gdal_rasterize with -add introduces additional values in the output

[ricardogsilva]

Expected behavior and actual behavior.

I'm trying to use gdal_rasterize with a bunch of linestrings as input. I want the output raster to be burned with the sum of each linestring and I also want it to be burned on all pixels that touch the linestrings. I am using the following incantation:

 gdal_rasterize \
    -burn 1 \
    -at \
    -add \
    -tr 1000 1000 \
    -ot Int16 \
    ~/test_rasterization.gpkg ~/burn_test_gdal_rasterize.tif

The outcome does not match what I expected. Here is a picture of the files loaded in QGIS:

In the image above, the red line represents my vector linestring. The raster is shown below the line. I would expect the raster to be made of only the values 0 and 1. However, I see that the lighter gray areas have the value 2. Since a single feature has been burned, it seems to me that the value 2 should never be a part of the raster.

Steps to reproduce the problem.

• Download the sample geopackage I've put on google drive:

  https://drive.google.com/file/d/1xE3miIrA6oatvukcF3U060Uua5NTQq33/view?usp=sharing

  Or make a simple vector linestring.

• rungdal_rasterize with the same parameters as shown above

Operating system

Ubuntu 18.04 64 bit

GDAL version and provenance

2.2.3 from the main ubuntu repos