Contents
Why are there no data values in QGIS?
This layer contains values of accumulated flow, a hydrological parameter. It contains those values only within the area of a given watershed, with no–data values outside of it. As you can see, the rendering is not very informative, due to the way values are distributed.
Why is catchment area not evenly distributed in QGIS?
To know why, you can have a look at the histogram and you will see that values are not evenly distributed (there are a few cells with very high value, those corresponding to the channel network). Calculating the logarithm of the catchment area value yields a layer that conveys much more information (you can do it using the raster calculator).
What happens if you increase initiation threshold in QGIS?
If you increase the Initiation threshold value, you will get a more sparse channel network. If you decrease it, you will get a denser one. With the proposed value, this is what you get. The image above shows just the resulting vector layer and the DEM, but there should be also a raster one with the same channel network.
What do the NaN values in GDAL mean?
The GDAL driver (v.1.9.1) works great, and valid data is visible in software that depends on GDAL (e.g., QGIS). The nan values are used to represent NoData.
How to run no data calculator in QGIS?
Select the DEM in the layers field and type a * 3.28084 in the formula field. For non English users: use always ”.”, not ”,”. Click Run to run the algorithm. You will get a layer that has the same appearance of the input layer, but with different values.
How to calculate the no data value inside a basin?
What we are doing in the first part of the formula here is to divide the accumulated flow layer by itself, which will result in a value of 1 inside the basin, and a no–data value outside. Then we multiply by the DEM, to get the elevation value in those cells inside the basin ( DEM * 1 = DEM) and the no–data value outside ( DEM * no_data = no_data)
How to calculate no data values in raster calculator?
It contains those values only within the area of a given watershed, with no–data values outside of it. As you can see, the rendering is not very informative, due to the way values are distributed. Using the logarithm of that flow accumulation will yield a much more informative representation. We can calculate that using the raster calculator.