0

I have generated contours in QGIS using point data (AutoCAD file) and I had to also smoothen the contours and remove noises (smaller contours) from the contour generated. I need to get this contour data or DEM for steady flow analysis in HEC-RAS. If I am not wrong HEC-RAS needs DEM (TIFF file) for using RASMAPPER. I, therefore, need to convert this smoothened contour I vector to a TIFF file. How can I do this?

I have seen suggestions regarding TIN interpolation, using GRASS, etc. While using TIN interpolation it just hangs if I do Clough toucher interpolation while linear interpolation gives a result but if I extract contour from it, it's different than the original contour. I am a bit confused if I use contour then should the under "type" should it be a point or breakline or structure? Because when I Used structure or break lines, it didn't work. As suggested in many other answers I tried to use grass. The main problem with GRASS for me is to set the region. It allows only rectangular region but the contour that I have does not fit in a rectangle (I mean if I select rectangle there will be empty areas as the river is meandering). This led to unrealistic contour in those areas. Does anyone have any idea how to select the "Region" in such a case? However, My main issue is to be able to get the smoothened contour vector to a format that can be used in the RAS-mapper. I am new to both QGIS and HEC-RAS.

3
  • 2
    Generally speaking, you don't want to model using the raster generated from a generalized contour, so it seems that you are "cruising for a bruising". Use the points to generate a TIN, and make a DEM from that.
    – Vince
    Dec 9, 2022 at 12:48
  • Yes, I Got DEM from that but when I used it in HEC-RAS, it showed cross-sections with three different elevations at the same location which HECRAS interpreted as vertical wall without width. It occurred in so many locations and almost all cross sections. So I thought it was the result of the extremely curly contours and smaller contours that DEM produced when I generated contour from it. So I tried smoothening it and removing contours of shorter length and then generating the DEM to use in HEC-RAS. Do you have any idea?
    – Usha
    Dec 10, 2022 at 16:06
  • This seems to be an XY Problem. You are asking a Question about a flawed solution instead of the root problem. "Curly" contours are a different issue entirely. and can be addressed by a low-pass filter.
    – Vince
    Dec 10, 2022 at 16:30

1 Answer 1

0

Adding to @Vince 's comment: you do not want to go round about making contours from points, then a DEM from contours. Interpolate the points directly to create your DEM. In GRASS GIS this can be done using the v.surf.rst module that uses a spline interpolation algorithm. If your point data is very dense, you can also use the faster v.surf.idw that implements Inverse Distance Weighting.

In either case you'll have to consider carefully what final resolution to choose for your interpolation. This will depend on the density of your point data and the size of your region.

7
  • Hi Micha, I understand that it's not a nice thing to do but when I used the DEM generated from points in HEC-RAS, the cross sections had issues while running steady flow analysis. It showed error-like cross-sections with three different elevations at the same location which HEC-RAS interpreted as a vertical wall without width. It occurred in so many locations and in almost all cross sections so.
    – Usha
    Dec 10, 2022 at 16:35
  • I thought it was the result of the extremely curly contours and smaller contours which I could see on extracting contours from the DEM generated with points. So I tried smoothening it and removing contours of shorter length and then going the reverse way of generating DEM from the contour. I had been using TIN interpolation for generating the DEM. Do you think if I use GRASS, the contours will be better? ( I mean using the point data).
    – Usha
    Dec 10, 2022 at 16:36
  • Also while using grass you have to create a "Region" right? As I mentioned in my question, as my river is meandering and I have data around it, I cannot select a rectangular region( as it will have blank areas). Do you have any idea how can i solve this problem?
    – Usha
    Dec 10, 2022 at 16:36
  • It's not a question of "nice thing to do". When you create contours, then a DEM you will always get poorer, less accurate results than directly interpolating the points.
    – Micha
    Dec 10, 2022 at 19:19
  • I don't get how your DEM had three different elevations at the same location. That doesn't really make sense. Can you post some of the points, and the code you used??
    – Micha
    Dec 10, 2022 at 19:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.