[Qgis-user] Do I need satellite raster layers relative to an extended UTM zone?

[Karl Zinglersen]

When you transform/reproject data across UTM zones, I'll again praise the
KMS Engsager Extended Mercator, which is now a part af PROJ.4 (thanks to
Thomas Knudsen and Frank Warmerdam et al), and consequently in GDAL. and is
relatively fast and very exact to within millimeters across multiple zones.
This should be default on the raster side, so far I know.

But on the vector side in QGIS, you'll need to set an environmental
parameter to get exact results: OSR_USE_ETMERC = YES. This can be done in
QGIS' menu Settings -> Options -> System -> and down to the Environment
section - or directly in regex so it takes effect for ogr2ogr in the
command prompt. Again, rasters should be OK. Otherwise put +proj=etmerc in
the proj-strin.

In the high latitudes of Greenland, we had 8-900 m transformation problem
across 5 zones. In ArcGIS, ESRI made a number of special UTM WGS 84 Zones
20-27 named "Complex" - just for Greenland. Which was needed, because while
working with data in the northern parts, we'll often need to cross multiple
zones.

If you are not working with data from the Arctic or Antarctic, it is
probably not a substantial problem, but it might save you some meters or
centimeters here and there.

Karl

2015-06-11 5:18 GMT-02:00 ardi <ardillasdelmonte at gmail.com>:

> Thanks a lot for your advice, Nicolas!
>
> ardi
>
>
>
> On Thu, Jun 11, 2015 at 1:09 AM, Nicolas Cadieux
> <nicolas.cadieux at archeotec.ca> wrote:
> > Hi,
> > It's never a good idea to delete files that you have reprojected.
> > Reprojection will, in most cases slightly alter the data although that
> > depends on the parameters used.  You could always test by reprojecting
> the
> > file back to the original projection and then by using raster math to see
> > the differences, if any.
> >
> > If you want to save space, I suggest you compress the data using
> "deflate"
> > parameters.  (You need to manually add support for bigtiff if the file is
> > over 4GB. ) Once you are done, you can always do raster math to make sure
> > data has not changed.  ( original layer - new layer should give zeros).
> >
> > For your second question, that is just the way it is! Since QGIS is open
> > source, not ever programmer puts reprojection in their code.
> >
> > Cheers.
> >
> > Nicolas Cadieux M.Sc.
> > Les Entreprises Archéotec inc.
> > 8548, rue Saint-Denis Montréal H2P 2H2
> > Téléphone: 514.381.5112  Fax: 514.381.4995
> > www.archeotec.ca
> >
> > On Jun 10, 2015 4:36 PM, "ardi [via OSGeo.org]" <[hidden email]> wrote:
> >
> > Hi,
> >
> > I've a satellite photo data set which spans across three UTM zones,
> > and I need to do some tasks in all three zones at once. Each image
> > tile from the data set is provided in two versions: one is projected
> > in the UTM coordinate system for the zone it actually belongs. The
> > other is projected over the central UTM zone, extending such central
> > zone.
> >
> > AFAIK, QGIS can deal with layers belonging to different UTM zones,
> > because you see a lat/long display and layers are reprojected to
> > lat/long on the fly.
> >
> > My question is: can I save disk space by storing the image tiles in
> > just one version, the version projected in its real UTM zone? Or would
> > I also need the versions projected into the (extended) central UTM
> > zone?
> >
> > Just out of curiosity, what's the use of the version projected into
> > the extended zone? Is it because some GIS packages don't support
> > on-the-fly reprojection? Or why?
> >
> > Thank you in advance
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-- 
Karl Brix Zinglersen
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