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Reliability of Area Mapping by
Delineation in Aerial Photographs
lau us-peter1 Gross and Petra ~ d l e ?
Abstract. - In a test series under standardised condlttions, findmgs between
individual photo-interpreters concerning forest stand boundaries
delineation were investigated. It was found that the dspersion of the
delineation was dependent on the degree of difficulty in defining a natural
border by the interpreter. The evaluation showed that 90% of delineated
polygon lines of a ,,normalu mapping project under central European
forest concbtions were found in a 10 meter buffer d a photo scale
1:7000 and required 22 meter buffer in a photo scale 1:20000 to fit 90%
of the delineated polygon lines.
INTRODUCTION
Aerial photographs are an important source of actual and detailed area
information. For decades the delineation of polygons of homogeneous
information from images for forestry or landuse application has been common
practice. After geometric correction using some kind of photogrammetric
procedure the stand polygons can be integrated as an information layer in a
Geographical Information System (GIs).
In dependence of the photogrammetric solution (low budget, advanced
photogrammetry), the positional error of the polygon lines can be estimated.
However, there is still uncertainty concerning the scope and possible variance of
interpretation, because:
the photogrammetric measurement of lines is less precise than the
measurement of point objects.
forests are natural objects, in general without well defined, clear
borderes.
the clearness of borders between different forest stands varies to a wide
degree.
Besides the photogrammetric accuracy the interpreter himself is responsible
for an unknown uncertainty. It is of great interest for GIs analysis to have an
quantitative idea of the influence of this human factor on the spatial data.
Forester, specialised in Remote Sensing (Ph.D.), Dept. of Remote Sensing and Landcsapinformationsystem,
University of Freiburg, Germany
Student of Forestry and Remote Sensing, University of Freiburg, Germany
MATERIAL AND METHODS
In the study aerial Color-Infrared (CIR) photographs of two different scales
were evaluated. The large scale photographs (1 :7000) show an intensively
managed forest district in Germany (Duvenhorst, 1995). Small scale aerial
photographs (1 :20000) were taken from a forest region with little management
activities in Italy (Gross, 1993). Both photo-flights were taken for use in an
intensive forest inventory based on remote sensing.
In a first step the degrees of difficulty to identify the forest borders was
estimated in three classes (easy, medium, difficult) using a representative
check of all polygon lines in the test sites deliniated in both inventory projects.
In a second step the reliability of the forest stand delineation was tested.
From the two test sites a representative study area was chosen for the different
photo scale. Ten experienced photointerpreters, students and staff members of
the Department had to delineate five forest stand boundaries in study area
using defined interpretation keys. For the mapping procedure a high precision
photogrammetric instrument (ZEISS Planicomp P3) was used to exclude the
influence of equipment errors.
A particular problem for the evaluation of the test series is the lack of
reference data. The borders of forest, as well as of many landscape objects, are
,,soft" and no true borderline can be mapped, using either terrestrial means or
aerial photographs. To handle this problem adequately and to quantify the
result of the test, a special GIs- application was chosen to evaluate the test
series:
All polygon section in which the test persons easily identified the same
stand border were cut out of the polygons for further evaluations. A
findamentaly different interpretation of a forest stand, e.g. the division of a
stand in smaller unites, was not of central interest in this study. These
variations can be influenced through training, interpretation guidelines and
the experience of the interpreters.
Starting from the minimal and maximal polygon line of the entire bundle,
the percentage of the total line length in 1 meter (1:7000) and 2 meter
(1 :20000) buffers was calculated.
The raw data were put in a new order, starting with the buffers with highest
density of polygon lines, independent of their localisation. The results
presented in this paper show the average between the results for maximum
and minimum polygons as cumulative histogram.
RESULTS
How difficult is it for experienced forest photointerpreters to identify and
delineate forest stand boundaries from aerial photographs? Independent of the
scale of the photographs it was found, that about 2/3 of all stand limits were
easy
ditllcutt
64%
19%
medium
S
!
Figure 1: Degree of difficulty to identify forest stand borders in aerial
photographs in the scale of 1:7000 (left) and 1:20000 (right).
clearly visible and can be identified without any doubt. Only about 20% of the
forest stands were found to be extremely difficult to delimit to neighbouring
stands. These borders were found to be more or less fluid (figure 1). Of course,
the visibility might change with natural forest conditions and silvicultural
concepts. For the situation in central Europe these results were considered
quite representative.
The delineation test series showed the following results for the two photo
scales:
In the large scale photographs the variation of easy to identify polygon
lines was a maximum of 5 meters. More than 80% of all lines were found in
a buffer of 2 meter width. The delineation of medium difficult to identifjr
lines varied for 91% in a 11 meter dispersion whereas a buffer width of 27
meter was necessary to fit 90% of all lines that were really difficult to
interpret (figure 2).
1 2 3 4
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 26 27
buffer width in meters
Figure 2: Variation of delineation results in three degrees of identification
difficulty (photo scale 1:7000)
The small scale photographs naturally show a higher dispersion. A buffer
of 10 meters fits 90% of all lines that were easy to detect. This makes 0.5
millimetres (mrn) in the photo scale 1:20000. About 90% of the medium
difficult stand borders were found in a 38 meter buffer and 90% of the
difficult to idente lines varied in a 46 meter width buffer. To fit all polygon
lines a buffer of 80 meter was necessary (Figure 3). The precision in
coordinates of the stereomodell is not so different fiom the large scale
photos. The 0 . 5 m in 1.20000 for easy delineated lines is similar to the
0.4in 1:7000. The difficult to interpret lines require 2.3mm (1:20000)
and 3.8rnm (1:7000) to fit 90% of all lines. It seems that the small scale
photographs may have even some advantages for the interpretation because
of the better overview.
2
4
6
8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48
buffer width in meters
Figure 3: Variation of delineation results in three degrees of identification
difficulty (photo scale 1: 20.000)
CONCLUSION
The central object of this study is the subjectivity of aerial photo
interpretation related to spatial accuracity of mapped forest stand border lines.
Only very few approaches can be found in the literature to quantify the
influence of the human factor (e.g. Congalton 1983, Glemser 1993, Scherrer
et al. 1990, Zihlavnik 1991). To identify the correct stand border well worked
and defined interpretation keys and sufficient training of the interpreters are
preconditions. But even when the interpreters identify the same stand border
and are willing to map the same line they have some freedom of action. The
resulting variations are strongly related to the clearness of the stand
delimitation. The photo scale has mainly an effect on the absolute precision in
ground coordinates.
When the delineation variations are transferred to the entire inventory
projects by weighting the data with the portion of identification difficulty, then
90% of all polygon lines in the 1:7000 photo flight were in a buffer width of
10 meters, in the 1:20000 photographs a width of 22 meters was required.
This means, speaking fiom the point view of a forester, under normal
conditions the accessible precision ranges fiom less than a tree crown to
maximal one adult broadleaf crown.
Nevertheless for some GIs procedures such as the intersection of stand
area with qualitative data, the existing uncertainy should be taken into
consideration. The authors propose to draw buffers with the same probability
around the mapped polygon lines with regard to the difficulty of identification.
REFERENCES
Congalton, R. G. 1983: A Quantitative Method to Test for Consistency and
Correctness in Photointerpretation. Photogrammetric Engineering and
Remote Sensing 49 (I), p. 69-74
Duvenhorst, J. 1995: Photogrammetry in Forestry - Tool for Effective
Inventory and Planning. in Fritsch, D.; Hobbie, D.(eds.): Photogrammetric
Week'95, Wichmann Heidelberg, p. 33 7-349
Glemser, M. 1993: Untersuchungen zur objektbezogenen geometrischen
Genauigkeit . Salzburger Geographische Materialien, Hefl 20, p. 97- 10%
Gross, C.P. 1993: Regionale Waldinventur zur Erfassung des Waldzustandes
mit kleinmaastabigen Color-Idiarot Luftbildem. Dissertation Universitat
Freiburg, 126 p.
Schemer, H. V.; Gautschi, H.; Hauenstein, P. 1990: Flachendeckende Waldzustandserfassung mit Infiarot-Luflbildern. Bericht der Eidgenossischen
Anstalt fbr das Forstliche Versuchswesen Nr. 3 1 8, Birmensdorf, 10 1 p.
Zihlavnik, S. 1991 : Utilization of Photointerpretation in the Determination of
Forest Stand Boundaries. Lesnictvi, p. 819-829
BIOGRAPHICAL SKETCH
Claus-Peter Gross is a gratuated forester (Ph.D.) Mer five years practical
work in the forest administration, he joined in 1987 the remote sensing team at
the Forest Faculty in Freiburg (Prof. Hildebrandt) and is now scientific
employee at the Department of Remote Sensing and LandscapeInformationsystem (Head Prof. B. Koch). He is specialised in forest remote
sensing applications and photogrammetry.
Petra Adler will finish her studies in forestry at the University of Freiburg
1996. In her studies she put a special emphasis on remote sensing.