coordinate system and the principal point one transforms the
measured pixel coordinates into an image coordinate system
given by the calibrated values of the fiducial marks. There can
be 4, 8 or more fiducial marks that have to be measured.
Other marks which have to be measured as well are réseau
crosses, which are positioned on a glass plate inside a special
camera, called réseau camera, and which are imaged to-
gether with the object. By means of these réseau crosses it is
possible to detect image deformations or deficiencies of the
used scanner and thereafter correct them. The mentioned
marks or crosses are all well-defined, and their image matrix
is known beforehand. It is called a template. By moving the
template in steps of one picture element over the image one
searches in the image for the imaged fiducial mark or cross.
At each position of the template a correlation coefficient is
calculated. The position where the correlation coefficient has
its greatest value is the searched one. The accuracy of the
determination is the size of one picture element. If subpixel
accuracy is required, a least square adjustment has to be
applied. A small shift of the template and two other parame-
ters (which apply corrections in the greyvalues of the pixels)
have to be calculated. This rather complex calculation leads
to a positional accuracy which is about 1/10 of the pixel size.
The measuring is completely automatic and fast.
The réseau camera is very handy and a pretty high accuracy
can be achieved. Réseau cameras and réseau techniques are
successfully used in industrial applications. (Dold, J. et al.,
1993).
2.2 The pedagogical concept
When a theme is selected, the information about this theme
(the so-called "knowledge") is presented. This occurs by
means of screen images with text and figures. A text is written
line by line on the screen, and additional information can be
displayed when it is wanted. The attached figures are dynami-
cal ones. The information is presented by small pieces so that
it can be grasped. The sequence of the text lines can also be
repeated. Several exercises ("tasks") are connected to the
themes. The student is active and manipulates a figure or
calculates values. The result of the "task" is sensed automati-
cally or typed in the keyboard and immediately checked by the
program. This immediate and neutral feed back is one of the
main strengths of this CAL program.
The user interface is graphical and very simple. There are only
few fields with their icons and texts to point to. The ones at the
bottom line are used for the navigation through the program,
the ones on the right side for the manipulation within the
displayed screen image (compare the figure). They are started
by means of the mouse or the track pad. A 'help' window
informs about the next steps to be taken. A simulated pocket
calculator can be used for the calculation of the results. The
language in the texts is English only.
2.3 The technical concept
The prototype of "LDIP" uses Macintosh hardware, the
models "2si" and "PowerBook 520". Both of them can be
connected to a CD 300 drive in order to read imagery from a
CD-ROM. The transportable PowerBook with a black and
'white screen can also be connected to a plasma screen
(LCD) which enables the projection on a large screen in
colour. The PowerBook can be upgraded to a PowerPC
which then also makes possible the emulation of the DOS
operating system so that PC programs can be used at a
reasonable speed.
The software is written in Think Pascal and runs under the
'System 7.1'. This operating system contains also toolboxes -
with numerous functions for the development of CAL software
such as pull down menus, input and message boxes, icons,
image and sound facilities.
THEME 1: AUTOMATIC MEASUREMENT gb
OF RÉSEAU CROSSES AND FIDUCIAL MARKS
SEARCH MATRIX
TARGET MATRIX
- THE POSITION OF THE TARGET (RÉSEAU CROSS OR
FIDUCIAL MARK) SHALL BE FOUND IN THE SEARCH MATRIX
- THE DENSITIES OF THE TARGET HAVE
NOISE AND SMEAR THE IMAGE
- THE TARGET MATRIX IS MOVED PIXEL BY PIXEL OVER
t THE SEARCH MATRIX
- AT EACH POSITION A CORRELATION COEFFICIENT (r) IS
CALCULATED
- THE POSITION WITH THE GREATEST VALUE OF r IS
THE REQUIRED POSITION
Information
HELP
P =) @
INFORMATION TASK REPEAT RETURN TO MAIN MENU STOP
Fig. 2. Screen image for 'Knowledge' in the CAL program
“LDIP”. At this moment of the sequence all of the text lines
and the additional information is displayed. The target matrix
of 5 x 5 pixels is situated within the search matrix at the
position where the correlation coefficient is r=0.28.
All image data shall be stored on an assembled CD-ROM
which has to be produced from imagery originally stored on
various media and at various formats. The image data is
loaded from an attached or integrated CD-ROM drive or from
various discs if CD-ROM drives are not available.
2.4 Description of the use
2.4.1 Automatic measurement of réseau crosses and
fiducial marks: Five text lines for this theme are displayed
sequentially, they are coordinated with the attached figure. In
this dynamic figure a target matrix moves over the search
matrix and the correlation coefficients are displayed at each
position (see fig. 2). The grey values of the two images
(search and target area) can be displayed as numbers or as
shades of grey. With the field "INFORMATION" the formulas
for the correlation coefficient can be displayed in addition.
Also, the different parameters in the formula are then explain-
ed in detail. All this information is necessary for carrying out
the related exercise (task 1.1). For this purpose, the field
"TASK" has to be activated.
2.4.2 Automatic measurement - task 1.1: The best fit of
two density profiles (target and search area) has to be found,
first manually by moving the target area over the search area.
The user clicks either on the icon "RIGHT" or "LEFT",
whereby the target area moves in the selected direction. At
the position of best fit a correlation coefficient has to be
calculated. The pocket calculator can be used for the compu-
tation. If the student has forgotten the formula for the compu-
tation of the correlation coefficient, the icon "KNOWLEDGE"
is clicked on which leads back to the screen image with the
formula. The computed value has to be typed in the keyboard
and is displayed at the input window. If the result is correct,
the program will indicate it by writing and saying "CORRECT!"
In the other case the written and spoken answer are
"WRONG!". By means of the icons "NEXT TASK" or "RE-
TURN TO MAIN MENU" the student can proceed. 2.4.3
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B6. Vienna 1996
Corr
abou
two fi
repee
and €
on tl
point
Corre
shifte
than
introc
deter
INFO
Fig. 3
progr
are di
print)
active
2.4.4
exerc
equat
set ir
calcu
watcl
accul
she L
(see f
and t
All inp
TH
TA
9171 92
“NOUSU AUD 5
Fig. 4
- One
deter
