Defining river boundaries so that TIN interpolates within those boundaries

Question

I am a new QGIS user just learning the product. As part of our works we take depth soundings around river bends. I have worked out how to do basic contours with colour bands in the background using TIN interpolation.

This works for straight sections of the river, however for areas where there are bends on the river, the interpolation does not keep within the boundaries of that bend, and interpolates straight across. I am looking for a way to keep the interpolation within the bounds of the river.

The process was as follow:

1. Enter the cooirdinates and depth, lat,long, depth, as CSV file

2. Open Street Map as Layer, where the points of the soundings show. Would after this go into layer properties of the CSV file, symbology, and reduce the size to zero, to get the dots to disappear

3. Do a TIN Interpolation, from the Interpolation group, on the toolbox menu

The resulting product as follows. As can be seen, it is not interpolating within the river boundaries. From this point can then extract basic contours, with the same result, not staying within the river boundaries. I understand QGIS can be vast, and there may be a simple way to do this, or I may require to do some further training to learn how to do this. Any advice?

Answer 1

Download OSM data from a rectangle selection using the OSM Downloader plugin:

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Turn off all features from the osm layers, except of the Natural - Water tag from osm multipolygons layer. Select the river geometry and export the selected features to a new datasource:

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Use the Split features from the Advanced digitizing toolbar to split the river around your area of interest:

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Select the recent created feature, invert the selection and erase all other selected features of the layer:

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Create a 0.0005 degrees (I don't like to do that, but I don't want to reproject the layer) buffer to the river:

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Densify by interval the buffered layer, with an interval of 0.0005 degrees (Idem, degrees are not a distance, but let it be):

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Extract the vertices of the densified layer:

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And save them, including a Z dimension in the destination datasource, and excluding the fid field (because it is not unique right now):

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The Z value of the outsides will be set to zero, I agree with that. You can Set other Z value for them, also you can use the geomnearest() function from refFunctions plugin to set the Z of the nearest depth point.

Interpolate them:

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Lastly, Clip the interpolated layer by the river mask layer:

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The result, is a digital depths model adjusted to the river boundaries: