iteration with the use of known image and object-
space coordinates of adequate control points, by
least square.
REALTIME IMAGE POSITIONING
As we know the LOOP program which runs with a cer-
tain frequency over 50 times per second in order to
dynamically keep the conjugate relationship of
identical object and left and right image points is
the core of an analytical plotter. It may include
the input of map or object-space coordinates, solu-
tion of left and right image coordinates, and
closed-service for driving the left and right pho-
tocarriages, and the drawing pen when on-line map-
ping, onto the positions just calculated. For frame
imagery, rotation matrice (consisting of direction
cosines) and exposure station coordinates are fixed
in all points of an image and image coordinates can
be easily solved through collinearity equations of
central perspective (Wang 5) from ground coordi-
nates. For SPOT imagery, however, since elements of
exterior orientation are y-dependent, exact image
coordinates cannot be computed simply by direct so-
lution of the equations (6) combined with (1).
Mathematically, time-consuming iteration, i.e.
y=0;
repeat:
calculate (1) and az, ba, cz, a3, bz, C5,
calculateAy equal to the right expression of
second equation in (6),
refine y=y+Ay,
until Ay is negligible (almost equal to zero);
calculate a; , bi, c;;
calculate x by first equation in (6).
is adopted to find the solution. This procedure
greatly slow down the execution frequency of LOOP
so that stereoscopic visual perception is apparent-
ly trembling or un-continuous. An approach that a
fictitious central perspective image whose six ele-
ments of exterior orientation are same as those of
center line of SPOT images is considered, and dif-
ferences between central perspective and SPOT geom-
etry are next compensated by bilinear interpolation
with regard to ground (or image plus height) coor-
dinates from tables computed before LOOP runs, as
described in detail by Konecny (1987) and Li
(1988), is substituted for the iteration algorithm.
In addition, before computing image coordinates,
Gauss-Kruger coordinates and height have to be con-
verted into TCS coordinates. Based on the method
here, a LOOP program for SPOT imagery whose opera-
tional sequence is shown in Figure 4 is coded in C.
SPOT Images
(Xg ,Yg,h) of
Control Points
Input of (Xg,Yg,h)
A
Transformation of (Xg,Yg,h) to TCS
i
Computation of
Central Perspective Image Coordinates
i
Image Coordinate Correction
by Interpolation
|
Fiducial Transformation
i
Driving Service of
Photocarriage and Drawer
Figure 4. Schematic Diagram of LOOP Program
It runs with 7ms per cycle on an AT personal com-
puter (10MHz) which controls the JX-3.
IMPLEMENTATION ON JX-3
According to the restitution discussed above, a
software running on the JX-3 was designed by the
author to produce topographic maps and digital ele-
vation models (DEMs) from SPOT stereo images. The
program structure and implementation procedure are
illustrated in Figure 5. Except those that closely
relate to hardware and hence are programmed in As-
sembly, all other subroutines and programs are de-
veloped in C (Microsoft C5.0 Version). All func-
tional modules are integrated and managed under a
main menu. No additional hardware is required.
EXPERIMENT AND CONCLUSION
A SPOT stereopair (duplicate) of the Tangshan area
taken in May, 1986, with a base-to-height ratio of
0.5 and a percentage overlap of 90%, was provided
for experiment. The ground coordinates of control
and check points are acquired from existing maps at
scales of 1:10,000 and 1:50,000 compiled in 1970s.
17 control points distributed as in Figure 6 were
used to compute the exterior parameters of images.
80 independent points were checked by comparing the
ground coordinates separately acquired from the
orientated stereoscopic model and the existing
maps, and statistic RMSEs (m) are
Scene Parameters \
and Ephemeris ]
E. erar
Stereoscopic Measurement
of Image Coordinates
Transformation of
(Xg,Yg,h) to TCS
J
Solution of Exterior Orientation Parameters
Construction of Image
Correction Tables for LOOP
i
Compilation of Line Map
Production of Orthophoto € - - —
on ZS-1
i
Registration of DEM
i
Generation of Control Data for ZS-1
Figure 5. Organization Chart of Implementation
a
Tr
