mercator projections

[William A Perkins]

>>>>> "Steve" == Steve Dzurenko <steve at ocean.oce.uri.EDU> writes:

    Steve> Hello,

    Steve> Is anyone a mercator projection location with GRASS?  I am
    Steve> trying to create a new location in a mercator projection
    Steve> and I am not having much luck.  In setting up the new
    Steve> region, GRASS asks me for the units and a conversion factor
    Steve> to meters.  The units for a mercator projection are
    Steve> degrees.  Does GRASS convert the degrees to meters?  And
    Steve> the conversion factor is different for lat and lon.  I am
    Steve> confused.  Has anyone successfully done this?

    Steve> Thanks.

    Steve> -Stephen Dzurenko
    Steve>  Ocean Mapping Division
    Steve>  Ocean Technology Center 
    Steve>  Graduate School of Oceanography
    Steve>  University of Rhode Island

    Steve>  ph: 401 792-6853
    Steve>  fx: 401 792-6849
    Steve>  e-mail: steve at ocean.oce.uri.edu

Steve,

I've digitized, in GRASS 4.1.3, some NOAA charts which used the
Mercator projection. When setting up the GRASS location, I specified
the `merc' projection and GRASS asked me for an ellipsoid, a ``Central
Meridian'', and a ``Latitude of True Scale'' (these are in quotes
because I'm not sure what they mean). I used the *default* central
meridian and the ellipsoid and latitude of true scale specified on the
chart.  After all that, it asked be for the ground units, which I
chose to be meters. I was later able to reproject the digitized charts
into UTM and state plane coordinates without difficulty. I can't tell
you how GRASS deals with the projection internally, but for me it
seemed to work.

Hope this helps.

Bill.

-- 
Bill Perkins
Battelle Pacific Northwest Laboratories
Environmental Sciences Center, Subsurface Flow and Transport
P.O. Box 999 MSIN K9-36
Richland, Washington, USA  99352    (509) 372-6131
wa_perkins at pnl.gov