[GRASSLIST:1616] Re: More projection problems...

[mberglund]

> I get exactly the same numbers 380339.35 1764858.00 using the same
> parameters you have (+ ellipsoid=wgs84)
I have been using wgs84 also.

> You can use m.proj to break down the projection into two steps:
Ok, I'm with you. Lets do a couple of steps together.

First the X and Y below are the ACCURATELY CONVERTED STP coordinates for
the original point in aea. I did the conversion to ll from his numbers. So
these are the expected coordinates for the other rounded point.

 X (feet)        Y (feet)        Longtitude      Latitude
-------------   --------------  -----------     ---------

 536507.00      1764928.00    81:53:08.131239W 29:11:20.067269N

By the way, my original estimate was wrong, I'm off by more like 156000
feet. Yikes!!

> My m.proj gives 
> 82d22'30.036W 29d11'17.659N 
> give (i assumed the ellipsoid wgs84). Does that make sense?
Ok. I got the ellipsoid(s) he is using (he wasn't certain of which but it
is one of the two) grs80 or wgs84. Stephen at FSU works on a number of
projects, so he couldn't remember which one off the top of his head. 
I've tried both, so I doubt that this is an issue.

> Note that r.proj nor m.proj will not make datum.shifts (yet), but datum
> differences should not give errors of the magnitude you mention.
Fortunately I now we know we don't have to do datum shifts.

> Also note that the proj parameters for STP projections are statically
> defined in <grass install directory>/etc/state27 and FIPS.code files. I
> have no idea about the accuracy of those formulas. Maybe someone with more
> knowledge about STP would like to check them.
I am not very knowledgeable, but as I understand it, these numbers are
gotten from PROJ.4. Those numbers have been good enough for me to go from
lat/long to stp from gps data, and present it on a map within a couple
foot accuracy. 
 
> Can you dump us your PROJ_INFO for both the input and output location, the
> DEFAULT_WIND for the output location and the cellhd file of the input map
> (a file with the same name as the input map, located in the cellhd
> directory of the input location.) Also please say what your expected STP
> co-ordinates for your test poit are exactly. This would help to do some
> more debugging.

The test point expected results(also in the chart above) are:
X: 	536507.00
Y:	1764928.00
NOTE: The original points were rounded for expediance. The resultant
points that were generated from the rounded points were still thousands of
feet more accurate than my conversion. 

STP Location:-------------------------
DEAFULT_WIND-
	proj:       99
	zone:       0
	north:      1907628
	south:      1657863
	east:       784820
	west:       473816
	cols:       311004
	rows:       249765
	e-w resol:  1
	n-s resol:  1

PROJ_INFO-
	name: State Plane
	proj: tmerc
	a: 0.63782064e+07
	es: 0.6768657997291094e-02
	x_0: 0.1524003048006096e+06
	y_0: 0
	k: 0.9999411764705882e+00
	lon_0: 82dw
	lat_0: 24d20'n

Aea Location:---------------------------
DEFAULT_WIND-
	proj:       99
	zone:       0
	north:      576795
	south:      569695
	east:       564255
	west:       557754
	cols:       6501
	rows:       7100
	e-w resol:  1
	n-s resol:  1

PROJ_INFO-
	name: Albers Equal Area
	proj: aea
	ellps: wgs84
	a: 6378137.0000000000
	es: 0.0066943800
	f: 298.2572235630
	lat_0: 27.4500000000
	lat_1: 24.0000000000
	lat_2: 31.5000000000
	lon_0: -84.0000000000
	x_0: 400000.0000000000
	y_0: 0.0000000000

Hope this helps and Thanks again,
Matt