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

The DCAR97 DEFLECTION OF THE VERTICAL MODELS
--------------------------------------------

  The DCAR97 model is a high resolution deflection of the vertical 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 DMEX97 deflection model has been designed specifically for            
  the Caribbean Sea.  Due to data coverage and computational issues, one will find     
  differences between DCAR97 and DEFLEC96 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

 DEFLEC.EXE  the deflection interpolation program (DEFLEC.FOR is source code)
                   (version 3.1 -- Written specifically for distribution
                    with DCAR97, but equally compatable with previous 
                    deflection models)

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

 AREA.PAR    text file of the filenames of pairs of deflection grids

 DCAR97NW.XII (.ETA) the DCAR97 pair for the Northwest Caribbean   21-28N, 86-71W
 DCAR97NE.XII (.ETA) the DCAR97 pair for the Northeast Caribbean   21-28N, 73-58W
 DCAR97CW.XII (.ETA) the DCAR97 pair for the Centralwest Caribbean 15-22N, 86-71W
 DCAR97CE.XII (.ETA) the DCAR97 pair for the Centraleast Caribbean 15-22N, 73-58W
 DCAR97SW.XII (.ETA) the DCAR97 pair for the Southwest Caribbean    9-16N, 86-71W
 DCAR97SE.XII (.ETA) the DCAR97 pair for the Southeast Caribbean    9-16N, 73-58W

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

To Install     (after uncompressing the files)
   1) Make a subdirectory on your hard disk    (example:  mkdir c:\dcar97).
   2) Copy the various deflection files into that subdirectory.
              copy *.*  c:\dcar97 /v        (for example)
   3) Repeat step 2) as required for your other sets of deflection 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 
            DEFLEC.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=46 (or a number greater than 46)
            in your CONFIG.SYS file.

   Note 3:  Do NOT put DCAR97 files in the same directory as other 
            deflection models (such as DEFLEC96).

To Execute
   Type  DEFLEC             , 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 DEFLEC Works
------------------------
   The deflection grids are stored in the ".XII" and ".ETA" files.  Program
DEFLEC will assume that the files in your local directory with .XII or .ETA
extensions are deflection files.  You can operate with as few as one .XII/.ETA
pair, or as many as 18.  When the program interpolates a given point, it
checks the AREA.PAR file for .XII/.ETA boundaries, and uses the earliest list 
entry whose boundaries contain that point.  The order in which the .XII/.ETA 
pairs appear on the opening screen indicates the order in which the .XII/.ETA 
pairs are searched.
   When running program DEFLEC.EXE, the latitude and longitude of each point
must be input.  The DEFLEC96 models are relative to the NAD 83 ellipsoid,
while all other recent deflection models (DMEX97 and DCAR97) are relative to
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 deflection values.  This does *not* imply that the deflections are 
relative to a different ellipsoid.  Using NAD 83 latitudes and longitudes 
interchangably with ITRF94/GRS-80 latitudes and longitudes is merely an acceptable 
horizontal approximation.  Each deflection model will always contain deflections
relative to a specific ellipsoid, stated above.
  Use of NAD 27 latitudes and longitudes is really only valid for DMEX97 and 
DCAR97, 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 deflection value in DEFLEC96.  
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
.XII/.ETA files are to be used.  If you have a favorite .XII/.ETA pair, put 
the basename at the top of the AREA.PAR list.  There is no problem in having 
overlapping .XII/.ETA files, nor is there any problem in having nested 
.XII/.ETA files.  The AREA.PAR file specifies which .XII/.ETA pairs are
available and their priority of use.

PLEASE NOTE:
   The AREA.PAR file we distribute with DCAR97 contains the names of all the 
DCAR97 grid pairs.  You may not have recieved them all; you may not want them 
all.  This is not a problem.  If a .XII/.ETA basename is in the AREA.PAR file, 
but not in the local directory, then a warning message is issued, and program 
DEFLEC proceeds with the files that are available and listed.  You must have 
an entry in AREA.PAR for each .XII/.ETA pair to be searched.

  An Example:
    You just wish to work with the DCAR97 - Southwest file.  So, 
    load AREA.PAR into your favorite line editor, and delete the lines in 
    AREA.PAR referring to the other deflection files.  You may now delete 
    those .XII/.ETA files (assuming you ever had them) without receiving any
    irritating warning messages on the opening screen of program DEFLEC.

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 supported, 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 DEFLEC.OUT, but you can use any legal file name you choose.  (A word
of advice:  Don't use misleading extensions such as .EXE, .XII .ETA, .BAT, 
etc.) The format of the output file is linked to the format of the input file 
to maintain consistency.

===================>  THE SENSE OF THE SIGNS  <======================

   A positive meridian component of deflection of the vertical (Xi) indicates
that the astronomic latitude will fall to the north of the corresponding
geodetic latitude of the point.

   A positive prime-vertical component of deflection of the vertical (Eta) 
indicates that the astronomic longitude will fall to the east of the 
corresponding geodetic longitude of the point.

   The computed Laplace correction (Hor.Laplace) should be ADDED to a 
clockwise astronomic azimuth, to obtain a "near-geodetic" Laplace azimuth.  
     Note: the deflection correction is usually negligible, yielding a 
           geodetic azimuth.
     Note: in many textbooks, the Laplace correction is shown with the 
           opposite sign and is subtracted from astronomic azimuth.

The DCAR97 Model
------------------
   The DCAR97 model was computed on June 20, 1997 using over seven hundred
thousand terrestrial and marine gravity values originally used in the computation 
of the MEXICO97 geoid height model.  The deflections are derived from spline 
models of the curvatures of CARIB97.  The result is a pair of deflection grids
with a 2' X 2' spacing in latitude and longitude, referred 
to the Geodetic Reference System 1980 (GRS 80) normal ellipsoid.  In order to 
have the deflections refer to the surface of the Earth (and not to sea level),
a correction for the curvature of the plumbline was included.
   One may expect to see larger tilts in the DCAR97 model near Cuba and Venezuela,
due to a lack of adequate 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 DCAR97, 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.

The Laplace Correction
----------------------
   An azimuth measured with a theodolite is an astronomic azimuth.  It refers 
to an azimuth expressed in a local horizon coordinate system aligned along 
the local gravity vector (line of the plumb bob).  A geodetic azimuth, on the 
other hand, is expressed about a local normal to the ellipsoid.  An astronomic 
azimuth is converted into a geodetic azimuth by applying the Laplace 
correction.  The Laplace correction typically takes one of two forms:

The Simplified Laplace Correction  (Hor. Laplace) :
   - Eta x Tangent(Geodetic Latitude)

The Complete (Extended) Laplace Correction:
   - Eta x Tangent(Geodetic Latitude)
   - [Xi x Sine(Azimuth) - Eta x Cosine(Azimuth)] x Cotangent(Zenith Dist.)

        The second term is known as the deflection correction, and it is 
        negligible for horizontal lines of sight.

   The output from DEFLEC provides not only the Xi and Eta components of
deflection of the vertical, but also provides the horizontal (simplified)
Laplace correction.  The sense of the sign is such that one should ADD the
correction from program DEFLEC (Hor. Laplace) to an astronomic azimuth that 
is measured clockwise, thereby obtaining a "Laplace Azimuth."  Such an azimuth 
does not contain the deflection correction, and is suitable for encoding into 
a *60* "Horizontal Blue-Book" record.

The CARIB97 and DCAR97 Models
-------------------------------
   The geoid is only one of an infinite number of equipotential surfaces of
the Earth -- each one is at a different height.  The geoid, by definition, 
corresponds to sea level.  Slopes of the geoid surface provide the components 
of deflection of the vertical at sea level.  However, deflections of the
vertical at high altitudes will not be the same as sea level deflections.
This difference is called "curvature of the plumb line", and can reach
several arc-seconds in areas of high altitude and rugged terrain.  DCAR97
contains a correction to provide surface deflections of the vertical.  For 
this reason, CARIB97 slopes will not exactly correspond to DCAR97 deflection 
components.

   In short, CARIB97 and DCAR97 are completely compatible in the sense that
they were computed by the same technique (FFT) from a common grid of gravity
data.  CARIB97 and DCAR97 are not compatible in the sense that CARIB97
refers to sea level (geoid height), whereas DCAR97 models deflections of
the vertical at the surface of the Earth.

The DEFGRD Utility Program
--------------------------
   DEFGRD  -- This converts to and from ".XII/.ETA" pairs of binary files 
              and ASCII text file pairs.  It can also be used to extract 
              subgrids in the process of conversion.  For example: one can 
              make a .XII/.ETA pair for the state of Quintana Roo by using 
              D96NWU.XII (.ETA), and "converting" from binary (.XII/.ETA) 
              into binary (.XII/.ETA) and specifying the appropriate area
              boundaries.  Remember two things: 1) If you do create new
              deflection grids, you must update your AREA.PAR file.  And,
              2) a total of only 18 pairs of deflection grids can be open
              at once with the PC version of DEFLEC.  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 DEFLEC
----------------------------------
   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 DEFLEC 3.00 (and
later versions, including the 3.10 version supplied with DCAR97) 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 deflection estimates in the 
future, in conjunction with an improved geoid height model.  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 DCAR97.  It is likely that this research 
will yield a significant improvement to our deflection models 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 DCAR97 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 our DCAR97 web site:

   http://www.ngs.noaa.gov/DEFLEC/dcar97.html

README file    16-July-1997   das/hjs