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Provincial forestry departments across the country
rely on Provincial Central Mapping Agencies to provide topographic base maps for
their needs. In most provinces outside of Newfoundland, 10,000 or 20,000 scale
topographic maps are available. These maps form the basis for forest inventory
(FI) mapping. Features such as water, roads and wetlands (bogs) are already part
of the topographic base so forestry departments are required to capture only
forested areas.
In Newfoundland, the largest scale topographic map
available for forestry use is 50,000. Because these maps do not contain
sufficient detail for our requirements, we capture all features during our
forest inventory mapping process. The topographic map is used for Ground Control
Point (GCP) purposes only. GCPs are used during the map making process to
position map features correctly with respect to their true position on the
earth’s surface. They are features on the mapping source, in our case 12,500 air
photography, for which the location (longitude, latitude) on the earth’s surface
is known.
Spatial accuracy, which is a measure of the distance that
features on the map are from their true position,
is affected by many factors, one being the accuracy of the Ground Control
Points. In our case, 50,000 NTS maps are rated at 25 meters accuracy, meaning
that the representation of any given feature may be as much as 25 meters off its
true position. This same degree of accuracy, or in-accuracy, is
transferred to our forest inventory maps.
In the past, the spatial accuracy of data was less
important than the relative accuracy of features to one another. It was more
important that a lake be properly positioned relative to a road than that either
the lake or road be positioned accurately with respect to the earth’s surface.
This has changed recently for two reasons. Firstly, the need to integrate data
from other agencies has become important. To ensure data from various agencies
overlay properly, these agencies must use a standard and accurate base.
Secondly, Global Positioning Systems (GPS) are being used to collect data for
incorporation into Forest Inventory Database. Because GPS data is very accurate
(5 meters) these data do not overlay very well with FI maps (25 meters). For
example, a cutover mapped with GPS may partially fall into a lake when overlaid
onto the FI maps.
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