47
Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986
Airphoto map control with Landsat - An alternative
to the slotted templet method
W.D.Langeraar
Euroconsult. Arnhem, Netherlands
ABSTRACT: Negative results experienced with the slotted templet method using aerial photographs that are
tilted and have varying scales, have prompted the development of a more suitable method to produce basemaps
under field conditions. Using Landsat as cartographic control it has become possible to assemble airphotos
into geographic basemaps with a mean accuracy of 1.5 mm on a photo scale of 1/20.000. This has been
empirically determined in south-eastern Irian Jaya using tilted airphotos with varying scales. Comparable
accuracy could have been obtained had slotted templets been assembled to a control density of one control
point per 85 accurately flown airphotos. Advantages of this approach over the slotted templet method are:
* Photo tilt and scale variations do not disturb map accuracy.
* Field data collected along survey lines can always be located on airphotos and thus integrated in the
interpretation.
* No ground control required prior to flying.
- Landsat control is always evenly distributed, and so
* Map compilation is much less time consuming.
In remote areas, the airphoto map control method using
for the production of an adequately accurate basemap to
1 INTRODUCTION
1.1 Purpose
The purpose of this article is not to provide the
remote sensing community with another mathematically
air-tight dissertation on certain geodetic
considerations inherent to any new mapping technique.
On the contrary, the purpose is to describe a
useful cartographic application of Landsat imagery
and discuss empirical accuracy tests.
As is the case with many innovations, the approach
discussed here was born out of a problem situation
where a practical solution was called for. The new
method was found to be cost effective and to save
time, while providing satisfactory mapping results.
For that reason, I feel that the airphoto map
control method using Landsat could be a useful and
beneficial technique for the mapping of remote
areas.
1.2 Background
During the last five years Euroconsult (a Dutch
firm mainly active in agricultural consultancy)
has built up considerable experience in integrated
rural development planning in the south-eastern
part of Irian Jaya, Indonesia. Euroconsult has
surveyed almost half a million hectares there, and
compiled the thematic data into maps with a scale
of 1/20 000.
Prior to the study, the Indonesian Government
made available to Euroconsult blocks of unrectified
aerial photographs not only as a source of thematic
information, but also as a basis for mapping. The
old Dutch maps of the region (scale 1/100 000) are
reasonably accurate only as far as the location of
major rivers is concerned. Geographic information
on the interfluvial areas is completely lacking on
these maps, which impedes their use as a mapping
base. Other existing maps are also unsuitable for
similar reasons. As a result, Euroconsult was
is map accuracy.
Landsat is thus found to be the best possible option
be used for supporting thematic mapping data.
obliged to produce its own basemaps. Until recently
the best option open to the consultant to achieve
this, was to assemble the airphotos into a mosaic
with the help of the slotted templet method and
then update the accuracy with the aid of traverse
measurements.
2 PROBLEM
2.1 Orientation on airphotos
Mapping which includes fieldwork in areas like
southern Irian Jaya is, at best, a cumbersome
exercise. Fieldwork areas are very difficult to
reach due to lack of infrastructure. The
environment is hostile for a variety of reasons
such as climate, flora and fauna.
A more serious mapping constraint follows from
the fact that aerial photography is flown
invariably before a start is made with fieldwork.
As a result, locations of observations in the
field cannot be readily identified on the
airphotos. If the consultant cannot correlate his
field data with the airphoto at all, then his
airphoto interpretation is reduced to guesswork
and the purity of his mapping units will suffer
proportionally. Also, the accuracy of the basemap
cannot be improved anymore.
A primary requisite for integrated rural
development studies is a detailed physiographic
land-unit map covering the study area, compiled
from airphoto interpration supported with field
observations. The individual mapping units should
be as pure and homogeneous as possible in order to
serve as ’building-blocks’ for subsequent maps on
soils, forest resources, land clearing plans, road
alignments, village structure plans, etc.
It is, therefore imperative that field
observations can always be exactly located on the
aerial photographs. To make this possible the
airphotos must be assembled into a planimetrically
correct photo-basemap on. which field observation
