HPGN Datum? (fwd)

[Frank Warmerdam]

On 4/25/05, Bill Binko <bill at binko.net> wrote:
> After looking into this, I'm a bit concerned: the .prj file has a false
> easting of 500000, but the espg file from proj.4 has this:
> 
> <26759> +proj=tmerc +lat_0=24.33333333333333 +lon_0=-82 +k=0.999941
> +x_0=152400.3048006096 +y_0=0 +ellps=clrk66 +datum=NAD27
> +to_meter=0.3048006096012192 +no_defs  no_defs <>
>
> Shouldn't the false easting and the x_0 match up?

Bill,

I believe that PROJ.4 +x_0 and +y_0 values are always in meters,
while the WKT (.prj) file has the false easting and northing in the 
units of the projected coordinate systems .. feet in this case. 

In any event, if you error is on the order of 100m then the
problem is likely not the false northing being off by 350000
(meters or feet).  That level of error is mostly likely related to
the datum shifting not being applied or possibly a mixup between
different kinds of feet.   I am doubtful it is related to HPGN since
my understanding is that the difference between NAD83 and HPGN
is small (less than 5 meters say), though I am not too sure about that
part. 

Best regards,
-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent