Gdal/QGIS resampling asc file

Question

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 1

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