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I am quite new QGIS user. I have point data from the field and I generated DEM and contour. However, the contours are a bit unrealistic. I have attached the image. The green contour is the contour generated from DEM generated by TIN interpolation. The maroon and grey contours are what it should be like.

I understand the surveyed points are not quite sufficient at places but that is what I have. The main problem is the contours generated are such that the point from the opposite bank is also considered so the contour goes around from one bank to the other. How can this be fixed?

enter image description here

enter image description here

I tried resampling using Bspline and in a coarse grid of 100 but the contour generated was even worse. Tried doing as suggested in Simplified polygons have overlapping lines by Michael Stimson. But I am confused, is it from the contour we generate the DEM again in higher ressolution?The contour generated after using Bspline and coarse grid of 100 looks even worse. enter image description here enter image description here The green line is the original contour generated and the brown/yellowish is after resampling as mentioned above.

Tried using breakline ( yellow line) for generating contour (TIN interpolation) but the contours generated were not changed. enter image description here I used some of the surveyed points (surveyed points marking HFL) to generate the breakline as Aldo suggested snapping the vertices. The attribute table of the breaklines shows no vertices/ elevation. Can somebody please suggest to me what I did wrong or how should it be done? enter image description here. In this picture the break line is magenta in colour

Usha
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    If you already mention "another Q&A", include a link to it. Also describe what steps from there you followed and where exactly you're stuck. Otherwise, it's difficult to help. If you're asking for "the steps", this is more a question for a tutorial, not a focused question. See [tour] – Babel Dec 03 '23 at 11:31
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    You state that you "saw ... about resampling and all" but not that you've actually resampled (though, technically, it's more of a low-pass smoothing filter than resampling) Our Focused question/Best answer model puts high value on a willingness to make an attempt to solve your problem. We are not a tutorial site, but more of a "fix a broken tutorial" site. – Vince Dec 03 '23 at 14:09
  • Sorry, if my question was misleading. I tried to read many questions and answers regarding contour generation from this website to solve the problem but I couldn't figure it out, maybe it's because I am new to QGIS. – Usha Dec 05 '23 at 04:33
  • https://gis.stackexchange.com/questions/100251/qgis-contours-smoothing-and-cleaning/100303#100303 . In the question, Michael Stimson answered it resampling up to higher resolution and then back to the original as I understand. I wanted to know if this procedure helps me or if there is something else I should do. Also, I am using QGIS 3.22.12 so whenever I try using SAGA it displays "SAGA version 7.8.2 is not officially supported. Algorithms may encounter issues" – Usha Dec 05 '23 at 04:33
  • I actually need good DEM to use in HECRAS – Usha Dec 05 '23 at 04:35
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    Even if you start with a good DEM, bodies of water such as wide rivers or harbours cause the same sorts of contour artifacts as you are finding above. You may find that you need to artificially set the height of the river area in the DEM before you create the contours. – Tom Brennan Dec 05 '23 at 06:25
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    Hi @Usha. Check Interpolation plugin. The non-realistic countour you describe is given by interpolations between points without using breaklines. Identify it and you DEM will improve, especially profiling the river – aldo_tapia Dec 05 '23 at 13:08
  • Also, breaklines aren't so common in GIS. For CAD and topography, is one os the base definitions before building a surface. Maybe this example could relate to you "realistic contour" definition: https://image.slideserve.com/919364/slide16-l.jpg – aldo_tapia Dec 05 '23 at 13:14
  • Hei Vince, I tried low pass smoothing filter , simple filter from SAGA but it didn't help. i tried checking both low pass and high pass but the contour is still in the river – Usha Dec 07 '23 at 11:22

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To avoid those extra interpolations, you need to set breaklines when creating the TIN model:

enter image description here

With that, you can mix the ground survey + river bathymetry. When you use a geodesic GNSS, you can use codes to identify a normal elevation point + breakline points (like left river border, right river border, sloped hill, etc), then is easy to join all breakline points

I don't have a river bathymetry to show the procedure, but I'll use an example of a small reservoir survey:

enter image description here

The red dots are geodesic GNSS data, they are distributed in the reservoir's bottom and top level, plus some extra points (ground points in your case).

Without using breaklines, the output is similar to yours:

enter image description here

In this case, the diagonal contours does not exist, so to get the correct DEM and contours, I added breaklines:

enter image description here

In this screenshot, all points are part of a breakline, this is not the usual case (ground level point are out of this capture). Here, breakpoints represent property limits, top/bottom inner/outer levels of the reservoir and a small channel connected to the reservoir. With this extra information, output contours are better than previous ones:

enter image description here

Same process done in CAD: enter image description here

aldo_tapia
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  • hei Aldo Tapia, – Usha Dec 07 '23 at 11:09
  • Thank you for your response Aldo. I might sound too stupid but I am very new to QGIS so I am not sure about how to create break lines in my case. I mean I need a vector layer and in my case, the elevation of the break line varies as I move along the river. How do I enter the elevation at different points along it? I have found answers to get it from DEM but the DEM itself is not correct and that is what we are creating so I am quite confused. Do I get it from DEM and then manually change it? If so, then it will be difficult to locate the exact point whose elevation I need to change. – Usha Dec 07 '23 at 11:19
  • You can digitalize the breaklines, you no need to add elevation value. I recommend you to activate snap to vertex option (in snapping options) for ease the process. First add GPS data, create breaklines, create TIN and then exoport to DEM – aldo_tapia Dec 07 '23 at 13:53
  • Hei Aldo, I digitized the bank/HFL but when interpolating with it as a breakline it didn't work, I mean the contour generated is the same. I digitized it as you said by snapping the vertices of the points. However when I check the attribute table of the break lines they don't show any vertices, is it not working because of this? – Usha Dec 10 '23 at 06:07
  • I don't know how to add data/excel sheet here or else I could have sent it to you the Aldo. – Usha Dec 11 '23 at 05:52
  • @Usha you can upload the data to google drive, dropbox, wetransfer, one drive or any other hosting service and post the URL here – aldo_tapia Dec 11 '23 at 11:32
  • Hei, The data is here. https://drive.google.com/file/d/1TY0SHal9Q9s6NoJf_pqBB-Y-Dne7ybeF/view?usp=sharing. – Usha Dec 12 '23 at 04:17
  • Hei, Aldo, The data is here. https://drive.google.com/file/d/1TY0SHal9Q9s6NoJf_pqBB-Y-Dne7ybeF/view?usp=sharing. My confusion is, Should the break line 3D polyline? I just drew the line. Did I have to merge the vertices with coordinates and elevation to the breakline layer? I am sorry if I am asking a basic thing but based on the information I see, the elevation of the vertices, they gather from the raster, and in my case that raster is not correct and that is why I am trying to draw the break line. Can you please kindly guide me on this? Thank you – Usha Dec 12 '23 at 04:23