Fisheries and Land Resources

Forestry and Agrifoods

Creation of Spatially Accurate Mapping

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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