New answers tagged gdalwarp
You can use gdal_merge which will combine the rasters. Have a look at the description of gdal_merge: This utility will automatically mosaic a set of images. All the images must be in the same coordinate system and have a matching number of bands, but they may be overlapping, and at different resolutions. In areas of overlap, the last image will be copied ...
Creating a mosaic from your source images takes only a few seconds if you use GDAL virtual raster as output. Read http://www.gdal.org/gdal_vrttut.html and http://www.gdal.org/gdalbuildvrt.html Often the artifacts like you have can also be avoided by taking care that the individually warped images are aligned to use a common canvas. This can be achieved by ...
Your zero values occur where you have no data (warping distorts the images and an output image must be rectangular, hence the noData pixels). You can force the output NoData value using -dstnodata In your case, -dstnodata None would create undefined NoData that will not be included in the range.
The source map comes with paletted colours, but only 12 are used. So I have set the nodata value to 255 in the clipper dialogue, and the nodata areas are not black, nor are the black parts set to nodata: If you intend to combine the rasters to a vrt, it is better to expand them to rgba (but only after you applied the nodata value).
If you have a polygon that represents the area you can use that as a Mask in QGIS. If you do not, you can create a polygon of the area you want. You could also type the coordinates into the Clipper tool in QGIS to ensure you do not clip into the black area (this will require you to analyze your quad to ensure you are using the best coordinates). Another ...
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