al
for the additional flying cost.
The whole process of the automated digital orthophoto pro-
duction process consists of the following nine steps:
1. Pre-Flight planning
The flight plan is very carefully developed, with
the required coordinates of each photograph pre-
programmed and entered in on-board computer sys-
tems. As the aircraft flies along the desired flight path,
the airborne GPS (AGPS) automatically triggers the
camera when it is closest to the pre-selected coordi-
nates. All relevant data for each exposure, including
time, exposure number, preliminary GPS coordinates
etc. are electronically printed on each negative.
2. Photo mission
In a dramatic departure from conventional 20% or 30%
sidelap photography, the photography is flown with
60% forward and side overlap, resulting in the photo
centers falling at the corners of almost perfect squares.
3. Scanning and image preprocessing
Instead of making diapositives or paper prints, we di-
rectly digitize the original negative film, using a Vexcel
scanner. |n future this will be done using a scanner
with automatic film feed and batch processing capa-
bilities. The whole image is scanned at a resolution of
154m, and an image pyramid for the further processes
is calculated.
4. Automatic interior orientation
The interior orientation is done fully automatically, us-
ing the software package AIO [cf. POTH Z., SCHICK-
LER W. 96], which has been developed by Bonn Uni-
versity in cooperation with Zeiss company. We have an
agreement with Zeiss to use this software for our own
internal purposes. The AIO automatically determines
how the image was oriented in the scanner by locating
an asymmetric feature. This has to be done only once
per film roll scanning process. The accuracy of the AIO
Is a tenth of a pixel for a single fiducial measurement,
and the sigma naught of the transformation estimation
is about 0.2 to 0.3 pixels.
5. Initialization
The initialization is necessary for the fully automatic
aerial triangulation. In this part approximate tie point
areas are found and the position of paneled control
points in all the images are automatically measured.
6. Automatic aerial triangulation
The aerial triangulation is done automatically using IN-
PHO's MATCH-AT [KRzvsTEK P. 96] software pack-
age followed by a manual checking of the results and
editing if necessary.
7. Automatic DTM Generation
For the DTM generation, we developed an extension
to INPHO's MATCH-T [KRZYSTEK ET AL. 91] soft-
ware package. This enables us to produce high preci-
sion DTM's in urban areas while reducing the neces-
sity of the manual or interactive part. Though this
process works mainly automatically there is still a need
for manual checking and possibly editing.
8. DTM editing
After the DTM has been created automatically, it is
very important to check that there are no gross errors.
This must be manually done in a stereo mode, with
tools available to modify the DTM where necessary.
At ASI we have developed stereo viewing and editing
techniques using our own proprietary production soft-
ware.
9. Digital differential rectification and mosaicking
At ASI we use proprietary ORBIT (Ortho Rectification
By Image Transformation) software to make the final
orthophotos. The only changes being made for the
new production system are conversion to the Silicon
Graphics platform and conversion to accommodate the
new geo-Tiff image format which is used in our AIO,
MATCH-AT, MATCH-T and the stereo editing soft-
ware.
The steps which lead from the film roll to an automatically
derived DTM (steps 3 to 7) are done mainly fully automati-
cally in batch processing for a whole block of aerial images or
a subset of a block. The most innovative parts are described
in more detail in the following sections.
2 Initialization
ASI, in collaboration with INPHO GmbH of Stuttgart, has
developed the initialization phase of MATCH-AT. This is the
process needed to find the image coordinates of
a) surveyed control points, and
b) bundle adjustment of tie point areas,
(these are approximate values in the sense of a search
space, for the automatic aerial triangulation )
in every photograph on which they fall.
Both steps are done automatically, and will be described in
the next subsections.
2.1 Finding tie point areas
This procedure is critical to the efficiency of the automatic
aerial triangulation adjustment. There are three critical ele-
ments:
e Coordinates of the camera lens, obtained from AGPS
to an accuracy of about 100 m.
e Orientation of the of the camera, obtained from a MO-
TIONPACK inertial measurement unit to an accuracy
of about 0.5 degrees.
e An existing digital elevation model. In the U.S. we are
fortunate in having the USGS DTM covering most of
the country, to an accuracy of 7 m vertical.
To obtain the image coordinates of the tie points, the classical
three von Gruber points of each photograph are projected
down to the DTM, and then projected back to all the other
photographs. After a check is made for proximity to other
tie points, a selection is made automatically of the best final
tie point position on the ground, and then points are re-
projected into all the relevant images. As a control process,
polygons containing water and dense forest (where these are
available from existing maps), where projected tie points fall
in the water or forest (or on steep slopes identified from the
DTM) they are weighted accordingly after checking if they
were slightly moved they could fall outside the polygons or
on flatter ground.
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International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
