[GRASS-user] I do not understand how region/projection/coordinate system are intertwined

[Stephen Sefick]

I am not an expert with coordinate projections.  I think that UTM 
coordinates are in meters from a false easting and northing.  This is a 
very good projection for a "small" part of an ellipsoid.  All other 
comments are inline.  I have reposted this to the list because maybe 
somebody with a better understanding of this could offer better assistance.

On 03/14/2012 07:24 AM, Fridtjof Schiefenhövel wrote:
> Hi!
>
> I am using Grass 6.4.2 on Mac OS X Snow leopard using a port by Kyngchaos. Thanks for the advice! I think I begin to understand the way Grass handles coordinates...
> I can't decide on my own whether coordinates are shown in meters / degrees, right? The projection dictates this, correct?
right.

> I am mostly working with geodata from new guinea, so even the lat/long projection would be not very much distorted because of the location being so close to the equator. Anyway, I'd like to work with a (transverse) mercator projection but use degrees as coordinates, not meters.

I don't know if there is a pre-made projection for this...  You may want 
to look into an equi-distance or equal area projection depending on what 
you are interested in, or use UTM if it will cover your area, and then 
convert to degrees when necessary.  I guess it depends on your problem.
>
> Thanks again!
>
> Fridtjof
>
>

You are more than welcome.  I hope I have helped and maybe somebody else 
will have better advice.

kindest regards,

Stephen
>
>
>
> Am 14.03.2012 um 11:26 schrieb Stephen Sefick:
>
>> Is the geotiff in degrees?
>>
>> If you are on unix use gdalinfo to find out the projection of the geotiff.  You will have to import in its native projection and then reporject into whatever coordinate system that you would like.  I am just drinking my coffee this morning, so I may not be awake yet.
>> HTH,
>>
>> Stephen
>>
>> On Wed 14 Mar 2012 02:27:39 AM CDT, Fridtjof Schiefenhövel wrote:
>>> Hello there!
>>>
>>> it seems I can't solve this problem:
>>>
>>> ->create a location with a mercator projection (i want to have degrees as the coordinate system)
>>>
>>> ->import landsat geotiffs
>>>
>>> ->have degrees as units showing (e.g. when moving the mouse)
>>>
>>> this is what I did:
>>>
>>> -created a location using the projection/datum from a georeferenced file (the landsat geotiff).
>>>
>>> -created a location selecting mercator projection
>>>
>>> here is what "g.region -p" showed:
>>> projection: 99 (Mercator)
>>> zone:       0
>>> datum:      wgs84
>>> ellipsoid:  wgs84
>>> north:      0
>>> south:      -12.5
>>> west:       125
>>> east:       150
>>> nsres:      12.5
>>> ewres:      12.5
>>> rows:       1
>>> cols:       2
>>> cells:      2
>>>
>>> -ran v.in.region in the source location
>>> {trying r.proj without it would give me an error saying the imported region is out of bounds}
>>>
>>> -imported the vector map in the target location
>>>
>>> -set region to imported vector map
>>>
>>> -ran r.proj to import/reproject the landsat geotiff from the source location.
>>>
>>> here is what "g.region -p" showed now:
>>> projection: 99 (Mercator)
>>> zone:       0
>>> datum:      wgs84
>>> ellipsoid:  wgs84
>>> north:      -375086.40793588
>>> south:      -583681.58556862
>>> west:       -122131.46631847
>>> east:       111149.12622161
>>> nsres:      12.49971103
>>> ewres:      12.50029968
>>> rows:       16688
>>> cols:       18662
>>> cells:      311431456
>>>
>>> I do not understand, how can these bound values be even accepted when I chose Mercator projection and degrees coordinate system in the first place?
>>>
>>> Thanks!
>>>
>>>
>>>
>>>
>>> _______________________________________________
>>> grass-user mailing list
>>> grass-user at lists.osgeo.org
>>> http://lists.osgeo.org/mailman/listinfo/grass-user
>> -- 
>> Stephen Sefick
>> **************************************************
>> Auburn University                                         Biological Sciences                                      331 Funchess Hall                                       Auburn, Alabama                                        36849                                                           **************************************************
>> sas0025 at auburn.edu                                  http://www.auburn.edu/~sas0025                 **************************************************
>>
>> Let's not spend our time and resources thinking about things that are so little or so large that all they really do for us is puff us up and make us feel like gods.  We are mammals, and have not exhausted the annoying little problems of being mammals.
>>
>>                                -K. Mullis
>>
>> "A big computer, a complex algorithm and a long time does not equal science."
>>
>>                              -Robert Gentleman
>>
>>
>

-- 
Stephen Sefick
**************************************************
Auburn University
Biological Sciences
331 Funchess Hall
Auburn, Alabama
36849
**************************************************
sas0025 at auburn.edu
http://www.auburn.edu/~sas0025
**************************************************

Let's not spend our time and resources thinking about things that are so little or so large that all they really do for us is puff us up and make us feel like gods.  We are mammals, and have not exhausted the annoying little problems of being mammals.

                                 -K. Mullis

"A big computer, a complex algorithm and a long time does not equal science."

                               -Robert Gentleman