Geography Reference
In-Depth Information
Box 43.3
Georeferencing for resource assessment: the role of GPS in community forest
resource mapping
Community forestry involves returning state-owned and
managed forests to villagers and local forest users, who
manage them for their subsistence needs of timber,
fuelwood, fodder, grazing and medicinal plants. There is
a growing interest in Nepal in obtaining information
regarding the quality of these resources: does the forest
improve or degenerate under village management? In
order to be able to gauge this reliable, baseline
information on the forest resource is required. Two critical
elements of this baseline information are an accurate
estimate of the position of the forest boundaries and
georeferenced air photographs that show the status of
the resource. In both cases, GPS was identified as a
cost-effective means of providing information (Jordan
and Shrestha 1998).
Forest boundary mapping used GPS as a survey
instrument. The receiver was taken around the forest
boundary and a geographical location recorded every
second. This information was stored in the GPS receiver,
and later downloaded into a computer, post-processed
and put into a GIS. In many cases, GPS data could be
used to map forest boundaries accurately and rapidly. The
feasibility (in terms of speed and accuracy) of this depends
largely on terrain: forests that have very steep slopes and
no clear delineation between community forest and other
land uses are difficult to map. This is due partly to the
physical difficulties and dangers of using GPS in this
terrain (in this region of the mid-hills of Nepal, 70 per cent
of the land has a slope angle greater than 30 degrees,
and slope angles in excess of 50 degrees are common),
and partly due to the obstructed view of satellites caused
by the hillsides.
To obtain useful results, kinematic DGPS was
necessary and ± 5-20 m accuracy was obtained. For
resource mapping in Nepal this is more than adequate
(and far more accurate than other spatial information
available). Absolute GPS did not provide a sufficient level
of accuracy, with positional fixes at one point being highly
variable. This was partially overcome by recording fixes
for 15-20 minutes at each point, and averaging the fixes,
but this is prohibitively slow.
To georeference air photographs, GPS techniques
were used to obtain an accurate geographical
location for an identifiable feature (such as a bend in
the road, house, temple or hilltop). This was used as
a control point. With georeferenced control points the
spatial information from the air photographs could be
corrected geometrically and entered into a GIS. This
technique provided a means for mapping approximate
forest boundaries rapidly (they are only approximate
due to the spatial distortion inherent in standard
aerial photographs). Georeferenced control points
were established using DGPS. In this project, five
minutes of readings were found to be adequate for
an accuracy of ± 2-5 m. The key was to identify
control points that could be identified easily on aerial
photographs, and in positions where the satellite
signal is not obstructed.
GPS users in developing countries face a lack of
georeferenced base stations in convenient and secure
positions. This problem was overcome in Nepal by locating
the GPS base station at a project building in the same
watershed as the mapping and resource assessment work.
The location was determined by averaging over 30,000
individual readings. Statistical analysis of the data collected
determined that the location was accurate to within
approximately ±5 m, more than adequate for this type of
work.
of field data at a variety of spatial scales for a
variety of applications. The digital format of GPS
data allows rapid transfer of information to GIS
or other computer mapping software. This in turn
permits data to be checked for accuracy while still
in the field (Carver
et al
. 1995; Smith
et al
. 1998).
In addition, work in Snowdonia has shown that
DGPS methods can supplement information
derived from orthophotography or other remotely
sensed data sets. Better-quality delineation of
features is possible by avoiding the limitations of
data resolution and allowing field knowledge to
be incorporated into the mapping process. DGPS
makes field mapping and higher-accuracy
surveying possible in difficult sites such as remote
and rugged mountain terrain and forested areas,
THE ADVANTAGES AND DISADVANTAGES
OF USING GPS TECHNOLOGY
GPS technology is proving to be a valuable tool
in a wide range of geographical studies. DGPS
static and kinematic methods are the most widely
used. The specific benefits of DGPS for the
geographer involved in field research are
summarised below.
Kinematic DGPS data collected using
pseudorange observations facilitate the creation of
digital maps delineating, for example forest and
glacial landform boundaries. Positional accuracies
of ±3 m can be achieved. Carrier phase DGPS
allows point locations to be defined to sub-metre
accuracy. This range of accuracy allows collection
