On Jan 2, 2009, at 4:38 PM, <grass-user-requ...@lists.osgeo.org> wrote:
Message: 2
Date: Fri, 2 Jan 2009 21:04:44 +0200
From: "Alex Bernstein" <pofi...@gmail.com>
Subject: [GRASS-user] Help with reprojection
To: grass-user@lists.osgeo.org
Message-ID:
<bf69017a0901021104n29f77efal2206f53128b08...@mail.gmail.com>
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Hi,
I'm new to GRASS, and am having trouble with reprojection. I've spent
two days trying to figure this out, and I feel that I'm close, but
reprojection is still failing. Could anyone clue me in what I'm
missing?
The input data set is Natural Earth II map of the world
<http://www.nacis.org/data/NE2/NE2_WDB1.jpg> 16200x8100 JPG in plate
carrée projection. I need to reproject it into Lambert azimuthal
equal-area projection centered on N0 E15. I'm using WinGRASS 6.3.0.
First, I've created GRASS location "geographic" with following region
and projection:
g.region -p
projection: 99 (Equidistant Cylindrical (Plate Caree))
zone: 0
datum: wgs84
ellipsoid: wgs84
north: 8100
south: 0
west: 0
east: 16200
nsres: 1
ewres: 1
rows: 8100
cols: 16200
cells: 131220000
g.proj -p datumtrans=0
PROJ_INFO-------------------------------------------------
name : Equidistant Cylindrical (Plate Caree)
datum : wgs84
towgs84 : 0.000,0.000,0.000
proj : eqc
ellps : wgs84
a : 6378137.0000000000
es : 0.0066943800
f : 298.2572235630
lat_0 : 0.0000000000
lat_ts : 0.0000000000
lon_0 : 0.0000000000
-PROJ_UNITS------------------------------------------------
unit : meter
units : meters
meters : 1.0
I then imported the JPG map, which got split into color channels and
composed the channels together into single map.
After, I created a new location "lambert":
g.region -p
projection: 99 (Lambert Azimuthal Equal Area)
zone: 0
datum: wgs84
ellipsoid: wgs84
north: 8100
south: 0
west: 0
east: 16200
nsres: 1
ewres: 1
rows: 8100
cols: 16200
cells: 131220000
g.proj -p datumtrans=0
-PROJ_INFO-------------------------------------------------
name : Lambert Azimuthal Equal Area
datum : wgs84
towgs84 : 0.000,0.000,0.000
proj : laea
ellps : wgs84
a : 6378137.0000000000
es : 0.0066943800
f : 298.2572235630
lat_0 : 0.0000000000
lon_0 : 15.0000000000
x_0 : 0.0000000000
y_0 : 0.0000000000
-PROJ_UNITS------------------------------------------------
unit : meter
units : meters
meters : 1.0
Finally, when I try reprojection I get "Input raster map is outside
current region" error:
r.proj input=NE2 location=geographic mapset=PERMANENT method=cubic
Input Projection Parameters: +proj=eqc +lat_0=0.0000000000
+lat_ts=0.0000000000 +lon_0=0.0000000000 +a=6378137 +rf=298.257223563
+no_defs +towgs84=0.000,0.000,0.000
Input Unit Factor: 1
Output Projection Parameters: +proj=laea +lat_0=0.0000000000
+lon_0=15.0000000000 +x_0=0.0000000000 +y_0=0.0000000000 +a=6378137
+rf=298.257223563 +no_defs +towgs84=0.000,0.000,0.000
Output Unit Factor: 1
Input raster map is outside current region
How can I make this work?
Your problem is that this map is an image, not a GIS map. It is in
what is called an xy "projection" in GRASS. That is, it is in no
projection at all. The coordinates of each cell are simply the xy
coordinates from the corner, not their position in the world.
In a 'real' GIS map, each cell would have some kind of real-world
geographic coordinates (latlon, UTM, etc).
To get from a graphic image like this to geospatial data, you need to
georectify the map. It would be a good idea for you to get an intro
GIS book so that you can understand what is going on with this if it
is unfamiliar to you.
The steps you need to follow in GRASS are...
1) create an xy location
2) import your jpg map into the xy location
3) open GRASS in your Lambert location
4) start the georectifier and georectify the map from the xy to
Lambert location.
Looking at the jpg, it has no latlon or other geographic markers. This
will make it difficult to georectify accurately. If you are looking
for nice geospatial data of world topography, it might be better for
you to download some data rather than trying to georectify a jpg
image. GTOPO30 or similar scale (1km or larger) topography can be
downloaded from a variety of places. Check the GRASS site to links to
geospatial data. Also check out the GLCF <http://glcf.umiacs.umd.edu/
>. For global-scale imagery, try MODIS satellite data. The good thing
about these data sets is that they are already georectified and can be
imported directly into GRASS as GIS data. You can then easily
reproject to something else.
Michael
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