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I have derived TWI using SAGA GIS, Have a look on the center of the image, a vertical line goes through it from top to bottom. Anyone else confronted this issue? TWI Raster

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To me it looks like a common error that occurs when multiple DEM tiles are mosaiced. This is particularly common if nearest-neighbour resampling is used for the mosaicing rather than a bilinear or cubic-convolution resampling scheme. Nearest-neighbour resampling is faster than either alternative, but should really only be used in the case of categorical data and not continuous data such as a DEM. Often these mosaicing errors are not apparent in the DEM but are only visible in data derived from the DEM, e.g. slope, aspect, curvature, TWI, etc. The first thing I'd do is check and see if the same line is apparent in an aspect/slope raster. If so, then it's the result of the DEM creation and not the TWI tool.

That said, more troubling to me than the vertical line is the fact that you have extreme discontinuities in TWI, i.e. the contouring effect apparent in the blue-orange transitions in your raster. These are often coincident in your image with what appear to be drainage divides. This shouldn't be the case for TWI. At first, I thought that this was an artifact of your palette, but the fact that the discontinuities occur at divides suggests otherwise. For reference, this is what a typical TWI image should look like:

TWI Image

Notice how it looks like a continuous surface with no localized breaks or discontinuities. The raster should take on similarly low values on either sides of divides/ridges.

The problem with your raster is most likely the result of the contributing area raster (TWI is a combination of slope and contributing area). It's highly probable that the topographic depressions (i.e. sinks) have not been properly removed from the DEM prior to calculating contributing area/TWI.

  • I am thankful for yours insight, it will help me to resolve the issue, I had filled sinks but didn't used resampling technique robust for continuous datasets. Surely, you have rightly pointed out. Thanks. – Ben Mar 20 '16 at 13:31
  • I have faced problems to derive TWI in various software, many others too. It will be very helpful for all of us if you could refer us to any specific tutorial or do manage to write (Snap) steps preferably in WhiteBox GIS software so everyone could produce acceptable results. It will be a great privilege for all of us. – Ben Mar 20 '16 at 13:54
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    @Nasir, Sure, I can write a tutorial on the creation of TWI rasters using Whitebox GAT when I get a free moment. I'll post a link to it here when I'm done. – WhiteboxDev Mar 20 '16 at 13:57
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    Sir this is really great privilege for us. This will help us to 'Do Right thing in Right Way', Especially for me, I tried to get acceptable results in ArcMap too but it didn't help me either. gis.stackexchange.com/questions/184998/… Getting TWI is common in many applications, therefore, this will be a great contribution. – Ben Mar 20 '16 at 14:03
  • I have resolved this problem by modifying above expression as; Ln(("Flow_acc"+ 1 ) * 30/"Tan_SlopeRad") There was nothing wrong with the Process Flow Diagram inserted above, however, I removed the 'Sinks' from the flow direction raster using WhiteBox and exported it in ArcGIS and use it to produce Flow Accumulation Raster. Although in ArcGIS, DEM was earlier pre-processed and all depression were removed by 'Filling' these imperfections using 'Fill' tool. However, now there are no anomalies in the derived raster and no cells contain 'No Data' values. – Ben Mar 23 '16 at 10:55

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