[GRASSLIST:1613] Re: More projection problems...

[Morten Hulden]

On Wed, 14 Mar 2001, mberglund wrote:

> In any event if some has a second and can verify that my conversion is
> correct, I would appreciate it.

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

You can use m.proj to break down the projection into two steps:

aea -> ll and then ll -> stp, and also do the reverse
stp -> ll and then ll -> aea

in order to find the place where the error creeps in. 

My m.proj gives 
82d22'30.036W 29d11'17.659N 
for the aea->ll projection with the values you
give (i assumed the ellipsoid wgs84). Does that make sense?

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.

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.

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.


regards
Morten Hulden

> 
> 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.
> 
> Thanks again,
> Matt Berglund
>