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

GRASS GRASS user
1

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!

2

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!

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--
Stephen Sefick
**************************************************
Auburn University Biological Sciences 331 Funchess Hall Auburn, Alabama 36849 **************************************************
sas0025@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

3

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@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/grass-user

--
Stephen Sefick
**************************************************
Auburn University Biological Sciences 331 Funchess Hall Auburn, Alabama 36849 **************************************************
sas0025@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@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

4

Fridtjof Schiefenhövel wrote:

it seems I can't solve this problem:

->create a location with a mercator projection (i want to have degrees as the coordinate system)

That's a contradiction. Only lat/lon locations use degrees as the
coordinate system. A Mercator projection (whether "normal" Mercator,
transverse, or oblique) has coordinates in distance units (usually
either metres or feet).

A Mercator projection /can't/ have coordinates in degrees, as
coordinates have to be linear while Mercator has one coordinate vary
non-linearly with the angle from the tangent circle.

--
Glynn Clements <glynn@gclements.plus.com>