I want to use python to operate a seriel remote sensing images.Though the gdal official site http://www.gdal.org/gdalwarp.html show a explaination, it's hard to apply because no grammar tutorial for each command.
For instance, this demo:
"gdalwarp -t_srs '+proj=utm +zone=11 +datum=WGS84 " -overwrite raw_spot.tif utm11.tif".
What's the means of code "+proj=utm +zone=11 +datum=WGS84"? Are there any practical examples?
There are several ways to describe the coordinate system using gdalwarp. In the example, it uses the proj.4 nomenclature, but you could also use the EPSG code or the name of a .prj file.
the proj.4 code in the example includes information about the projection type (utm= Universal Transverse Mercator projection), the projection parameter (with UTM, giving the zone number - 11 - is sufficient to describe the projection, but usually you need more parameters) and the geographic coordinate system (datum) used as a base for the projection (WGS84 is a standard GCS, so it is sufficient to provide its name alone).
In practice, I usually copy-paste the string from a spatialreference.org and modify some of the parameters from a similar projection if needed. Most of the world's projection are available on this site (typing EPSG + name of the projection is another way to quickly find the string that you need).
Note that it is recommended to specify both source and target references for accurate results.
The link in description is really specific. -t_srs is target CRS or coordinate reference system. Also, includes description in each command:
-t_srs srs_def: target spatial reference set. The coordinate systems that can be passed are anything supported by the OGRSpatialReference.SetFromUserInput() call, which includes EPSG PCS and GCSes (i.e. EPSG:4296), PROJ.4 declarations (as above), or the name of a .prj file containing well known text.
By the other hand, code is Proj.4 notation of WGS 84 UTM 11N. SpatialReference.org is a excellent repository of all kind of CRS.
"+proj=utm +zone=11 +datum=WGS84"
In my opinion, you do not need a GDAL tutorial, you need a strong background of GIS. You can read Open Source Geospatial Tools (book) to understand how GDAL or OGR works, or check this good introduction to GDAL processing.
That string is a proj4 set of parameters. PROJ.4 is the library that lots of tools use to project from one coordinate system to another - it's used by QGIS, Grass, SAGA and many others.
The proj4 documentation has been improved recently (since I last looked, anyway!), and gives a lot of detail about the parameters
This is worth a look even if you're familiar with Proj4 strings! I just noticed that you can reverse axis directions and handle antimeridian wrap-arounds :-)
