The grass developers insist doing so would introduce artifacts and some distortions when taking measurements off the overlayed data, so we have to deal with a 'one location = one projection/zone/datum/ellipsoid' combination.
How do we get our raster images to underlay or overlay our vectors? The golden rule when using GRASS is if in doubt create a new 'location' any time you want to import or use data in a different projection.
I mainly work with just two projections, EPSG:27700 (OSGB36) and EPSG:4326 (WGS 84) and In my case I would define two locations and associated mapsets, one for my everyday working projects in (OSGB36 here in the UK) and one for my working landsat data in (WGS84).
can easily change between these two locations any time and re-project (r.proj) data from one the other. For example I would simply open the (OSGB36) location and use (r.proj) module to overlay data from the (WGS84) location. Lets look at how this is done.
1. Let's open a GRASS GIS session and create our first new 'location' and 'mapset' for our (OSGB36) location using the 'Grass Location Wizard'. This will be my everyday working project location here in the UK.
|When the 'Splash' screen opens click on the left 'New' location button|
|Select first (default) option then Click 'Next'|
|Use the magnifier to find your code if you know it.|
|Select the option 'Used in Great Britain'|
|That's it... the 'Summary' confirms success.|
|Ignore the fact this is showing 'OSGB362' yours will show 'OSGB36'|
|I'm only going to import one layer from my 'sqlite' database stored inside my 'PERMANENT' directory.|
|Zoom to the full extent of your layer simply to check it is viewable in the projection.|
|Again Zoom to the full extent of your layer to check it is viewable in this projection.|
|Change the 'location' to OSGB36 and 'mapset' to 'PERMANENT'|
|Click on the '-' magnifier about 4/5 times.|
8. 'r.proj' your Landsat image/s you wish to overlap enter text as below and then hit 'RUN'.
|Source Tab is 'WGS84' and 'PERMANENT' and the 'Input' is the raster you wish to load across into the 'OSGB36' projection.|
|Target Tab - leave output raster blank, leave interpolation as 'nearest' and add the resolution to 15m (this may be 30m) depending on what yours is.|
If we import (instead of project) any new raw landsat images through 'v.in.ogr' whilst inside the 'OSGB36' location the new images will misalign vertically because these images are still hardcoded to their 'WGS84' projection/datum parameters even though we are importing them into a 'OSGB36' projection.
|Notice the vertical misaligned vector data in the bottom of the window because i have 'imported' the landsat image (WGS84) (top) into 'OSGB36' location.|
THE GOLDEN RULE: Small 'extents' fit into bigger 'extents', Bigger 'extents' will not fit into smaller 'extents'.
If you wish to check your grass environment files (those which control your Grass session in the 'PERMANENT' directory) you can use the following as a guide for checking. In other words the contents of your 'WIND', 'DEFAULT_WIND', 'PROJ_EPSG', 'PROJ_INFO and 'PROJ_UNITS' files if using Landsat images.
Checking your settings (optional) for 'OSGB36' projection.
type 'g.gisenv -n' in the command console.
type 'g.region -p' in the command console.
projection: 99 (Transverse Mercator)