THE USGG2003 README FILE
                         ------------------------

Version: January 16, 2004   drr

The USGG2003 GEOID MODEL
----------------------
You have received these models on CD-ROM, or downloaded them from 
the National Geodetic Survey (NGS) web site or the NGS FTP site.

Files you may have received include:

 INTG.EXE (PC) or   The geoid interpolation program (source code is
 INTG     (Sun)     INTG.FOR)

 XNTG.EXE (PC) or   Program for extracting, translating (ascii/binary)
 XNTG     (Sun)     and yielding statistics of geoid files (source code
                    is XNTG.FOR)

The following file names are valid for binary files (if, however, 
you downloaded the ASCII versions of these files, the suffix will 
be ".asc" rather than ".bin"):
 s2003u01.bin       USGG2003 grid #1 for CONUS (40-58N, 230-249E)
 s2003u02.bin       USGG2003 grid #2 for CONUS (40-58N, 247-266E)
 s2003u03.bin       USGG2003 grid #3 for CONUS (40-58N, 264-283E)
 s2003u04.bin       USGG2003 grid #4 for CONUS (40-58N, 281-300E)
 s2003u05.bin       USGG2003 grid #5 for CONUS (24-42N, 230-249E)
 s2003u06.bin       USGG2003 grid #6 for CONUS (24-42N, 247-266E)
 s2003u07.bin       USGG2003 grid #7 for CONUS (24-42N, 264-283E)
 s2003u08.bin       USGG2003 grid #8 for CONUS (24-42N, 281-300E)
 (note: the USGG2003 model only exists for the conterminous United States)

To Install:
   1) Make a subdirectory on your hard disk.
   2) Copy the various geoid files into that subdirectory.  You need 
      not put the geoid files in the same directory as the programs.
      (If you have also received USGG2003 model files, you may safely 
      place them in the same directory as GEOID03, if you like.)

To Execute
   (PC or Sun) Type INTG            , and follow the prompts.

To Terminate
   You can stop the program at any time using <Control> C.

BUT, PLEASE DON'T START YET.  PLEASE KEEP READING THIS DOCUMENT.

Check The Byte Counts of all Downloaded Files
---------------------------------------------
    Before beginning, it will be useful to ensure that all files
you have received are the correct size.  (Download problems are
often manifested by incorrect byte counts in the files).  Check
with the list below to make sure your files match these numbers
exactly.  These values are good for the PC and Sun versions of 
the data.

PC or Sun Data:
     g2003u**.bin    4,933,728 bytes
ASCII Data:
     g2003u**.asc   12,488,896 bytes (uncompressed)
FORTRAN Source Code:
     INTG.FOR           88,980 bytes
     XNTG.FOR           31,158 bytes
PC executables:
     INTG.EXE          299,008 bytes
     XNTG.EXE          282,624 bytes
Sun executables:
     INTG              466,692 bytes
     XNTG              426,924 bytes

How Program INTG Works
-----------------------
   The various geoid height grids are stored in the ".bin" files.  
Program INTG will prompt you for the name of the directory where you 
have chosen to store the .bin files, as well as prompting you for which 
geoid model you wish to use.  You can operate with as few as one .bin
file, or as many as 8 for USGG2003 and 14 for GEOID99.  When the program 
interpolates a given point, it checks an internal list of .bin 
boundaries, and uses the earliest list entry whose boundaries contain 
that point.  The order in which the .bin file names appear on the 
opening screen indicates the order in which the .bin files are searched.

   When running program INTG.EXE (PC) or INTG (Sun), the latitude and 
longitude of each point must be input.  The GEOID03 models are heights 
above the NAD 83 ellipsoid.  However, latitudes and longitudes in the 
ITRF97/GRS-80 and WGS84(G873) systems are very close to those of the 
NAD 83 system (with only 1-2 meters of horizontal shift.)  So any of 
these types of latitude and longitude (NAD 83, ITRF00, WGS84) may be 
input, without affecting the interpolated geoid value.  This does *not* 
imply that the geoid heights are heights above a different ellipsoid.  
Using NAD 83 latitudes and longitudes interchangeably with ITRF00/GRS-80 
Or WGS84 latitudes and longitudes is merely an acceptable horizontal 
approximation.  GEOID03 geoid heights, will always be above the NAD83
ellipsoid.

Do *NOT* use NAD 27 latitudes and longitudes.  The horizontal shifts 
between NAD 83 and NAD 27 can exceed 100 meters, causing a noticeable 
difference in the interpolated geoid value.  To convert from NAD 27 to 
NAD 83 latitudes and longitudes you may use programs NADCON or CORPSCON, 
available from NGS.

Data Input
----------
   You can key data by hand, point by point, or you can create an input 
File using a text editor.  Several file formats are provided, including 
the NGS "Blue Book" format.  These formats are detailed in a "Help" menu 
option which appears if you specify that you wish to use an input file.

Data Output
-----------
   Results may be collected into an output file.  There is no default
output file name.  The format of the output file is linked to the format 
of the input file to maintain consistency.  If, however, you input your
data by keyboard, and ask for an output file, the format of that output
file will be in the format known as "Free Format, Type 1".

The USGG2003 Model
----------------
   The USGG2003 model is known as a gravimetric geoid model, as it makes
use purely of gravimtric information, and does not rely on GPS ellipsoid
heights on leveled bench marks.  The USGG2003 model refers to a GRS-80
shaped ellipsoid, centered at the ITRF00 origin.  It does not support 
direct conversion between NAD 83 GPS ellipsoidal heights and NAVD 88 
orthometric heights.  

   When comparing the USGG2003 model with GPS ellipsoidal heights in the 
ITRF00 reference frame and leveling in the NAVD 88 datum, one can 
discern a systematic offset at a 50 cm level.  It is likely that this 
offset is inherent in the definition of the EGM96 geopotential surface. 
In addition, long-wavelength systematic errors are evident in the 
comparisons.  These errors are a composite of error in the NAVD 88 
elevations, error in the GPS ellipsoidal heights, and error in the 
USGG2003 model itself.  Since the errors are long-wavelength, they can 
be modeled locally as a plane; usually at a 1 to 2 part-per-million 
level. 

Deriving Orthometric Heights From GPS
-------------------------------------
   One key problem is deciding which orthometric height datum to use.
NGVD 29 is not a sea-level datum, and the heights are not true 
Orthometric heights.  The datum of NAVD 88 is selected to maintain 
reasonable conformance with existing height datums, and its Helmert 
heights are good approximations of true orthometric heights.  And, while 
differential ellipsoidal heights obtained from GPS are precise, they are 
often expressed in the NAD 83 datum, which is not exactly geocentric.  
In addition, USGG2003 rests upon an underlying EGM96 global geopotential 
model, and EGM96 does possess some error of commission.  

This leads to a warning:
   Do not expect the difference of a GPS ellipsoidal height at a point 
and the associated USGG2003 height to exactly match the vertical datum 
you need. However, one can combine the precision of differential carrier 
phase GPS with the precision of USGG2003 height differences, to approach 
that of leveling.  

   Include at least one existing bench mark in your GPS survey 
(preferably many bench marks).  The difference between the published 
elevation(s) and the height obtained from differencing your adopted GPS 
ellipsoidal height and the USGG2003 model, could be considered a "local 
orthometric height datum correction".  If you are surveying an extensive 
area (100+ km), and you occupy a lot of bench marks, then you might 
detect a trend in the corrections up to a one part-per-million level.  
This may be error in the USGG2003 model.

   We do not currently consider geoid-corrected GPS orthometric heights 
as a substitute for geodetic leveling in meeting the Federal Geodetic 
Control Subcommittee(FGCS) standards for vertical control networks.  
Studies are underway, and many less stringent requirements can be 
satisfied by geoid modeling.  Widespread success has been achieved with 
the preceding models, GEOID99, GEOID96, GEOID93 and GEOID90.

The XNTG Utility Program
------------------------
   The XNTG program can perform various functions, none of which are
required to use the INTG program.  The functions of XNTG are the 
extraction of sub-grids from the provided geoid grids, the translation 
between ASCII and binary grids, and the reporting of basic statistics 
for geoid grids.

Future Plans
------------
New gravimetric and hybrid geoid models will be generated in the next
year (USGG2004 and GEOID04) for all U.S. regions including the 
conterminous United States. These models may adopt newer global gravity 
field models derived from the GRACE mission as well as improved terrain 
and gravity field information.

For More Information
--------------------
    For Products Available From the National Geodetic Survey:

                     Information Services Branch
                     National Geodetic Survey, NOAA, N/NGS12
                     301-713-3242     fax: 301-713-4172

    For Information on USGG2003 and Future Research:

                     Dr. Daniel Roman
                     National Geodetic Survey, NOAA, N/NGS6
                     301-713-3202
                     Internet:   dan.roman@noaa.gov

                     Dr. Yan Ming Wang
                     National Geodetic Survey, NOAA, N/NGS6
                     301-713-3202
                     Internet:   yan.wang@noaa.gov

Visit our web site:
   http://www.ngs.noaa.gov/GEOID/USGG2003/index.html