README file    9-oct-96   dgm/das

The GEOID96 GEOID MODELS
------------------------
You have received these models on CD-ROM, or downloaded them from the National
Geodetic Survey (NGS) web site, the NGS FTP site, the NGS bulletin board system,
or have received the models on individual floppy disks.

Among the files you have received are:

   GEOID.EXE     the geoid interpolation program  (GEOID.FOR is source code)
                   (version 3.0)

   DOSXMSF.EXE   32-bit DOS extender  (needed for GEOID.EXE)

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

   GEO96NE.GEO   the GEOID96 grid for the Northeastern U.S.  36-50N,  89- 66W
   GEO96NC.GEO   the GEOID96 grid for the Northcentral U.S.  36-50N, 107- 84W
   GEO96NW.GEO   the GEOID96 grid for the Northwestern U.S.  36-50N, 125-102W
   GEO96SE.GEO   the GEOID96 grid for the Southeastern U.S.  24-38N,  89- 66W
   GEO96SC.GEO   the GEOID96 grid for the Southcentral U.S.  24-38N, 107- 84W
   GEO96SW.GEO   the GEOID96 grid for the Southwestern U.S.  24-38N, 125-102W
   GEO96AN.GEO   the GEOID96 grid for North Alaska           60-72N, 179-128W 
   GEO96AS.GEO   the GEOID96 grid for South Alaska           51-63N, 179-128W 
   GEO96HW.GEO   the GEOID96 grid for the Principal Hawaiian Islands
   GEO96PR.GEO   the GEOID96 grid for Puerto Rico - Virgin Islands

   GEOGRD.EXE    utility program for sub-area extraction and format conversion 
                    (GEOGRD.FOR is the source code)

To Install     (after uncompressing the files)
   1) Make a subdirectory on your hard disk    (example:  mkdir c:\geoid96).
   2) Copy the various geoid files into that subdirectory.
              copy *.*  c:\geoid96 /v        (for example)
   3) Repeat step 2) as required for your other sets of geoid files.
      (If you have also received G96SSS model files, do not place them in the
       same subdirectory as your GEOID96 files.)
      (If you have installed from floppies, the files are put in the designated
       subdirectory.)

   4) Check your AUTOEXEC.BAT and CONFIG.SYS files to insure compliance with
      the following notes:

   Note 1:  DOSXMSF.EXE must either be present in the same directory as 
            GEOID.EXE, or, it must be in a directory in your DOS PATH 
            environment variable. (such as:  c:\dos, for example) DOSXMSF.EXE
            may be freely reproduced and distributed, without royalty.

   Note 2:  You must have a statement FILES=25 (or a number greater than 25)
            in your CONFIG.SYS file.

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 the files in your local directory with a .GEO
extension are geoid height files.  You can operate with as few as one .GEO
file, or as many as 15.  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 indicates 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 which geoid files are available and their priority of use.

PLEASE NOTE:
   The AREA.PAR file we distribute contains the names of all the GEOID96
grid files.  You may not have received them all; you may not want them all.
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.  You must have an entry 
in AREA.PAR for each .GEO file to be searched.

   An Example:
      You just wish to work with the GEOID96 - Northwest file.  So, load 
      AREA.PAR into your favorite line editor, and delete the lines referring 
      to the other geoid regions.  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.  That file doesn't need to
exist if you are only going to look at the supported formats in the "Help".

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 GEOID96 Model
-----------------
   The GEOID96 model was computed on October 1, 1996 using over 1.8
million terrestrial and marine gravity values.  The method of computation
uses a Fast Fourier Transform (FFT) technique to compute the detailed geoid
structure, which is then combined with an underlying EGM96 geopotential model.
The result is a gravimetric geoid height grid with a 2' X 2' spacing in latitude
and longitude (2' x 4' in Alaska), referred to the Geodetic Reference System 
1980 (GRS 80) normal ellipsoid in an International Terrestrial Reference System
1994 (ITRF94) frame.  Then, by means of NAD 83 GPS ellipsoidal heights on
NAVD 88 benchmark data, plus known relationships between NAD 83 and the ITRF94 
reference frames, a conversion is applied to generate the final GEOID96 geoid 
model.  This conversion causes the GEOID96 model to be biased relative to a 
geocentric ellipsoid; but, this bias is deliberate.  The GEOID96 model was 
developed to support direct conversion between NAD 83 GPS ellipsoidal heights a
nd NAVD 88 orthometric heights.  

   When comparing the GEOID96 model with GPS ellipsoidal heights in the NAD 83 
reference frame and leveling in the NAVD 88 datum, it is seen that GEOID96 has 
roughly a 3-cm accuracy (one sigma) in the regions of GPS benchmark coverage.  
In those states with sparse (150km+) GPS benchmark coverage, less point accuracy
may be evident; but relative accuracy at about a 1 to 2 part-per-million level,
or better, should still be obtained.  For users with less stringent accuracy 
requirements, simple height conversions with GEOID96 in the conterminous United
States can be sufficient.  For users with more stringent accuracy requiements,
please see the section entitled "Deriving Orthometric Heights From GPS", later
in this document.  Users should be aware that GPS ellipsoid height error, by 
itself, can be significantly greater than error in geoid height differences.

States with Sparse GPS Benchmark Coverage
-----------------------------------------
   As of the date of computation of GEOID96, the states with sparse GPS 
benchmark coverage are:  Arkansas, Illinois, Indiana, Iowa, Kansas, Minnesota,
Missouri, North Dakota, South Dakota, and West Virginia.  This does not mean 
that the GEOID96 model can not be used in these states.  It does mean that
users may not see the same absolute accuracy when compared to other parts of the
United States with denser GPS benchmark coverage.  As stated above, relative
accuracy may reach 1-2 PPM.  Even so, the major components of the datum 
relationships between NAD 83 and NAVD 88 in all of the lower 48 states have been
incorporated into the GEOID96 geoid model.  As a rule, one can expect better
results with GEOID96, relative to GEOID93, in any part of the United States.


Alaska, Hawaii, Puerto Rico and the Virgin Islands
--------------------------------------------------
   It must be emphasized that the GEOID96 models in Alaska, Hawaii, Puerto
Rico, and the Virgin Islands were NOT, repeat, NOT computed by incorporating
a conversion surface based on GPS benchmarks.  This was due to a shortage of 
reliable NAD 83 GPS ellipsoidal heights on NAVD 88 benchmarks in these regions.
The GEOID96 geoid models provided in these areas are relative to a geocentric, 
GRS80 ellipsoid; as were earlier GEOID93 and GEOID90 models.  For this reason,
users should refer to the section entitled "Deriving Orthometric Heights From 
GPS", later in this document.

   Due to poorer data coverage, error estimates for GEOID96 in these regions are
larger.  Long-wavelength errors may be as large as 4-5 parts-per-million in some
areas.  Particular care must be used in computing heights in the tectonically 
active areas in southern Alaska.  Crustal motion may exceed 1 meter even after
accounting for the shift of the 1964 Prince William Sound Earthquake.

The National Imagery and Mapping Agency
---------------------------------------
   The National Imagery and Mapping Agency (NIMA), which incorporates the 
former Defense Mapping Agency (DMA), has been of immense help in this
endeavor.  NIMA has provided a major portion of the NGS land gravity data
set.  NIMA has also been instrumental in the creation of the various 30" and 3"
elevation grids in existence.  And, NIMA was a partner in the joint project
to compute the new global geopotential model, EGM96.  Although the work of the 
NIMA generally precludes public recognition, their cooperation is gratefully
acknowledged.

GSFC/NIMA Geopotential Model, EGM96
-----------------------------------
   The Goddard Space Flight Center (GSFC) and the National Imagery and Mapping
Agency (NIMA) have been engaged in a joint project to compute an improved global
spherical harmonic model of the Earth's geopotential.  This model incorporates 
the latest satellite tracking data, as well as altimeter data from 
TOPEX/Poseidon, ERS-1, and the Geosat Geodetic Mission.  EGM96 also incorporates
new surface and marine gravity data covering the globe, including the former 
Soviet Union.

   EGM96 is a global geopotential model expressed as spherical harmonic 
coefficients 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.  We thank the many members of the project
team for making this model available.

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, GEOID96 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 GEOID96 height to exactly match the vertical datum you need.
The results will be close when converting NAD 83 GPS ellipsoidal heights into
NAVD 88 elevations; but, maybe not accurate enough for your requirement.

   However, one can combine the precision of differential carrier phase GPS with
the precision of GEOID96 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 GEOID96 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 benchmarks, then you might detect a trend in the corrections 
up to a one part-per-million level.  This may be error in the GEOID96 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 preceeding models, 
GEOID93 and GEOID90.

The GEOGRD 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 GEO96NW.GEO, "converting" from
              binary, .GEO into binary, .GEO, and specifying the Colorado
              state boundaries.

A Technical Note on Program GEOID
---------------------------------
   Some users prefer to write their own interpolation software.  If you do,
please be aware that there is a loss of precision in the grid file headers
for grid spacings of 2' (or 4').  This is accomodated in program GEOID 3.00
by internally re-computing the grid spacing in subroutine GRIDS.  You might
need to place similar code in your interpolation software, depending upon
how it was written.

------------------  (Example Fortran 77 code)  -----------------------------

*** patch for inexact headers (due to 2' spacing) 

          idx1=idnint(DX1*3600.d0)
          DX(NAREA) = dble(idx1)/3600.d0

          idy1=idnint(DY1*3600.d0)
          DY(NAREA) = dble(idy1)/3600.d0

*****     DX(NAREA) = DX1     **** old code
*****     DY(NAREA) = DY1     **** old code

----------------------------------------------------------------------------

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 measurements, and the
study of error in GPS ellipsoid heights and in the NAVD88 vertical datum.
It is likely that this research, in conjunction with the completion of the
state upgrade GPS surveys, will yield a significant improvement to our geoid 
model in 1999.

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

                     Information Services Branch
                     National Geodetic Survey, NOAA, N/NGS12
                     1315 East-West Highway, SSMC3, Station 9202
                     Silver Spring, MD 20910-3282
                     301-713-3242     fax: 301-713-4172

    For Information on GEOID96 and Future Research:

                     Dr. Dennis G. Milbert
                     National Geodetic Survey, NOAA, N/NGS5
                     1315 East-West Highway, SSMC3, Station 9349
                     Silver Spring, MD 20910-3282
                     301-713-3202
                     Internet:   dennis@ngs.noaa.gov

                     Dr. Dru A. Smith
                     National Geodetic Survey, NOAA, N/NGS5
                     1315 East-West Highway, SSMC3, Station 9316
                     Silver Spring, MD 20910-3282
                     301-713-3202
                     Internet:   dru@ngs.noaa.gov


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

Best Wishes!


README file    9-oct-96   dgm/das
