[GRASSLIST:1625] Re: More projection problems...

[Eric G. Miller]

On Wed, Mar 14, 2001 at 12:36:16PM -0500, mberglund wrote:
> Ok, I'm a heck of a lot closer than I have been with my projection
> problems.
> 
> I have done a point conversion on one of the points in a .tfw file from
> albers to stp, and I get the wrong x coordinate out. The Y is right on the
> money.
> 
> In any event if some has a second and can verify that my conversion is
> correct, I would appreciate it.
> 
> Input----
> 
> Projection: Albers
> Units: Meters
> Datum HPGN
> 1st standard parallel: 24 00 00
> 2nd standard parallel: 31 30 00
> Central Meridian: -84 00 00
> Latitude of projection's origin: 24 00 00
> False easting (meters):  400000.0
> False northing (meters): 0.0
> 
> The point in albers is
> Easting: 557754.976629873621278
> Northing: 576793.941004194319248
> 
> Output----
> Projection: State Plane
> Units: Feet
> State FIPS code: 12(Florida)
> County FIPS code: 83(Marion)
> 
> My result has been:
>   X_in (meters)   Y_in (meters)   X_out (feet)    Y_out (feet)
>   ------------    -----------     -------------   -------------
> 
>     557754.00       576793.00       380339.35      1764858.00
> 
> -------------------------------------^
> This is off by roughly 120000 feet!!! (According to the map I am
> converting into.) This presents a small problem for me, and explains why
> grass complains that the "Input map is outside the current region" when I
> try to do a r.proj on the incoming map.
> 
> If any one can give me a handle on what I am doing wrong I would
> appreciate it.
> 
> I would also like to thank the 4 or 5 people who have helped me get
> rolling with grass. The docs are pretty good, but the people here make
> this thing work.

I don't know if this is the source of the problem, but I tried out
defining a State Plane for Florida/Marion using nad83/grs80, but it
still culls the projection parameters for the nad27/clark66.  Unless I'm
mistaken, you want the NAD83 State Plane for Florida???  However, the
state plane interface for GRASS doesn't seem to let one decide.  Also,
should the output units be meters as well?  I never use this State Plane
thingy...

There are significant differences in the false easting between stp27 and
stp83.  The difference in just false easting, not accounting for other
variables is roughly 157440 feet (converted from the x_0 parameter
which is in meters) -- close to your 120000.

The state27 and state83 files live in grass5/etc if you want a look.  It
won't help you much though...

I'll look into it more if I can...

-- 
Eric G. Miller <egm2 at jps.net>