README file    3-oct-91   dgm

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

   DEFLEC.EXE  the deflection interpolation program (DEFLEC.FOR is source code)

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

   D90NE.XII (.ETA)  the DEFLEC90 pair for Northeastern U.S.  36-50N,  87- 66W
   D90NC.XII (.ETA)  the DEFLEC90 pair for Northcentral U.S.  36-50N, 106- 85W
   D90NW.XII (.ETA)  the DEFLEC90 pair for Northwestern U.S.  36-50N, 125-104W
   D90SE.XII (.ETA)  the DEFLEC90 pair for Southeastern U.S.  24-38N,  87- 66W
   D90SC.XII (.ETA)  the DEFLEC90 pair for Southcentral U.S.  24-38N, 106- 85W
   D90SW.XII (.ETA)  the DEFLEC90 pair for Southwestern U.S.  24-38N, 125-104W
   D90PR.XII (.ETA)  the DEFLEC90 pair for Puerto Rico/V.I.   15-21N,  70- 63W

To Install
   1) Make sure your original floppies are write protected!
   2) Make a subdirectory on your hard disk (DEFLEC will also run on floppies)
              md deflec90       (for example)
   3) Go into the new subdirectory .
              cd deflec90       (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!

   7) How many files can be open at the same time? 
       a) Go to the main directory and list CONFIG.SYS
            > c:
            > cd \
            > type config.sys
       b) Look for a "FILES=..." entry.
            eg:  FILES=20
       c) DEFLEC can open up to 20 files at one time (with 6 pairs of grids).
          If you do not have a "FILES=..." command, then you need to add one
          to CONFIG.SYS using a text editor.
            Add the line:   FILES=20
          If you do have a "FILES=..." command, but the number of files is
          less than 20, then you need to edit that line using a text editor.
            Edit the line to read:   FILES=20
      You can read more about CONFIG.SYS and FILES in your DOS manual.

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 six.  When the program interpolates a given point, it
checks an internal list of 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 indicate the order in which the .XII/.ETA pairs
are searched.

The AREA.PAR File
   AREA.PAR is a vanilla, 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 on your floppies contains the basenames of six DEFLEC90
grid pairs.  You may not have gotten all six; you may not want all six.  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.  There are NO default
names for .XII and .ETA files.  You must have have an entry in AREA.PAR for 
each .XII/.ETA pair to be searched.

   An Example:
      You just wish to work with the DEFLEC90 - Southwest file.  So, load 
      AREA.PAR into your favorite line editor, and delete the lines referring 
      to D90SC, D90SE, D90NW, D90NC, and D90NE.  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.

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 DEFLEC90 Model
   The DEFLEC90 model was computed on May 22, 1991 using the nearly 1.5 
million terrestrial and ship gravity values originally used in the computation 
of the GEOID90 geoid height model (Therefore, we retain the "90" designation).  
The method of computation follows that of GEOID90;  by using a Fast Fourier 
Transform (FFT) technique to compute the detailed deflection structure, then
combining with an underlying OSU89B geopotential model.  The result is a pair 
of deflection grids with a 3' X 3' spacing in latitude and longitude, referred 
to the Geodetic Reference System 1980 (GRS 80) normal ellipsoid.  By comparing 
the DEFLEC90 model with observed astronomic latitude and longitudes at 3421 
control points (NAD83) throughout the United States, it has been established 
that DEFLEC90 has a 1 arc-second RMS accuracy.  However, it is likely that
the typical user will obtain even better results, particularly at lower
elevations and in less rugged terrain at high elevations.  One study
indicates that one may achieve 0.5 arc-second (one sigma) accuracy in the
Eastern part of the United States.

Other Programs
   Depending on which disks you requested, you may have received some other
programs which are nice to have.

   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 Wyoming by using 
              D90NW.XII (.ETA), and "converting" from binary, .XII/.ETA 
              into binary, .XII/.ETA, and specifying the Wyoming state 
              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 6 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.

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 GEOID90 and DEFLEC90 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, 
is at 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.  DEFLEC90
contains a "normal curvature of the plumb line" model to provide approximate
surface deflections of the vertical.  For this reason a meridional (N-S)
GEOID90 slope will not exactly correspond to a DEFLEC90 Xi component.

   In short, GEOID90 and DEFLEC90 are completely compatible in the sense that
they were computed by the same technique (FFT) from a common grid of gravity
data.  GEOID90 and DEFLEC90 are not compatible in the sense that GEOID90
refers to sea level (geoid height), whereas DEFLEC90 models deflections of
the vertical at the surface of the Earth.

   Future deflection of the vertical grids from the National Geodetic Survey
will be using more sophisticated techniques to obtain better estimates of
surface deflections of the vertical.

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 
measurments.  It is likely that this research, in conjunction with greater 
availability of high precision GPS surveys, will yield a significant upgrade 
to our deflection and geoid height models in the next five years.

For More Information

    For Products Available From the National Geodetic Survey:

                     National Geodetic Information Center
                     N/CG174, Rockwall Bldg, Room 26
                     National Geodetic Survey, NOAA
                     Rockville, MD 20852
                     301-443-8631

    For Information on DEFLEC90, GEOID90, and Future Research:

                     Dr. Dennis G. Milbert
                     N/CG113, Rockwall Bldg, Room 513
                     National Geodetic Survey, NOAA
                     Rockville, MD 20852
                     301-443-8528
                     Internet:   dennis@hecate.ngs.noaa.gov
  

Best Wishes!

README file    3-oct-91   dgm