[OSRS-PROJ] Pseudo X/Y values, forward and reverse

[Clifford J Mugnier]

Frank,

It's the implementation.  I think John P. Snyder's book and formulae were
used for PROJ4.  That would mean that the formulae for TM are to the 5th
derivative.  That barely gets to +/- 3 degrees from the central meridian.
7th derivative gets almost to 4 degrees difference.  Fifteenth derivative
gets to +/- 24 degrees from the central meridian with 1 mm "round-trip"
computational accuracy.

Cliff Mugnier
LOUISIANA STATE UNIVERSITY
William Hersman wrote:
> Input    -175.1234589    59.7654305  DD
> Return   -4054878.4814   2990301.082 ASP 4 FEET
> Input    -4054878.4814   2990301.082 ASP 4 FEET
> Return   -175.1487387    59.7638269  DD


William,

It would clearly help alot if you could provide the details of the
coordinate system.  I tried this:

warmerda at gdal2200[237]% proj -I -v +init=nad27:5004 +units=ft
#Transverse Mercator
#       Cyl, Sph&Ell
# +init=nad27:5004 +units=ft +proj=tmerc +datum=NAD27 +lon_0=-150 +lat_0=54
# +k=.9999 +x_0=152400.3048006096 +y_0=0 +no_defs +ellps=clrk66
# +nadgrids=conus,ntv1_can.dat
-4054905.40     2990291.47             (input)
175d7'10.261"W  59d45'56.295"N         (output)

warmerda at gdal2200[238]% proj -v +init=nad27:5004 +units=ft
#Transverse Mercator
#       Cyl, Sph&Ell
# +init=nad27:5004 +units=ft +proj=tmerc +datum=NAD27 +lon_0=-150 +lat_0=54
# +k=.9999 +x_0=152400.3048006096 +y_0=0 +no_defs +ellps=clrk66
# +nadgrids=conus,ntv1_can.dat
175d7'10.261"W  59d45'56.295"N          (input)
-4054186.84     2990083.90              (output)

I see there is an over 700 ft difference in the positions.  Far more than
I would have expected.  In this case the longitude is 25 degrees away from
the central meridian for the zone, but I wouldn't have thought the
reversibility of the equations would have been so poor at that distance.

Cliff, you mentioned that the TM coordinates would not be reversible at
great distances.  Is this the nature of the TM equations or a weakness in
their implementation with PROJ.4?

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


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