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

Randy Bradley Randy.Bradley at ars.usda.gov
Tue Mar 30 17:09:49 EDT 2010


    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
name       : Latitude-Longitude
datum      : wgs84
towgs84    : 0.000,0.000,0.000
proj       : ll
ellps      : wgs84
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
³Vector>Develop map>Reproject vector² menu item and selected the vector from
the above location.  Here is what the new location and default region looks

> g.proj -p
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
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 ³Zoom display to selected map² at which point my nice Albers map gets
displayed.  Or I can ³zoom out² about 20 times to make the Albers map start
showing up near the top of the screen.  Obviously I¹m doing something wrong
here but I¹m not sure if its the units (meters vs miles) or something more

Drowning in a sea of GRASS and loving it!

Thanks for any

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