

		  Fort Hood Image Distribution
			 Second Data Set
			  Version 2.1
       	                 August 11, 1995


	                 Chris McGlone
                   Digital Mapping Laboratory
	           School of Computer Science
	           Carnegie Mellon University
	             Pittsburgh, PA.  15213	



1. CONTENTS

This tar file contains the third distribution of the second set of 
image data taken of Ft. Hood, TX, for the RADIUS program.  The first
set of Fort Hood imagery was taken on December 10, 1991 and consisted
of four images, two vertical and two oblique.  The second
data set  includes 7 vertical and 19 oblique images,
which cover a larger portion of the Fort Hood base area
and span a far greater range of viewing angles and spatial resolutions.
The images and initial orientations were distributed at the RADIUS 
meeting at SRI, March, 1995.  The second set of orientations were 
distributed in June, 1995.

The motivation for this distribution is the realization that the 
elevations used for the control points and the image altitudes 
in the previous distribution were relative to sea level (the geoid), 
instead of WGS84 (the ellipsoid).  This can cause some confusion 
if the images are used with DMA DTED data, which is defined relative
to the WGS84 ellipsoid.  The difference between the geoid
and the ellipsoid in this area is 27.386 meters, with the geoid
being higher than the ellipsoid.  (A brief explanation of the
difference in in Section 2.) 

This correction was applied to all the control point elevations
and the simultaneous resection re-run.  As would be expected, 
this resulted in changing the image altitudes by almost exactly 
the same amount--in fact, you can add 27.386 meters to the 
existing image elevations instead of importing these new 
TEC header files, if you like.

The contents of this tape are:

	README3 (this file)

	all-wgs.resect, all-out-wgs.resect		
		input and output .resect (control point image measurement)
		files for the block adjustment (see format description below)
	
	for each image:
		TEC image header file, with RADIUS extensions

2.  GEOIDS AND ELLIPSOIDS

The type of model used to describe the earth depends upon the 
requirements of the application and the level of detail required.

The geoid is a model of the shape of an gravitational equi-potential
surface (sea level).  This is used for field work, since all surveying
and geodetic measurements (including satellite measurements) are 
made relative to this surface.  The geoid is irregularly shaped, due 
to variations in the earth's density and topography, and must be
determined by actual measurement.

An ellipsoid, in the context of geodesy, is a mathematical model of the earth's
shape.  An ellipsoid is calculated by fitting an ellipsoid of
revolution to the observed geoid shape.  This may be done only within
a region of interest, such as the continental United States, or may be
done for the entire world, as is the case for WGS84. 

So, at any given point on the earth's surface, there are two different
reference surfaces to deal with, the geoid and the ellipsoid. 
Surveyors use the geoid (mean sea level), since that's what's relevant
to their field work.  Using the ellipsoid is simpler for dealing
with data over large areas.  


3. IMAGE MEASUREMENT (.RESECT) FILES

Included with the data set are two .resect files, one (all.resect)
containing the input data to the block adjustment and 
the other (all-out.resect) containing the final coordinates and 
covariances of the ground points.

The format of the file is pretty simple:

	point -- (type of information -- always point in this case)
	<pt-id>
	2 -- (means sexagesimal coordinates)
	<N | S> <lat-degrees> <lat-minutes> <lat-seconds> <lat-thousandths of sec>
	<W | E> <lon-degrees> <lon-minutes> <lon-seconds> <lon-thousandths of sec>
	<elevation>
	<std-dev-X> <correlation XY> <correlation XZ> <std-dev-Y> <correlation YZ> <std-dev-Z>
	<n> -- (number of images point is measured on>
	<image-id> <row> <col>   (for each image) 
	...
	
The image coordinates are given as <row>, <col>, where the origin is
the upper left corner of the image, rows increase downward, and
columns increase to the right.   
***This is not the RCDE image coordinate system!!!***
The column coordinate is the same as RCDE 'v' coordinate.
To convert the row coordinate to the RCDE 'u' coordinate, subtract
it from the number of rows in the image.

PSI's point number scheme gives tie points a 4 or 6 digit number. The
6 digit numbers always begin with 90.  The first 3 digits are the
photo number on which the point appears, while the last digit is the 
point number on that photo.  Point number 2 is in the middle of the
image, while points 1 and 3 are at the edges.  So, for instance, 
point 907052 is a tie point identified on photo 705, near the middle 
of the image.  Tie points are artificial points, marked in stereo 
("pugged") on overlapping images.   Without the marked film, there is 
no way to identify the point used in the image.

Control points have 2 or 3 digit numbers, such as 94 or 102.  These 
are photo-identifiable points that have either been surveyed on the
ground or measured in the imagery to use as control for the oblique 
images.  

We also measured additional tie points between images, using 
photo-identifiable features in the images. These have arbitrary 
names, usually pass**  or t**. 

