qgis
Using ' Raster -> Extract -> Raster Cut ' in QGIS and 'Nodata = 0', I cut the black part of the Landsat8 image as shown below.
However, when I saved it as "tif file", it seems that the black area is still there.
So I tried to put the tif file in Google Earth, and it appeared as shown below.
I want to save and show only the image that I cut. Please give me some advice.
You should not be getting null or empty values. How exactly did you try to calculate TWI? In SAGA, module Terrain Analysis - Hydrology -> SAGA Wetness Index should work, so long as all of your input files are correct. i.e slope needs to be in radians, what about sinks?, drainage networks need to be correct, DEM needs to be projected (good discussion on projections here), etc...
The general equation:
TWI/CTI = ln(a / (tan (beta))
where; a=Upland contributing area: [(flow accumulation + 1 ) * (cellsize)]
beta=slope in radians.
References:
Moore, ID., P.E. Gessler, G.A. Nielsen, and G.A. Petersen (1993) Terrain attributes: estimation methods and scale effects. In Modeling Change in Environmental Systems, edited by A.J. Jakeman M.B. Beck and M. McAleer Wiley , London, pp. 189 - 214.
Gessler, P.E., I.D. Moore, N.J. McKenzie, and P.J. Ryan. (1995). Soil-landscape modeling and spatial prediction of soil attributes. International Journal of GIS. 9(4):421-432.
For rough build up areas:
For the example image the result processed by python script looks like this:
Some graphics and CAD software has good tools to convert technical drawings lines to polylines and can be scriptable but it is far from perfect and needs much additional manual work:
At the end for these kind of tasks if you need good quality vector polygons you need to do that manually.
Best Answer
As Vince's comment suggests, the best way is to make the
Nodatavalues transparent. If you can't do that, you may try the following:You can cut out a part of the image from the full scene (in this case you will lose some of the image pixels that are not equal to
Nodata).You can pick up such a projection so that the image is no longer rotated, then save the image (in practice, picking up such a projection is difficult or almost impossible).
In any raster editor rotate the image and crop the black borders (the geographical reference will be lost).