README.txt file    16-july-1997   das/hjs

The CARIB97 GEOID MODELS
------------------------
 
  The CARIB97 model is a high resolution geoid height model covering the   
  region 9-28N, 86-58W.  It has been prepared in a cooperative effort with the       
  National Imagery and Mapping Agency (NIMA).  The CARIB97 geoid model has 
  been designed specifically for the Caribbean Sea.  Due to data coverage and 
  computational issues, one will find offsets between CARIB97 and either 
  GEOID96 or G96SSS in regions of overlap.                                                               
 

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 3.10 -- Written specifically for distribution
                    with CARIB97, but equally compatable with previous 
                    geoid models)

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

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

   CAR97W.GEO    the CARIB97 grid for the Western Caribbean Sea  9-28N, 86-71W
   CAR97E.GEO    the CARIB97 grid for the Eastern Caribbean Sea  9-28N, 73-58W

   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:\carib97).
   2) Copy the various geoid files into that subdirectory.
              copy *.*  c:\carib97 /v        (for example)
   3) Repeat step 2) as required for your other sets of geoid 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.

   Note 3:  Do NOT put CARIB97 files in the same directory as other 
            geoid models (such as MEXICO97, G96SSS or GEOID96).  This will cause an
            error in program GEOID.EXE.


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 15.  When the program interpolates a given point, it
checks the AREA.PAR file for .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.
   When running program GEOID.EXE, the latitude and longitude of each point
must be input.  The GEOID96 models are heights above the NAD 83 ellipsoid,
while all other recent geoid models (G96SSS, MEXICO97, and CARIB97) are heights 
above a GRS-80 ellipsoid, centered at the ITRF94(1996.0) origin.  However, 
latitudes and longitudes in the ITRF94/GRS-80 system are very close to those 
of the NAD 83 system (with only 1-2 meters of horizontal shift.)  So either 
type of latitude and longitude (NAD 83 or ITRF94) 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 
interchangably with ITRF94/GRS-80 latitudes and longitudes is merely an acceptable 
horizontal approximation.  Each geoid model will always contain geoid heights above 
the specific ellipsoid, stated above.
  Use of NAD 27 latitudes and longitudes is really only valid for MEXICO97 and 
CARIB97, where no NAD 27 to NAD 83 conversion is readily available.  The horizontal 
shifts between NAD 83 and NAD 27 can exceed 100 meters in the United States, causing 
a noticable difference in the interpolated geoid value in GEOID96 and G96SSS.  To 
convert from NAD 27 to NAD 83 latitudes and longitudes in the U.S., you may use 
programs NADCON or CORPSCON, available from NGS.

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 file we distribute with the CARIB97 model contains the names 
of all the CARIB97 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 CARIB97 - West 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 CARIB97 Model
-----------------
   The CARIB97 model was computed on June 20, 1997 using over seven hundred
thousand 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, referred to the Geodetic Reference System 1980 
(GRS 80) normal ellipsoid in an International Terrestrial Reference System 1994
(ITRF94(1996.0)) frame.  The EGM96 model was evaluated to conform to a 
"tide-free" system, insofar as the Earth's permanent tide effect is concerned.

   When comparing the CARIB97 model with GPS ellipsoidal heights in the ITRF94
reference frame and tidal benchmarks (in various individual island datums), one 
may discern individual offsets, on an island by island basis, with an average
offset at the -50 cm level.  Many different signals contribute to these island-wide
offsets.  First, the difference between Mean Sea Level (MSL, which is one possible
sea level measured at a tide guage) and the geoid level is non-zero, and  depends 
partly on the permanent ocean circulation around each island.  Also, the actual 
sea level measured at each tide guage (mean sea level, mean low water, etc) was not 
clearly identified, which will contribute to relative differences between differing 
tide guages.  Finally, the treatment of offshore gravity data in the CARIB97 model 
will contribute to these offsets.  

   In addition to the -50 cm offset, localized systematic errors are evident
on some islands.  These systematic errors may appear as extreme (30+ ppm) tilts, 
over very short ranges, island by island.  These sort of short of localized
extreme tilts are generally too large to be geoid error.   GPS height error is in 
the 2.5 cm RMS range, indicating that these systematic errors are probably caused by
relative errors between neighboring tide gauge measurements.  

   Finally, some long-wavelength systematic errors are evident in the 
comparisons.  These errors are a composite of error in the tidal benchmark elevations, 
error in the GPS ellipsoidal heights, and error in the CARIB97 model itself.  
Since the errors are long-wavelength, they can be modeled locally as a plane; 
perhaps at a 5 to 6 part-per-million level.  One may expect to see larger tilts
in the CARIB97 model near Cuba and Venezuela due to a lack of gravity coverage.


The National Imagery and Mapping Agency (NIMA)
----------------------------------------------
   The National Imagery and Mapping Agency (NIMA) has often been a significant
contributor of data in previous NGS geoid models.  For CARIB97, NIMA joined
NGS as a full partner, contributing time, data and personnel to help ensure
the success of this program.  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 with 
NASA in the joint project to compute the new global geopotential model, EGM96.  
Although the work of the NIMA generally precludes public recognition, their 
partnership with NGS is gratefully acknowledged and both agencies are looking 
to the future for further partnerships.


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 that each Caribbean island may have it's own datum, or
no defined datum at all.  In addition, for the continental US, NGVD 29 is not 
a sea-level datum, and the heights are not true orthometric heights.  The 
datum of NAVD 88 for conterminous North America is selected to maintain 
reasonable conformance with existing height datums, and its Helmert heights 
are good approximations of true orthometric heights.  In addition, CARIB97 
contains local and regional vertical datum offsets, itself. 

This leads to a warning:
   Do not expect the difference of a GPS ellipsoidal height at a point and
the associated CARIB97 height to exactly match the vertical datum you need.

However, one can combine the precision of differential carrier phase GPS with
the relative precision of CARIB97 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 CARIB97 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 
perhaps a six part-per-million level.  This may be error in the CARIB97 model.

   The United States does not currently consider geoid-corrected GPS 
orthometric heights as a  substitute for geodetic leveling in meeting our 
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, GEOID96, 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
              island of Jamaica by using CAR97W.GEO, "converting" from
              binary (.GEO) into binary (.GEO) and specifying an area that
              surrounds Jamaica.  Remember two things: 1) If you do create
              new .GEO grids, you must update your AREA.PAR file.  And,
              2) a total of only 15 geoid grid files can be open
              at once with the PC version of GEOID.  If you construct many
              different grid pairs, consider creating different sub-
              directories with different combinations of grid pairs in them.

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'.  This is accomodated in program GEOID 3.00 (and
later versions, including the 3.10 version supplied with CARIB97) 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 various island datums
in use.  Additionally, more gravity data is being collected, and we are 
investigating the use of additional NIMA gravity data for an improvement
to CARIB97.  It is likely that this research will yield a significant 
improvement to our geoid model in 1999.

Acknowledgements
----------------
  We would like to thank all of the agencies that have contributed to the 
NGS databases.  We thank the United Nations Environment Programme (UNEP)
for their GTOPO30 global 30 arcsecond digital terrain data, which was used
everywhere but in Florida.  We also thank the National Geophysical Data Center 
(NGDC) for their TOPO30 digital terrain data (TOPO30) which was used in Florida.

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 Products Available From the National Imagery and Mapping Agency:

                     NIMA(GIMG), Mail Stop L-41
                     Geodesy and Geophysics Department
                     National Imagery and Mapping Agency
                     3200 South Second Street
                     St. Louis, MO 63118-3399
                     314-260-1174     fax: 314-260-1071
                     

    For Information on CARIB97 and Future Research:

                     Dr. Dru A. Smith
                     National Geodetic Survey, NOAA, N/NGS5
                     301-713-3202
                     Internet:   dru@ngs.noaa.gov

                     Mr. Howard J. Small
                     NIMA
                     Attn: H. Small
                     3200 S. Second Street
                     St. Louis, MO 63118-3399
                     314-260-1052
                     Internet:  smallh@nima.mil


Visit the CARIB97 web site:

   http://www.ngs.noaa.gov/GEOID/carib97.html


README file    16-July-1997   das/hjs

