Hi,
I am a new starter for GRASS. I create a new location and its mapset in a new
directory (database). The region was set as 0-10000 (W-E) and 0-10000 (S-N)
when I did the creation. But after a map was imported (.jpg file, downloaded
from website), this imported map's region was -915-0 (W-E) and -587-0 (S-N), in
the -/- area (the third quadrant of Cartesian coordinate system?). I tried
serveral times with similar downloaded maps, the imported maps always falled in
this area. How to sovle this problem? How to let the map "sit" in a region as I
want? Thanks for help.
Lizhong Jin
Lizhong, so it sounds like there are a few things you need to know about GIS (I hope I am not being presumptuos). The first thing is that all information is geocoded, or it isn't (not very profound). Suppose a raster file has geocoding information in it, if you import it into a location that has the same projection parameters (i.e. latlong, wgs84), then the map will show up in the part of the world it is supposed to. Every location has a unique projection. If your raster does not have projection data associated with it (like all jpegs, gifs, etc.), then you need to import it into an XY location (this location has no projection). When importing into XY locations, all rasters come in with the top right corner at the origin (just because). The reason it came in as -915-0 etc. is because your jpeg is a 915x587 pixel image.
Generally once you have a map in the XY location, you will want to project it to another location that has projection parameters that match some kind of real world map. This is why you would use a GIS. If you just want to edit a raster without a projection, then GRASS is probably the wrong application. I hope this is helpful. If you have more specific questions about how to fit your jpeg into another location, there is info in the archives, or write back.
G'luck
-Ian
On Feb 11, 2005, at 7:34 AM, ljin@hunter.cuny.edu wrote:
Hi,
I am a new starter for GRASS. I create a new location and its mapset in a new
directory (database). The region was set as 0-10000 (W-E) and 0-10000 (S-N)
when I did the creation. But after a map was imported (.jpg file, downloaded
from website), this imported map's region was -915-0 (W-E) and -587-0 (S-N), in
the -/- area (the third quadrant of Cartesian coordinate system?). I tried
serveral times with similar downloaded maps, the imported maps always falled in
this area. How to sovle this problem? How to let the map "sit" in a region as I
want? Thanks for help.
Lizhong Jin
hello nyc..
When importing into XY locations, all rasters come in
with the top right corner at the origin (just because). The reason it
came in as -915-0 etc. is because your jpeg is a 915x587 pixel image.
It uses the top-right of the image as the origin because r.in.gdal sets
it that way if the image doesn't contain geocoding. Older modules in
GRASS 5.0 used the bottom left as the origin (eg r.in.tiff and r.in.png).
You can reset it with r.region if you want.
Hamish