where a little fill-in brush tool "À la Photoshop" would be nice.  On a more serious note, you could  isolate the area and use a flood model to fill it in. Then use raster math to stick it all back together.  Posting a picture of you problem could help. 

Nicolas


On 2018-11-08 8:49 AM, Ulrich Kriechbaum wrote:
Dear all!
 
I investigate a Glacial-Lake Outburst Flood and try to back-calculate the event using the simulation tool r.avaflow. During the event there was a big trench eroded. I have a post-event DEM with a clearly visible eroded trench. What i want to do now is to "flatten" out the trench so that i get a pre-event DEM to start my simulation. What i tried to do is to create the contour lines of the post-event DEM, change them so that they would not show the trench and then create a pre-event DEM out of this contour lines. I already asked the question, if anyone knows how to do that, i already got some nice answers but unfortunately none of the inputs really work for me....
 
So now to my actual question: Does anyone know another method how to "flatten" out the trench in the DEM? I would be really glad if someone would have an answer, as i have ben struggling a while now with these problem..
 
Best regards,
 
Ulli


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Re: Manipulating DEM

Rick Rupp
In reply to this post by Ulrich Kriechbaum

Hi Ulli,

If the trench is narrow (in terms of pixels), you can run a smoothing filter over that part of the DEM. Then replace the original trench pixels with those from the smoothed DEM using a conditional statement (gdal_calc with a ‘where’ statement).

Regards, Rick

 


From: Qgis-user <[hidden email]> on behalf of Ulrich Kriechbaum <[hidden email]>
Sent: Thursday, November 8, 2018 5:49:12 AM
To: [hidden email]
Subject: [Qgis-user] Manipulating DEM
 
Dear all!
 
I investigate a Glacial-Lake Outburst Flood and try to back-calculate the event using the simulation tool r.avaflow. During the event there was a big trench eroded. I have a post-event DEM with a clearly visible eroded trench. What i want to do now is to "flatten" out the trench so that i get a pre-event DEM to start my simulation. What i tried to do is to create the contour lines of the post-event DEM, change them so that they would not show the trench and then create a pre-event DEM out of this contour lines. I already asked the question, if anyone knows how to do that, i already got some nice answers but unfortunately none of the inputs really work for me....
 
So now to my actual question: Does anyone know another method how to "flatten" out the trench in the DEM? I would be really glad if someone would have an answer, as i have ben struggling a while now with these problem..
 
Best regards,
 
Ulli

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