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Acquisition of Spatial Data by Forest
Management Agencies: a Review
Michael
J o C o
Weir1
Abstract. - Forest management agencies have a long tradition of
surveying and mapping for the acquisition of a wide range of spatial data.
These include: i) external (cadastral) boundaries; ii) permanent, internal,
forest boundaries; iii) forest stand and other impermanent boundaries; and
iv) roads and other topographic features. In recent years, the
well-established processes of graphical map compilation and revision have
been increasingly replaced by digital mapping techniques.
Based on a limited survey, together with published information, this paper
reviews the surveying and mapping techniques currently employed by
forest mapping agencies for the task of digital database establishment and
maintenance. Accuracy tolerances specified for different categories of
information vary considerably. The survey showed that not all agencies
apply checks to ensure that specified tolerances are achieved. It appears
also that some agencies apply techniques whose achievable accuracy is
inconsistent with specified tolerances.
INTRODUCTION
The acquisition of spatial data to assess the state of forest resources has its
origins in central Europe during the 15th century (Griess, 1989). Since the early
19th century, when forest management based on reliable area statistics gradually
became common practice in Europe, forestry agencies have employed personnel
and established facilities for the specific tasks of surveying and map production.
In Canada alone, about 30 million hectares are inventoried annually by provincial
forestry agencies. These agencies maintain more than 37,000 map sheets at scales
varying from 1:10,000 to 1:20,000 (Leckie and Gillis 1995). Spatial data
acquisition by forest management agencies worldwide therefore accounts for a
significant portion of mankind's total effort and investment in resource mapping.
The aim of this paper is to present a review of the surveying and mapping
techniques currently employed by forest mapping agencies for digital database
establishment and maintenance. The presented information is mainly based on data
Lecturer, lnternalional Institute for Aerospace Survey and Earth Sciences (ITC), Enschede. The Netherlands.
309
from a small survey of forestry agencies known to use GIs in production (research
organizations, were not included). The aim of this survey was to determine: i) how
forest management agencies acquire spatial data for input to GIs; ii) the accuracy
tolerances (if any) applied; and iii) policies concerning the updating of the spatial
database. A questionnaire covering these topics was sent to 33 agencies in various
parts of the world and 25 replies were received.
The remainder of the paper is divided into three sections. The first section
briefly describes the main categories of spatial data required by forest management
agencies. The technology used by the 25 surveyed agencies to acquire these data
is summarized. In the second section, specified accuracy tolerances are examined
in the light of the surveying and mapping technology employed. The paper
concludes with some remarks on spatial data accuracy and error modelling in
forest management.
SPATIAL DATA ACQUISITION
Although maps are probably the documents most frequently consulted during
forest operations (Moser 1970), technical details their production are not widely
published, neither in the forestry nor in the surveying and mapping literature.
Leckie and Gillis, (op. cit.) provide extensive details of current forest mapping
practice in Canada. A recent report (Kennedy et al. 1994) summarizes forest
mapping activities in Europe. Probsting (1994) provides a useful review of
applications of aerial photography in European forest mapping.
Information content
The management of forests and forest land requires a wide range of spatial
information, usually obtained from a variety of sources. For example, a GIs to
support the formulation of "Indicative Forestry Strategies" in Scotland uses 34
sources of biophysical and administrative data (Aspinall et al. (1993). Although
requirements vary from agency to agency depending on forest conditions and
management goals, maps and GIs databases generally comprise four main
categories of spatial data, namely: cadastral boundaries, "internal" administrative
(for example, compartment) boundaries, topographic details and information on
the forest resources. Information on spatial data acquisition by the 25 management
agencies was compiled for each of these categories of information (table 1). It
should be noted that the data in the table refer only to the number of instances
where a particular technique is mentioned as being used. The data do not consider
the area concerned or the amount of equipment employed. Furthermore, many
agencies apply more than one technique to acquire certain categories of spatial
data.
Table 1. - Sources of spatial data required by 25 forest management agencies
(number of occasions mentioned).
Source
Cadastral
boundaries
Topographic
features
Internal
boundaries
Forest
boundaries
Supplied by other
agencies as:
maps
digital data
Acquired by the forest
agency using:
Land survey
GPS
Graphical transfer
from aerial photographs
Graphical stereoplotting
Digital stereoplotting
Orthophotography
Satellite imagery
Not required
Cadastral information
An exact knowledge of the area and limits of the forest land is essential for
proper management. In many parts of Europe, large scale cadastral maps are the
source documents for the compilation of the "basic forest map" (German
Forstgrundkarte). Forest management agencies generally obtain inforrnation on
land ownership boundaries from cadastral survey agencies. In some countries,
however, the forest management agency itself may have official cadastral survey
tasks such as the determination and demarcation of parcel boundaries on forest
land (Wander 1981).
Topographic information
General topographic inforrnation is required for planning forest operations such
as road construction and timber harvesting and also to provide a metric framework
within which sample plots for forest inventory can be located. Most importantly,
details such as roads or drainage features may define (parts of) forest stand
boundaries. Topographic information therefore forms an integral part of any
(graphical or digital) forestry database rather than simply a background against
which details, such as forest types, are mapped. The survey of 25 forest agencies
showed that they obtain topographic information primarily from other agencies.
However, surveys by forestry agencies themselves are generally required to map
additional details not shown on general purpose topographic maps and "new"
features such as recently constructed forest roads. Land surveying, GPS and a
range of photogrammetric methods are all employed for this purpose. When
relatively approximate methods of graphical transfer from aerial photographs are
employed, these additional topographic features are likely to be acquired with a
spatial accuracy lower than that achieved by the national mapping agency, leading
to inconsistencies in spatial data quality among different sources.
Permanent boundaries established by forest agencies
Many forestry organizations establish and maintain a "quasi cadastral" survey
of the land under their jurisdiction. This may take the form of compartments or
timber concession areas, the boundaries of which must be demarcated, surveyed
and mapped. The survey showed that a wide range of surveying and mapping
techniques are employed to acquire data on permanent "internal" forest boundaries.
Unlike "external" property boundaries which, for reasons of accuracy, are
primarily surveyed in the field, permanent forest boundaries need not be surveyed
to cadastral mapping standards. Photogrammetric techniques are therefore feasible
and, in fact, apparently predominate over field surveys.
Forest information
Information on stand boundaries and other "specialized" forest information is
not generally available from existing sources. Acquisition of this category is
therefore one of the main tasks of any forest surveying and mapping agency. Just
over half of the agencies employ simple methods of graphical transfer from aerial
photographs for this purpose. The operating costs of these "simple" graphical
methods are not significantly lower than those associated with the use of rigorous
photogrammetric methods (Weir 1981). It is therefore surprising to find this labour
intensive approach still so widely used by forestry agencies in developed
countries. This technique is employed by many provincial forestry agencies in
Canada (Leckie and Gillis, op. cit.), although some are now replacing it by more
efficient photogrammetric methods such as digital mono plotting.
As table 1 indicates, forest management agencies make only limited use of
satellite imagery for forest type mapping. Clearly, the spatial resolution of the
present generation of earth observation satellites is inadequate for mapping at the
scale, level of detail and positional accuracy generally required by agencies
practicing intensive forest management. In many developing countries, however,
satellite imagery may be the only source of timely spatial data on forest resources,
although informal discussion with foresters from these countries suggest that there
too, aerial photography remains the principal source of spatial data.
Frequency of revision
Unlike a soil or geological map which, if carefully produced, may serve as a
useful document for many decades, a forest management map must be updated at
regular intervals. Consequently, map revision is an important aspect of forest
management mapping. In the intensively managed forest of western Europe, where
management plans are traditionally prepared every ten years, management maps
are revised on a corresponding 10-year cycle.
Nowadays, map revision is increasingly carried out as a process of (digital)
spatial database maintenance. Table 2 gives details of the frequency of updating
employed by 16 of the surveyed agencies and indicates a tendency towards
continuous or annual updating. Corrie et al. (1994) describe the database
maintenance procedures employed by a timber company for 2.4 million hectares
of productive forest in British Columbia. The database, which includes 250 base
and thematic layers, is continuously updated, with all changes distributed to local
offices on a nightly basis.
Table 2.
-
Frequency of database updating by 16 forest management agencies.
Continuous
Several times per year
Annually
Every 2 - 5 years
Every 10 years
Cadastral
boundaries
Topographic
features
7
9
1
3
1
1
3
2
2
Internal
boundaries
Forest
boundaries
In fact, experience with database updating is still limited. At the time of the
survey (mid-1994), only two systems had been in operation for more than ten
years. Eight agencies had being using their system for less than five years and
some agencies had not yet done any updating. Interestingly, a few agencies
indicated that forest stand information is updated by means of a complete
remapping rather than by considering only the actual changes. Although this
would appear to negate the advantages of having a digital database, there may be
a limit to the amount of revision which can take place. One agency, which has
been employing GIs since 1977, updates the database on a 2-year cycle and
makes completely new overhaul every ten years.
ACCURACY STANDARDS
In forest management, spatial data are required to provide not only locational
information but also, most importantly, area data for the calculation of timber
volume and other quantities. Ideally, therefore, the errors associated with these
area data should be known. Leckie and Gillis (up. cit. p81) report that most
Canadian forest inventory maps are considered to have accuracies "in the 10 to
25m range", although as Gernrnell et al. (1991, p22 1) note, "most existing forest
inventories do not have estimates of the accuracies of their polygon labels and
boundaries".
The survey of 25 forest management agencies suggested that, among the four
main categories of information, cadastral boundaries tend to have relatively
stringent tolerances. As indicated in table 1, however, this category of data is
generally provided by other agencies according to predetermined standards. The
other three categories of information are mostly all acquired to the same standards
(although some agencies relax the tolerances for - relatively uncertain - forest
stand boundaries). Actual values vary markedly from one agency to another or are
expressed as a range (for example, 10 to 30m) within which map accuracy should
fall. Tolerances for stand boundaries set by individual forest management agencies
in Europe are generally in the range 2 - 10m rmse relative to nearby fixed detail.
Although virtually all of the 25 surveyed agencies indicated that they set
accuracy standards for the acquisition of spatial data, almost half (12) indicated
that they do not apply any quality control measures to check if accuracy tolerances
are actually being met. Interestingly, eight of these 12 agencies employ simple
graphical methods for mapping forest boundaries from aerial photographs. Tests
using a variety of instruments, photo-scales and operators (Weir, op. cit.) have
shown that the accuracy of these methods is highly sensitive to terrain conditions,
base map quality and operator skill. Their performance (under optimal conditions,
0.8 - 1.3mm rms plotting error) is generally inconsistent with required standards.
CONCLUSIONS
This paper has shown that many forestry agencies already work to, or at least
aim at, "near professional" standards of surveying and mapping. In some cases,
however, there is a need to define these standards more clearly and, in particular,
to apply appropriate quality control measures.
In forestry, spatial data acquisition is not simply a matter of map production.
Data on the areas of individual forest stands form an essential input for forest
management planning. Spatial data quality therefore has a direct impact on forest
management operations and business decisions (Prisley 1994). The establishment
and maintenance of a spatial database can represent a considerable investment for
a forestry agency, and it may be difficult to justify measures to improve spatial
data quality (Keefer 1994).
In recent years, GIs researchers have laid much of the ground work needed
to handle spatial error in natural resource databases. A lot of the research on error
propagation (see, for example, Heuvelink 1993) is, however, concerned with
situations in which the accuracy of the basic data is predetermined. Because they
acquire a large part of their primary data themselves, forest management agencies
are in the fortunate position of being able to chain backwards through the various
GIs operations in order to define the accuracy of primary data acquisition needed
to achieve the desired quality of the information product. In this way, the methods
and costs of spatial data acquisition can be better matched to the value of the
resource being mapped.
ACKNOWLEDGEMENT
This paper is based on information about spatial data acquisition kindly
supplied by forestry agencies in many parts of the world.
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BIOGRAPHICAL SKETCH
Michael Weir is a lecturer at the International Institute for Aerospace Survey
and Earth Sciences (ITC), Enschede, The Netherlands. He is mainly responsible
for teaching surveying, photogrammetry and GIs to foresters and other resource
specialists from developing countries.