[Qgis-user] Support for (wrong) GEOCOVER projection for S hemisphere in qgis

[Agustin Lobo]

Lack of raster OTF reprojection is definitely not the
problem, although, paradoxically, raster OTF reprojection would be a 
solution.

The question is that if the raster CRS is defined
as 18N but the raster has negative coordinates (as for Geocover images), 
we should see it
displayed in the Southern hemisphere even if the project is defined
as 18S. The current situation is that, with raster as 18N and project as 
18S, the raster layer is not displayed at all, even after "zoom to layer 
extent".

Note that the current "solution" (both raster and project as 18N) is 
bad, as all layers are coincident because they are all reprojected to a 
wrong projection. Also, note that additional correct raster data on UTM 
18S (most raster layers for Peru) would not be correctly displayed with 
these settings.

The only good solution is modifying geocover
products to be on a standard CRS, in this case
18S with positive coordinates.

Agus

Micha Silver wrote:
> Agustin Lobo wrote:
> 
>> No, what I was doing was different:
>> I was setting the project to 18S (not 18N) with OTF reprojection, the 
>> raster to 18N
>> and the vector layers got reprojected to 18S but the raster
>> was not displayed.
> Since QGIS doesn't deal with on-the-fly reprojection of rasters, we know 
> that you have to set the project CRS to whatever projection the raster 
> is in. In this case, since the Landsat are (wrongly) set to UTM N, I 
> guess you have to stay with that in order for vectors to be properly 
> registered.
> Cheers,
> Micha
>> I was expecting the raster to be located in the S hemisphere because
>> the coordinates are negative. In TNTmips, as the coordinates are 
>> negative, even if the raster CRS is set
>> to 18N, the raster is displayed in the S hemisphere. Not sure if this
>> is what qgis should do, though, as we are dealing with wrongly annotated
>> raster layers: in UTM, S is S, not negative N.
>>
>> On the other hand, I assumed that if you set the project to 18N, then 
>> the vector layers would projected to the N hemisphere.
>> But have tried defining the project as 18N as you say, used Coordinate 
>> capture to check lon,lat, and actually get correct lon,lat coordinates 
>> (in the S
>> hemisphere). The vector layers are reprojected to
>> 18, no matter the S or N, the original latitude correctly defines the 
>> hemisphere.
>>
>> So it works "your way", thanks!
>>
>> Agus
>>
>>
>>
>> Micha Silver wrote:
>>> Hi Agus:
>>> I am actually able to do just that. I also am using some Landsat 
>>> GEOCover images from below the equator, projected, as you say in UTM 
>>> 39_N_.
>>> I load the raster, set the project CRS to that (UTM Northern) 
>>> projection, and enable OTF projection. Then I can overlay vector 
>>> layers, which are WGS84 Lon-Lat., and they are correctly located.
>>>
>>> It's usually not very helpful to say "works for me" but...
>>> Anyway, HTH,
>>> Micha
>>>
>>>
>>> Agustin Lobo wrote:
>>>> The global set of Landsat images on GEOCOVER
>>>> https://zulu.ssc.nasa.gov/mrsid/mrsid.pl
>>>> is on UTM/WGS projection
>>>> but with the Southern hemisphere tiles as UTM zones in the N with
>>>> negative coordinates. For example, a tile that should be at 18S is
>>>> actually 18N with negative coordinates
>>>> ("Non-standard UTM definition: For the southern hemisphere, the 
>>>> GeoTiff files
>>>>   contain positive zone numbers with negative northing coordinates")
>>>> https://zulu.ssc.nasa.gov/mrsid/docs/GeoCover_circa_2000_Product_Description.pdf 
>>>>
>>>>
>>>> As we have some vector layers on geographic coordinates WGS84 and a 
>>>> Geocover tile for 18S,
>>>> I've set the project as 18S, enabled CRS reprojection on the fly, 
>>>> and set
>>>> the raster CRS to 18N. Unfortunately, QGIS does not display the 
>>>> raster, even
>>>> selecting zoom to layer extent. (Using Mimas).
>>>>
>>>> This is probably not a problem of QGIS, the Geocover projection is 
>>>> just wrong,
>>>> but would like to know if there could be a way to circumvent this 
>>>> problem to
>>>> be able to use GEOCOVER tiles of the S hemisphere within QGIS.
>>>> A way could be making a world file with the Georeferencer plugin, 
>>>> but perhaps
>>>> there is a more general way.
>>>>
>>>> Thanks
>>>>
>>>> Agus
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