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mosaic was required. The approach of one photo making
one map has always been the standard practice for
producing photomaps in Taiwan. In light of such
requirement, aerial photos were taken with flight lines
oriented north and south along the centerline of map
quadrangles. Although it requires larger end laps (e.g.,
80-90 percent as opposed to the typical 60 percent), the
added costs are easily outweighed by the savings from
no mosaicking.
Aerial photo films were scanned at 22.5 um, a resolution
deemed optimum based on a technical evaluation and
backed by experiment. It should be pointed out that such
resolution is applicable only when Intergraph-Zeiss
PhotoScan PS1 is being used as this resolution is a
multiplier of 7.5 um. When other comparable scanners
such as Lecia DSW 200 or Vexcel VX3000 Plus are
used, a resolution in the range of 20-25 um may be
appropriate. At 22.5 um, it offers 4/10 meter pixel
resolution and requires approximately 100 megabytes
(Mbytes) of data storage for each image. From a
practical standpoint of view, file size of this order of
magnitude is still manageable when considering large
volume productions. The 1.3 GB optical disk cartridges
were used for data storage of the scanned images.
However, the 8mm magnetic tape could have been an
option but was not used in daily operations due to its
slow speed of data retrieval.
A number of various types of large data storage media
are available in today's market. For example, there are
magnetic tapes such as the 8mm Exabyte tape, the 4mm
DAT tape and the 9-track tape, etc., and the Magneto-
Opticals (MOs) such as the 5.25" optical disk cartridge,
just to name a few. In general, magnetic tapes are
relatively inexpensive and are good media for archives.
However, they are slow in data retrieval and may not be
effective in the day-to-day operations in a production
environment. The use of optical disks provides an
effective means for quick data retrieval instead.
Map grids are based on Transverse Mercator Projection
with two (2) degrees wide zone instead of the six (6)
degrees for UTM. It should be pointed out that map
projection with a two-degree wide.zone is unique to
Taiwan.
Information regarding magnetic declinations was
obtained from the U.S. Geological Survey (USGS) via on-
line information system. The International Geodetic
Reference Field (IGRF90), a geomagnetic field
mathematical model, covering the worldwide regions
with date range from 1945 to 1995 was chosen for
Taiwan. The IGRF90 model shows that Tai-Chung had a
magnetic declination of about 3°07" W in 1995 and an
average annual change of approximately 1.2 min
westerly.
Aerotriangulation was measured on Zeiss P-2
Stereoplotter using PAT-M for block adjustment. A total
of 4208 photogrammetric pass points were measured
Which yielded a sigma naught of 6.7 um for horizontal
block and 10.8 um for vertical block, respectively.
369
Aerotriangulation of this accuracy is well within the 18
um tolerance, a condition required for meeting the
contract.
Contour lines on mountainous areas were generated
automatically from Intergraph ImageStation while
contours in relatively flat terrain were manually plotted
from Zeiss P-2 and P-33 stereoplotters, respectively. In
light of the fact that images are representing only the
tree tops, manual adjustment of tree heights has to be
taken into consideration when contour data are
generated automatically from ImageStation. A 5-meter
contour interval was used. The DTM wasthen derived by
interpolation from contour data coupled with breaklines
and spot elevations at 20-meter center, a density
equivalent to 4mm at the map scale. The DTM was
recorded and saved in ASCII format for general
applications.
Rectification. With respect to the orthorectification, the
DTM was generated automatically from Intergraph
ImageStation at 9-meter center, a density deemed
necessary for the best results of rectification. The data
was used as an input to ImageStation for orthophoto
rectification. It should be noted that tree top elevations
are the basis for rectification of the images. Storage of
data before and after rectification was on 1.3 GB optical
disks.
The linework including ground features such as roads,
rivers, contour lines, text and map symbols was digitized
in AutoCAD and subsequently converted to Microstation
DGN. The vector was then rasterized in order to overlay
with photo images. In the past, to add topographic line
map onto orthophoto would require multiple sheets of
film negatives for printing. Not only does it add costs to
the reproduction, but also invites mistakes due to
possible human errors. Through the use of the
Sophisticated Map Publisher, photo images along with
the rasterized topographic linework were output to a
single TIFF (Tagged Image File Format) file per map for a
filmwriter production. This would eliminate possible
misalignment of multiple sheets due to an operator error.
The Map Publisher does the necessary work including
mask, fill, alignment, cut and paste, all in one single
softcopy process.
Outputs. Screen SG-618A, a high end filmrecorder, was
used for generating the film negatives. To prepare for an
output plotting of orthophotos, files were saved in TIFF
format and then exported from UNIX workstation to PC
via PC/TCP(Transmission Control Protocol) network.
Through the use of Photoshop, the orthophoto image
was checked for quality and edited as needed for final
hardcopy output with a film writer.
In addition to hardcopy orthophotos, softcopy output is
also in high demand and offers a wide range of
applications. The softcopy orthophotos can be used in
conjunction with the geographic information systems
(GIS). However, often time data transfer may become an
issue when original files being transferred are not
compatible with the new system. This is due to a number
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
