artifacts in SRTM contour layers processed with GDAL tools and rendered with mapnik

Here is my workflow for importing individual SRTM tiles into postgis using the GDAL utils and rendering a png using mapnik:

1. gdal_fillnodata.py -srcnodata -32768
2. gdal_contour -i 10 -a ele -snodata -32768
3. ogr2ogr -f PostgreSQL -nln srtm3 -append -t_srs ‘EPSG:900913′
4. (some SQL to split the data into 10m, 50m and 250m tables)
5. nik2img.py [stylesheet] [file] –srs 900913 -d [width] [height] –no-open -f png -v –center [center] –zoom [zoom]“

Most of the time this works perfectly. However, large areas with mountainous terrain are often completely distorted:

Huge swaths of landscape are covered with strange (often linear) patterns that shouldn’t be there. No way the SRTM data is that messed up. What am I doing wrong? How can I avoid this? Ideally there should be a command-line solution. Thanks in advance!