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Remote sensing for resources development and environmental management
Damen, M. C. J.

lines (cut through the jungle after the photos are
flown), can be exactly reconstructed.
Considering the quality of the provided airphotos
in terms of variations in scale and tilt, the
slotted templet method was in this case found to
yield unsatisfactory results, even though the
assembly of templets was fitted to pass-points
identified and linked to traverses by
Euroconsult's topographical team. Although the
location and orientation of each individual
airphoto was in theory in this way fixed relative
to the planimetrie map of traverses, in actual
fact it appeared that locations of field
observations could not always be traced back. This
was especially valid for parts of the terrain
remote from pass-points.
2.2 Use of Landsat
Since the Australian Landsat Station (ALS) went on
stream early in 1981, ample coverage by Landsat
MSS imagery of southern Irian Jaya has become
available. Although Landsat has been around since
1972, up to the establishment of ALS only a
one-time acquisition of the region existed and
then mostly covered with clouds. In recent years,
however, consultants operating in the area have
had at their disposal more and more cloud-free
Landsat MSS imagery to complement and connect the
existing scattered blocks of aerial photography.
As will be shown below, with this development
the most important mapping constraint mentioned
above was overcome by Euroconsult. Landsat MSS
bulk processed photographic products appeared
sufficiently true to scale and shape as to serve
as planimetric control for the construction of
airphoto-derived basemaps.
3.1 Principles
The method is based on the assumption that
directions to objects, when reckoned from the
centre of a vertical aerial photograph, are
constant under all conditions of elevation
difference and scale change. Under this principle
a scale adjustment can be made for a block of
photographs by allowing corresponding points on
adjacent airphotos freedom of movement away from
and towards their centre points. The block of
airphotos can thus be adjusted to scale, by making
it fit a number of control points (Slama et al,
The slotted templet method is a mechanical
solution based on this principle. Although this
method is obsolete as regards modern
photogrammetric triangulation, in practice the
slotted templet method is often still used in the
field as the best option open to achieve a more or
less usable result (depending on the quality of
the photography).
3.2 Conditions for use
A prerequisite for the proper working of slotted
templet method is that the centres (principal
points.) should more or less coincide with the
nadir points of the airphotos. Another condition
is that the individual scales of the photos should
not fluctuate too much. In other words, the aerial
photographs should be flown carefully, with
minimum tilt and altitude variations.
Unfortunately, this is not always the case with
aerial photography made available to consultants
working in developing countries. In addition to
the fact that tilt and scale variations hamper the
use of the slotted templet method, the method also
requires a tremendous amount of time and patience
to physically assemble the entire set of templets.
4.1 Principle
The principle on which the Landsat airphoto map
control method is based, is the merging of
Landsat's high planimetric accuracy over large
areas, with the high local detail of aerial
Ideally, merging of these two characteristics is
best done by registration of the airphoto onto the
Landsat scene with a rectifier. Next, the settings
which compensate tilt and scale are freezed.
Finally, the airphoto is blown-up to basemap
scale. Though airphoto and Landsat can be matched
optimally in this way, the procedure clearly is
not suited for application under field conditions.
As a result, we have resorted to an approach in
which corresponding control points are sought on
Landsat scene and airphotos. This basically
approximates the above procedure. It is this
technique that is described in the next Section.
4.2 Description
The following is a step-by-step guide on how to
use Landsat in combination with airphotos to
produce a basemap under field conditions.
* A Landsat scene covering the area of study is
ordered in a conveniently large scale. (At ALS,
1/I6th of an MSS scene can be ordered at a scale
of 1/50 000 in false-colour, with excellent
results.) The date of aquisition should be close
to that of the aerial photography to facilitate
the next step.
* Salient landmarks discernible on both airphoto
and Landsat are pinpointed and allocated a number.
* Temporary and arbitrary x-y coordinates of the
points, scaled off the Landsat scene with the aid
of translucent millimetre grid on a stable base,
are multiplied by the scale ratio factor so that
the points can be plotted on the basemap scale
(nominal photo scale). The resulting matrix of
points serves as a field of control points for the
assembly of the air photographs in a map.
* A minimum of three control points per airphoto
is required in order to correct not only scale but
also tilt variations. More than three control
points enable a better check on compatibility
between Landsat and corresponding airphoto points
and increase the matching accuracy between Landsat
and airphoto.
* In actual practice an optical pantograph can
best be used to mate the control points on the
airphotos with their corresponding points on the
basemap. Pantographs that allow the introduction of
a certain amount of distortion are to be preferred
(e.g. the Stereo Facet Plotter by O.M.I.) as they
make it possible to approximately adjust for
* Using this instrument the principal points of
the photographs can be transferred to the basemap,
including all relevant geographic information
visible on the aerial photographs.
The resulting map contains pertinent, highly
detailed, geographic information true to scale and