Hello all,
Being completely new to grass, I am having a hard time figuring out an appropriate method to convert a USGS DRG from Albers projection to UTM.
The DRG has a .tfw forld file in Albers coordinates. The .tfw file only has coordinates for the South-East corner and the meters per pixel value.
So would I set up my database in UTM or Albers? How do I associate the .tfw world file with the DRG?
When I attempted to set up the Albers projection parameters, I was prompted to enter a value for the Central parallel. I was not given a Central parallel parameter, just the 1st and 2nd standard parallel values. Here are all of my known Albers projection parameters:
units: meters
1st standard parallel 34 00 00 (would this be 34N?)
2nd standard parallel 40 30 00
Central Meridian -120 00 00 (would this be entered as 120W or -120 ?)
Latitude of Origin 00 00 00
False easting 0
False northing -4000000
Datum NAD27
Sheroid Clarke, 1866
Note that I did not notice anywhere to input the "False nothing" or "False easting" values.
Does anyone know how I can figure out what the value for the Central parallel?
Sorry for so many questions!! I would greatly appreciate an outline of suggested events to complete this re-projection.
Thanks in advance,
Jeshua Lacock
Cartographer/Owner
http://SierraMaps.com
http://3dTopoMaps.com
Telephone: (760) 935-4481
Hi Jeshua,
I noticed no one had responded to this on the list yet, so I throw in some
general ideas
Try to create a location corresponding to the projection of the map/data
to be imported. You may need to know a lot a parameters to create that
location. If you don't have them you can always create an XY-location (and
maybe later manually change the PROJ_INFO and DEFAULT_WIND files in the
PERMANENT subdirectory for that location) This will at least let you view
the imported map in the grass monitor (which _may_ give some more hints
about missing parameters).
So would I set up my database in UTM or Albers?
In Albers (Equal Area) first. Then import to that location. Then create
the UTM location you want and convert the map to that location. You have
to quit from grass and restart it in order to create/switch locations.
Remember that with each conversion some data is lost, so keep the Albers
location around so you have access to the original data in case you want
it into some other projection later.
1st standard parallel 34 00 00 (would this be 34N?)
2nd standard parallel 40 30 00
Central Meridian -120 00 00 (would this be entered as 120W or -120 ?)
Latitude of Origin 00 00 00
g.setproj accepts either hexagesimal (120:30:00W) or decimal
(-120.5) format.
Does anyone know how I can figure out what the value for the Central
parallel?
Latitude of origin = Central Parallel is usually but not necessarily
midway between the the 1st and 2nd standard parallels. The zero value
above is incorrect, try 37N.
Use one of the 'in' tools to import, then switch to the UTM location
and use r.proj and v.proj to convert the maps. (r.proj may not work
properly for all projections but there is a replacement r.proj.new in the
CVS repository)
hope this helps
MHu
On Mon, Nov 20, 2000 at 07:46:26PM +0100, Morten Hulden wrote:
> Does anyone know how I can figure out what the value for the Central
> parallel?
Latitude of origin = Central Parallel is usually but not necessarily
midway between the the 1st and 2nd standard parallels. The zero value
above is incorrect, try 37N.
I think in this case, it actually *is* Latitude of Origin (0 0 0.0). At
least, I've been using it that way for some time and it seems to work.
--
Eric G. Miller <egm2@jps.net>