How to Quickly Transform a Bounding Box from One CRS to Another Using QGIS

Today I needed to convert a bounding box for a tilemill project that I want to bring into QGIS as a tile layer (more on that in a future post if I get it to work…). I needed to convert a bounding box from EPSG:4326 (‘Geographic’) coordinates to EPSG:3857 (Spherical Mercator). Fortunately it is a fairly trivial process if you don’t mind writing a few lines of python in the QGIS python console:

box = QgsRectangle(-19.6875,-37.9962,59.0625,37.4400)  
source_crs = QgsCoordinateReferenceSystem(4326)  
dest_crs = QgsCoordinateReferenceSystem(3857)  
transform = QgsCoordinateTransform(source_crs, dest_crs)  
new_box = transform.transformBoundingBox(box)  
new_box.toString()  
u'-2191602.4749925746582448,-4578889.0142234507948160 : 6574807.4249777207151055,4500615.8633687794208527'

It really is quite trivial to do arbitrary once-off things in QGIS if you roll up your sleeves and get to grips with the python API!