[GRASS-user] Newbie... Locations, Projections, Regions, choices? (long)

[Micha Silver]

On 03/31/2010 12:09 AM, Randy Bradley wrote:
> All,
>
>     I need to create 6 regional maps of the continental U.S., north 
> and south by west, central, and east.  My first thought was to create 
> 7 locations (one for the entire continental US) but my second thought 
> was to create just one location with the default region as the entire 
> continental US and 6 other regions. Is this sound logic?
>
>     First I downloaded the States data from the National Atlas.  I 
> then created a latitude - longitude location and imported the shape 
> file excluding Alaska, Hawaii, Puerto Rico, and the Virgin Islands.
> > g.proj -p
> -PROJ_INFO-------------------------------------------------
> name       : Latitude-Longitude
> datum      : wgs84
> towgs84    : 0.000,0.000,0.000
> proj       : ll
> ellps      : wgs84
> -PROJ_UNITS------------------------------------------------
> unit       : degree
> units      : degrees
> meters     : 1.0
>
> > g.region -p
> projection: 3 (Latitude-Longitude)
> zone:       0
> datum:      wgs84
> ellipsoid:  wgs84
> north:      49N
> south:      25N
> west:       125W
> east:       67W
> nsres:      0:00:30
> ewres:      0:00:30
> rows:       2880
> cols:       6960
> cells:      20044800
>
>     The map displays fine but I want to change the projection to 
> Albers Equal Area or Lambert Conformal Conical since it is such a 
> large area.  So I created a new location: aeaConUS with one caveat. 
>  It said the eastern value had to be larger than the western value so 
> I (perhaps wrongly) used –125 for the western boundary and –67 for the 
> eastern.  Using the GUI selected the “Vector>Develop map>Reproject 
> vector” menu item and selected the vector from the above location. 
>  Here is what the new location and default region looks like:
>
> > g.proj -p
> -PROJ_INFO-------------------------------------------------
> name       : Albers Equal Area
> datum      : wgs84
> towgs84    : 0.000,0.000,0.000
> proj       : aea
> ellps      : wgs84
> a          : 6378137.0000000000
> es         : 0.0066943800
> f          : 298.2572235630
> lat_0      : 23.0000000000
> lat_1      : 29.5000000000
> lat_2      : 45.5000000000
> lon_0      : -96.0000000000
> x_0        : 0.0000000000
> y_0        : 0.0000000000
> -PROJ_UNITS------------------------------------------------
> unit       : mile
> units      : miles
> meters     : 1609.3440000000
>
> > g.region -p
> projection: 99 (Albers Equal Area)
> zone:       0
> datum:      wgs84
> ellipsoid:  wgs84
> north:      49
> south:      25
> west:       -125
> east:       -67
> nsres:      0.05
> ewres:      0.05
> rows:       480
> cols:       1160
> cells:      556800
>
>     When I first try to display this map, nothing is displayed unless 
> I select “Zoom display to selected map” at which point my nice Albers 
> map gets displayed.  Or I can “zoom out” about 20 times to make the 
> Albers map start showing up near the top of the screen.  Obviously I’m 
> doing something wrong here but I’m not sure if its the units (meters 
> vs miles) or something more fundamental.
>
It looks like you're off to a good start. All you need to do, in the 
aeaConUs location, is save a few regions. Begin with:
g.region vect=<states_vect> save=ConUS
You might want to save this Continental zoom as the default with
g.region -s ConUS
Then zoom to each of the E,Central,W etc areas and save an additional 
region, i.e. (after zooming to central region):
g.region save=Central
You can alternatively specify the region bounds with something like:
g.region save=Central west=-120 east=-110 ...
This all can be done in the GUI with Config->Region.

Now you "jump" to each of the saved regions with
g.region reg=Central
etc...


>
> Drowning in a sea of GRASS and loving it!
>
> Thanks for any
> suggestions,
>
> Randy
>
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>
>
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>    


-- 
Micha Silver
http://www.surfaces.co.il/
Arava Development Co.  +972-52-3665918

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