2.0 THE POTENTIAL FOR SMALL FORMAT DIAPOSITIVES
Aerial photography produced for mapping is used for many other purposes by
people from various disciplines such as foresters, town planners,
engineers, etc. Mostly the information extracted by these people is by
means of approximate techniques since they seldom have access to accurate
equipment nor do they have the time to become skilled at using mechanical
analogue equipment. In addition it is usual for maps to be reviewed on
say a ten to fifteen year cycle, but photography is to be taken on say a
two to three year cycle so that the photography represents current
information.
A mapping organization could cheaply produce a microfilm data base for an
area in addition to their normal products. Such a data base would consist
of reduced diapositives together with a diskette of orientation parameters
computed from the block adjustment. in this way the the organization
would be making their expertise more readily available, and users of
photogrammetric data would have direct access to more accurate
information.
3.0 INITIAL MEASUREMENTS
The Central Mapping Authority of New South Wales maintains a
photogrammetric test range in the Bathurst Town Area. The co-ordinates of
over 60 natural features have been measured by conventional surveying
techniques and photography at various scales has been taken of the area.
Negatives from a typical stereo-pair were selected and rephotographed
using a Hassablad camera fitted with a 100 mm copying lens. The camera
was mounted on a tripod, the negative on an illuminated table, and the
principal axis of the camera was set normal to the negative by first
pointing to a mirror placed on top of the negative. When the photographs
were taken the mirror was replaced by a glass grid plate with the grid
squares in contact with the emulsion of the negative.
Photography was on Kodak Technical Pan film 2415 (ESTAR-AH Base) and it
was processed to achieve maximum resolution according to the manufacturers
instructions.
The original photography was at a flying height of 2530 metres with an 80%
overiap giving an air base of 1096 metres.
Initial measurements of the fiducial marks were made in the MPS to
determine the reduction scale. This value was used to compute fiducial
co-ordinates and the effective focal length of the reduced diapositives
using data from the aerial camera calibration certificate.
An orientation was then performed on the MPS and after misidentified
points were removed, the standard errors in X, y and z were about 0.3
metres.
The photos were then measured using the MPS in stereocomparator mode to
record plate co-ordinates of 52 points as well as the camera fiducials.
The camera fiducials were measured twice to check the precision of
pointing and this was inside 4 microns.
24