Eliminate small polygons from shapefile

Question

I downloaded GTOPO30 *.DEM files, and extracted a contour line at 2000m altitude using gdal_contour. In the resulting shapefile, I have many small polygons in areas where the altitude is close to that contour line, and the image looks quite noisy.

How can I, preferably using gdal/ogr or QGis, somehow 'downsample' these contours, so that the picture looks cleaner? I basically want to eliminate all small features.

PS: One way would be to downsample the grid before doing the contour extraction; is this the way to go, or should i rather downsample the shapefile?

Answer 1

Actually, I solved this using a combination of GDAL and QGis:

with GDAL:

Merge the different GTopo30 tiles into one file

gdal_merge.py -o gtopo30_usa.tif *.DEM

Down-sample this merged file to 0.1°

gdalwarp gtopo30_usa.tif -tr 0.1 0.1 gtopo30_usa_downsampled.tif

Extract the contour line

gdal_contour -fl 2000 gtopo30_usa_downsampled.tif gtopo30_usa_contour2000.shp

in QGis:

• create new project
• add vector layer, select the gtopo30_usa_contour2000.shp shapefile
• Vector->Geometry->Lines to Polygons
• Vector->Geometry->Add Geometry Columns
• Layer->Query-> select an area threashold

Answer 2

When you say "small polygons" I assume they are actually LINESTRING features that form closed boundaries? THese are probably the contours encircling peaks that are just above 2000 m.

A possible solution is to import the shapefile into GRASS, and use the module v.edit tool=delete where="len< (some threshold) So the steps would be, after you have created a suitable GRASS Location and Mapset:

#Import the shapefile
v.in.ogr -e dsn=contours.shp out=contours_2000 type=line
# Create attrib table and add line lengths
v.db.addcol contours_2000 col="len double precision"
v.to.db contours_2000 option=length units=me col=len
# Now v.edit to delete all lines shorter than some threshold
# say 5000 meters
v.edit contours_2000 tool=delete where="len<5000"
# and export back out to shapefile
v.out.ogr contours_2000 dsn=contours_clean.shp

HTH