Gdal/QGIS resampling asc file

I have a DEM (.asc) file with a resolution of 1000 meters per pixel. Therefore the file has a lot of columns and rows. I would like to chance the resolution per pixel from 1000 to 3000 meters per pixel. So i would like to resample it.

which gdal command do i use for this? i know it's gdalwarp but how is the exact command line with the extra options?

if you know how to do it in QGIS I'm also happy with it.

Answer

The task could feel trivial by reading the gdalwarp documentation http://www.gdal.org/gdalwarp.html and GDAL AAIGrid -- Arc/Info ASCII Grid driver documentation http://www.gdal.org/frmt_various.html. The target pixel size is three times bigger than the native resolution 0.008333333333 degrees/pixel (not 1000 m/pixel, see the comments).

gdalwarp -of AAIGrid -tr 0.024999 0.024999 input.asc output.asc

However, it is a bit more difficult than that.

• gdalwarp does not support AAIGrid format as direct output format


• The default resampling method of gdalwarp is nearest neighbor which does not suit well for DEM

Therefore the conversion must be done in two steps and with a better resampling method.

First step is to create an interim output as GDAL Virtual raster (.VRT) with "average" resampling

gdalwarp -of VRT -r average -tr 0.024999 0.024999 input.asc interim.vrt

Second step is to convert the interim DEM into new ASCII Grid file with gdal_translate

gdal_translate -of AAIGrid interim.vrt output.asc

Note:

The formats which are supported for output can be listed with gdalwarp --formats command. Formats which support direct output are marked with "+" character. However, all of them will not really work (NTv2 Datum Grid Shift file for example).

On Windows

gdalwarp --formats|find "+"
  FITS -raster- (rw+): Flexible Image Transport System
  HDF4Image -raster- (rw+): HDF4 Dataset

  netCDF -raster- (rw+s): Network Common Data Format
  VRT -raster- (rw+v): Virtual Raster
  GTiff -raster- (rw+vs): GeoTIFF
  NITF -raster- (rw+vs): National Imagery Transmission Format
  HFA -raster- (rw+v): Erdas Imagine Images (.img)
  ELAS -raster- (rw+v): ELAS
  MEM -raster- (rw+): In Memory Raster
  BMP -raster- (rw+v): MS Windows Device Independent Bitmap
  PCIDSK -raster,vector- (rw+v): PCIDSK Database File
  ILWIS -raster- (rw+v): ILWIS Raster Map

  SGI -raster- (rw+): SGI Image File Format 1.0
  Leveller -raster- (rw+): Leveller heightfield
  Terragen -raster- (rw+): Terragen heightfield
  ISIS2 -raster- (rw+v): USGS Astrogeology ISIS cube (Version 2)
  ERS -raster- (rw+v): ERMapper .ers Labelled
  RMF -raster- (rw+v): Raster Matrix Format
  RST -raster- (rw+v): Idrisi Raster A.1
  INGR -raster- (rw+v): Intergraph Raster
  GSBG -raster- (rw+v): Golden Software Binary Grid (.grd)
  GS7BG -raster- (rw+v): Golden Software 7 Binary Grid (.grd)

  PNM -raster- (rw+v): Portable Pixmap Format (netpbm)
  ENVI -raster- (rw+v): ENVI .hdr Labelled
  EHdr -raster- (rw+v): ESRI .hdr Labelled
  PAux -raster- (rw+): PCI .aux Labelled
  MFF -raster- (rw+): Vexcel MFF Raster
  MFF2 -raster- (rw+): Vexcel MFF2 (HKV) Raster
  BT -raster- (rw+v): VTP .bt (Binary Terrain) 1.3 Format
  LAN -raster- (rw+v): Erdas .LAN/.GIS
  IDA -raster- (rw+v): Image Data and Analysis
  GTX -raster- (rw+v): NOAA Vertical Datum .GTX

  NTv2 -raster- (rw+vs): NTv2 Datum Grid Shift
  CTable2 -raster- (rw+v): CTable2 Datum Grid Shift
  KRO -raster- (rw+v): KOLOR Raw
  ADRG -raster- (rw+vs): ARC Digitized Raster Graphics
  SAGA -raster- (rw+v): SAGA GIS Binary Grid (.sdat)
  PDF -raster,vector- (rw+vs): Geospatial PDF