README file    12-jul-94   dgm/dgs

The GEOID93 and ALASKA94 GEOID MODELS

You have probably received one to five floppies, depending upon your request.
Among the files on the floppies are

   GEOID.EXE      the geoid interpolation program  (GEOID.FOR is source code)

   AREA.PAR       text file of the filenames of geoid height grids

   ALA94.GEO      the ALASKA-94 grid for Alaska          51-72N, 179-128W 
   GEOID93E.GEO   the GEOID93 grid for the Eastern U.S.  24-50N,  89- 66W
   GEOID93C.GEO   the GEOID93 grid for the Central U.S.  24-50N, 107- 84W
   GEOID93W.GEO   the GEOID93 grid for the Western U.S.  24-50N, 125-102W
   HAW93.GEO      the GEOID93 grid for the Principal Hawaiian Islands
   PRVI93.GEO     the GEOID93 grid for Puerto Rico - Virgin Islands

To Install
   1) Make sure your original floppies are write protected!
   2) Make a subdirectory on your hard disk (GEOID will also run on floppies)
   3) Go into the subdirectory .
              cd geoid93       (for example)
   4) Copy the first floppy disk into your subdirectory.
              copy A:*.* *.* /v
   5) Repeat step 4) as required for your other floppies.
   6) Put the original floppies in a safe place!

To Execute
   Type  GEOID             , and follow the prompts.

To Terminate
   You can stop the program at any time by the Control C key combination.

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

How Program GEOID Works
   The various geoid height grids are stored in the ".GEO" files.  Program
GEOID will assume that any file in your local directory with a .GEO
extension is a geoid height file.  You can operate with as few as one .GEO
file, or as many as eight.  When the program interpolates a given point, it
checks an internal list of .GEO boundaries, and uses the earliest list entry
whose boundaries contain that point.  The order in which the .GEO file names
appear on the opening screen indicate the order in which the .GEO files are
searched.

The AREA.PAR File
   AREA.PAR is a plain, ASCII text file.  It specifies the order in which
.GEO files are to be used.  If you have a favorite .GEO file, put it at the
top of the AREA.PAR list.  There is no problem in having overlapping .GEO
files, nor is there any problem in having nested .GEO files.  The AREA.PAR
file specifies their priority of use.

PLEASE NOTE:
   The AREA.PAR on your floppies contains the names of the six GEOID93
grid files.  You may not have gotten all six; you may not want all six.
This is not a problem.  If a .GEO file name is in the AREA.PAR file, but
not in the local directory, then a warning message is issued, and program
GEOID proceeds with the files that are available.  And, if a .GEO file is in
your local directory, but is not listed in your AREA.PAR file, then the .GEO
file will still be used.  As mentioned above, all .GEO files in your local
directory will be used.  The AREA.PAR file allows you to specify the
priority of use.

   An Example:
      You just wish to work with the GEOID93 - West file.  So, load AREA.PAR
      into your favorite line editor, and delete the lines referring to 
      GEOID93C, GEOID93E, HAW93, ALA94, and PRVI93.  You may now delete those
      .GEO files without receiving the warning messages on the opening screen 
      of program GEOID.  Save the updated AREA.PAR as plain ASCII text.

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 an input file name.

Data Output
   Results are collected into an output file.  The default name of these
files is GEOID.OUT, but you can use any legal file name you choose.  (A word
of advice:  Don't use misleading extensions such as .EXE, .GEO, .BAT, etc.)
The format of the output file is linked to the format of the input file to
maintain consistency.

The GEOID93 Model
   The GEOID93 model was computed on January 26, 1993 using over 1.8
million terrestrial and ship gravity values.  The method of computation
uses a Fast Fourier Transform (FFT) technique to compute the detailed geoid
structure, which is combined with an underlying OSU91A geopotential model.
The result is a geoid height grid with a 3' X 3' spacing in latitude and
longitude, referred to the Geodetic Reference System 1980 (GRS 80) normal
ellipsoid.  By comparing the GEOID93 model with combined GPS and leveling,
the GEOID93 has roughly a 10-cm accuracy (one sigma) over length scales of
100 km.  Better accuracy is seen over shorter lengths.  At transcontinental
spacings the accuracy of GEOID93 will be governed by the accuracy of the
underlying global geopotential model, OSU91A.  In some locations of the
country, long-wavelength errors in GEOID93 up to a 1 to 2 part-per-million
level may occur.  Because of better data quality and coverage, and better
computational procedures, GEOID93 possesses better accuracy in mountains 
when compared to GEOID90.

The ALASKA94 Model
  The ALASKA94 model was computed using the same basic methods as GEOID93.
Because of meridian convergence the geoid height grid has a 3' by 6' spacing
interval in latitude and longitude respectively.  The same general trends in
accuracy issues are found.  Because of poorer overall data coverage compared
to GEOID93 for the continental US, error estimates for Alaska are larger.  
Long-wavelength errors may be as large as 4-5 parts-per-million in some
areas.  In addition, over short distances (100km), the Alaskan geoid model
approximately has a 20 cm. accuracy.  Particular care must be used in 
establishing GPS-derived orthometric heights using ALASKA94 in the tectonically
active areas in southern Alaska.  Crustal motion may exceed 1 meter even
after coseismic events such as the 1964 Prince William Sound Earthquake
are modeled and removed.  Improvement in modeling the causative effects
of crustal motion will change this situation in the next couple of years.

The Defense Mapping Agency
   The Defense Mapping Agency (DMA) has been of immense help in this
endeavor.  DMA has provided a major portion of the NGS land gravity data
set.  DMA has also been instrumental in the creation of the various 30" 
elevation grids in existence.  Although the work of the DMA generally
precludes public recognition, their cooperation in this work is gratefully
acknowledged.

The OSU91A Model
   The OSU91A model was computed by Prof. Richard H. Rapp, Yan Ming Wang, 
and Nikolaos K. Pavlis, at the Department of Geodetic Science and Surveying, 
The Ohio State University, Columbus, Ohio.  This is a global geopotential 
model using spherical harmonics complete to degree and order 360.  Therefore, 
the shortest wavelength this model can exhibit is one degree, and its 
resolution is one-half degree (about 50 km).  Although this model does not 
reproduce geoid structure at very fine resolution, it is global.  Its 
accuracy is estimated at around 60-cm (one sigma).

A Useful Utility Program
   GEOGRD  -- This converts to and from ".GEO" binary files and ASCII text
              files.  It can also be used to extract subgrids in the process
              of conversion.  For example:  one can make a .GEO grid for the
              state of Colorado by using GEOID93W.GEO, "converting" from
              binary, .GEO into binary, .GEO, and specifying the Colorado
              state boundaries.

Deriving Orthometric Heights From GPS
   One key problem is deciding which orthometric height datum to use.
NGVD29 is not a sea-level datum, and the heights are not true orthometric
heights.  The datum of NGVD88 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, an ellipsoidal height at a single point does
not display the same precision.  In addition, GEOID93 rests upon an 
underlying OSU91A global geopotential model, and OSU91A 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 GEOID93 height to exactly match the vertical datum you need.

   But, one can exploit the precision of differential carrier phase GPS with
the precision of GEOID93 height differences to approach that of leveling.  
Include at least one existing benchmark in your GPS survey (preferably many 
benchmarks).  The difference between the published elevation(s) and the 
height obtained from differencing your adopted GPS ellipsoidal height and 
the GEOID93 model, could be considered a "local orthometric height datum 
correction".  If your are surveying an extensive area (100+ km), and you
occupy a lot of benchmarks, then you might detect a trend in the corrections 
at a one part-per-million level.  This may be error in the GEOID93 model.

   We do not currently consider geoid-corrected GPS heights as a substitute
for geodetic leveling in meeting the FGCS standards for vertical control
networks.  Work is underway, and many less stringent requirements can be
satisfied by geoid modeling.  Widespread success has been achieved with the
preceeding model, GEOID90.

Future Plans
   A research effort is underway to improve geoid height estimates in the
future, perhaps at the 1-cm accuracy level.  One important direction is
integrating gravity data with GPS and geodetic leveling measurments.  It is
likely that this research, in conjunction with greater availability of high
precision GPS surveys, will yield a significant upgrade to our geoid model
in 1996.

For More Information

    For Products Available From the National Geodetic Survey:

                     National Geodetic Information Center
                     301-713-3242

    For Information on GEOID93 and Future Research:

                     Dr. Dennis G. Milbert
                     301-713-3202
                     Internet:   dennis@hecate.ngs.noaa.gov

                     Donald G. Schultz
                     301-713-3202
                     Internet:   donald@charybdis.ngs.noaa.gov
  

Best Wishes!

README file    12-jul-94   dgm/dgs