You could use osr's CoordinateTransformation to evaluate the new location of 
the four corners in the destination spatial reference:
http://gdal.org/python/osgeo.osr.CoordinateTransformation-class.html

From there a min/max on the four coordinates will give you the extent of the 
new image space.

If the input and output CRS are very dissimilar you might want to evaluate a 
regular grid instead to get a more representative extent.


Benjamin Deschamps 

Physical Sciences Specialist
Canadian Ice Service, Meteorological Service of Canada
Environment and Climate Change Canada / Government of Canada 
benjamin.descha...@canada.ca / Tel: 613-943-4293 

Spécialiste des sciences physiques
Service Canadien des Glaces, Service Météorologique du Canada, 
Environnement et Changement climatique Canada / Gouvernement du Canada 
benjamin.descha...@canada.ca / Tél. : 613-943-4293



-----Original Message-----
From: Bang Pham Huu [mailto:b.pham...@jacobs-university.de] 
Sent: May 17, 2018 5:34 AM
To: gdal-dev@lists.osgeo.org
Subject: [gdal-dev] How to estimate the width/height size of output image in a 
target CRS without running gdalwarp?

Hi,

I have a hard problem to find the width/height of a 2D GeoTiff file which is 
reprojected from a source CRS (e.g: EPSG:4326) to target CRS
(e.g: EPSG:3857) without running gdalwarp. Because, the file could be large 
(GBs) and I don't need the projected output file, just need the width/height of 
it.

What could be done with gdalwarp is:

gdalwarp  -s_srs EPSG:4326 -t_srs EPSG:3857 full.tif full_3857.tif

gdalinfo full_3857.tif

Size is 879, 811 (width/height).

I've searched a lot and what seems to be good without doing the real projection 
is this gdal function: GDALSuggestedWarpOutput2 
https://github.com/OSGeo/gdal/blob/106c8288e7a05f4efc1a588c5a3b2da7ec52d915/gdal/alg/gdaltransformer.cpp#L354.
However, it doesn't help because my application developed in Java and it uses 
gdal-java (GDAL JNI) 
https://search.maven.org/#artifactdetails%7Corg.gdal%7Cgdal%7C1.11.1%7Cpom
as library. Unfortunately, I cannot find this GDALSuggestedWarpOutput2() from 
gdal-java http://gdal.org/java/overview-summary.html then cannot invoke this 
C++ function from my application to test.

Can anyone please give me a hint to solve this problem?

Thanks,




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