                         THE GEOID99 README FILE
                         -----------------------

Version: September 30, 1999   das/drr

The GEOID99 GEOID MODELS
------------------------
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 (PC) or INTG.F (Sun))

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

 DOSXMSF.EXE (PC)   32-bit DOS extender (needed for running INTG.EXE on a 386 PC)

The following file names are valid for PC or Sun (if, however, you downloaded the
ASCII versions of these files, the suffix will be ".asc" rather than ".bin"):
 g1999u01.bin       GEOID99 grid #1 for CONUS (40-58N, 230-249E)
 g1999u02.bin       GEOID99 grid #2 for CONUS (40-58N, 247-266E)
 g1999u03.bin       GEOID99 grid #3 for CONUS (40-58N, 264-283E)
 g1999u04.bin       GEOID99 grid #4 for CONUS (40-58N, 281-300E)
 g1999u05.bin       GEOID99 grid #5 for CONUS (24-42N, 230-249E)
 g1999u06.bin       GEOID99 grid #6 for CONUS (24-42N, 247-266E)
 g1999u07.bin       GEOID99 grid #7 for CONUS (24-42N, 264-283E)
 g1999u08.bin       GEOID99 grid #8 for CONUS (24-42N, 281-300E)
 g1999a01.bin       GEOID99 grid #1 for Alaska (60-72N, 172-204E)
 g1999a02.bin       GEOID99 grid #2 for Alaska (60-72N, 202-234E)
 g1999a03.bin       GEOID99 grid #3 for Alaska (49-61N, 172-204E)
 g1999a04.bin       GEOID99 grid #4 for Alaska (49-61N, 202-234E)
 g1999h01.bin       GEOID99 grid #1 for Hawaii (18-24N, 199-206E)
 g1999p01.bin       GEOID99 grid #1 for Puerto Rico/VI (15-21N, 291-296E)

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 G99SSS model files, you may safely place
      them in the same directory as GEOID99, if you like.)
   3) If you are using a PC, check your AUTOEXEC.BAT and CONFIG.SYS files 
      to insure compliance with the following notes:

   Note 1:  If you are running a 16-bit PC (such as a 386), then DOSXMSF.EXE 
            must either be present in the same directory as 
            INTG.EXE, or, it must be in a directory in your DOS PATH 
            environment variable (such as:  c:\dos).   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
   (PC or Sun) Type INTG            , 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.

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.

PC or Sun Data:
     g1999u**.bin    4,933,728 bytes
     g1999a**.bin    5,540,208 bytes
     g1999h01.bin      607,968 bytes
     g1999p01.bin      434,688 bytes
ASCII Data:
     g1999u**.asc   12,488,896 bytes (uncompressed)
     g1999a**.asc   14,024,273 bytes (uncompressed)
     g1999h01.asc    1,539,044 bytes (uncompressed)
     g1999p01.asc    1,100,429 bytes (uncompressed)
PC executables:
     INTG.EXE          161,280 bytes
     XNTG.EXE          165,888 bytes
     DOSXMSF.EXE       393,942 bytes
Sun executables:
     INTG              107,320 bytes
     XNTG               67,840 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 14.  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 (EXCEPTION: In the northwest USA, there
is a small overlap between the CONUS g1999u01.bin file and the
Alaska g1999a04.bin file  In this case, if the CONUS file is available, 
it will take precedence over the Alaska file).  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 GEOID99 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, ITRF97, 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 ITRF97/GRS-80 or
WGS84 latitudes and longitudes is merely an acceptable horizontal 
approximation.  GEOID99 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 GEOID99 Model
-----------------
   The GEOID99 model is known as a hybrid geoid model, combining
gravimetric information with GPS ellipsoid heights on leveled bench marks.
The GEOID99 model was developed to support direct conversion between 
NAD 83 GPS ellipsoidal heights and NAVD 88 orthometric heights.  

   When comparing the GEOID99 model with GPS ellipsoidal heights in the NAD 83 
reference frame and leveling in the NAVD 88 datum, it is seen that GEOID99 has 
roughly a 4.6 cm absolute accuracy (one sigma) in the regions of GPS 
on Bench Mark coverage.  In those states with sparse (150km+) GPS on Bench Mark 
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 GEOID99 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.

GPS on Bench Mark Coverage
-----------------------------------------
   As of the date of computation of GEOID99, all 48 of the Conterminous
United States had a complete High Accuracy Reference Network (HARN).  
Additionally, a few states had already had re-observations of their
HARNs completed.

Alaska, Hawaii, Puerto Rico and the Virgin Islands
--------------------------------------------------
   It must be emphasized that the GEOID99 models in Alaska, Hawaii, Puerto
Rico, and the Virgin Islands were NOT, repeat, NOT computed by incorporating
GPS on leveled bench marks.  This was due to a shortage of 
reliable NAD 83 GPS ellipsoidal heights on NAVD 88 bench marks in these regions.
The GEOID99 geoid models provided in these areas are relative to a geocentric, 
GRS80 ellipsoid.  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 GEOID99 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.

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, GEOID99 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 GEOID99 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 GEOID99 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 GEOID99 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 GEOID99 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, 
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
------------
   Due to the quickly expanding availability of GPS data on leveled
bench marks in the NGS database, there is some incentive to providing
annual geoid models which reflect the latest GPS/BM data.  In addition,
from the gravimetric geoid side, there is always ongoing research
to improve the theory of geoid determination.  This research moves
at a slower pace than GPS/BM data acuqisition, so while annual 
*hybrid* geoid models (like GEOID99) may be provided, there may
be bi- or tri-annual *gravimetric* geoid models (like G99SSS).

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 GEOID99 and Future Research:

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

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


Visit our web site:
   http://www.ngs.noaa.gov/GEOID/GEOID99/geoid99.html
  


