- from menu: Raster / Extraction / Contour
- define output name path/to/output.shp
- alter GDAL call for single contour line at 900 m asl:
gdal_contour -fl 900 "path/to/dem_raster.asc" "path/to/output.shp"
- for removing small poplygons or lines add area or length field (attr table / field calc or vector / geometry / add geometry)
- query by length or area to deselect unwanted iso-lines
Finally, you can export the contours as KML and check it in Google Earth:
I downloaded my DEM-rasters (Nord-Tirol, Österreich) HERE