Topic 7.4: Topographic Maps

Good site maps are a prerequisite for sound management planning.  Where topographic maps are unavailable, unreliable, or at too small a scale, 1:5000 or at most 1:10,000 maps need to be prepared before roads and main skid trails can be laid out in a cost-effective and environmentally-sound manner.  The costs of map preparation can almost invariably be recouped during road building, harvesting, hauling, silvicultural treatments, and site closure activities as well as in the future when the site is again opened for harvesting.  Consider the financial (and environmental) savings accrued if accurate maps result in shortening roads by 10% or in 10% more roads being located on more level terrain.

A variety of methods are available for drawing accurate maps, ranging from using hand held compasses and inclinometers to technologically sophisticated methods involving satellite imagery, side-scanning radar, and etc.  While traditional field-based methods are still the norm in most of the tropics, use of geographical information software (GIS) in conjunction with global positioning systems (GPS) is rapidly becoming more common.  While GIS and GPS can increase the accuracy of maps, for the forester responsible for developing management and harvest plans, there is still no substitute for substantial first-hand familiarity with the site.

Inventories and stock-mapping

Several different sorts of inventories are often needed in a single management unit.  Where concessions are granted on the basis of competitive bidding, potential investors need at least rough estimates of the volumes and grades of harvestable timber or other forest products.  The forest owner may need more accurate estimates of forest product values to assess rents due and to avoid excessive profits going to whomever is carrying out harvesting operations. Detailed stock maps of trees to be harvested are required to assure that skid trails are located so as to facilitate efficient timber yarding.  Finally, estimates of sustainable harvesting rates need to be based on site-specific data from permanent inventory plots.

Large scale, low intensity (e.g., 5-10% of area) inventories are often carried out using 20 m-wide strips spaced at 100-200 m intervals.  A common alternative is using prism plots at 25 m or 50 m intervals, depending on stocking, prism angle, and area to be surveyed.

Forest management decisions, like lengths of cutting cycles or types of silvicultural treatments, need to be based on more detailed information derived from permanent or continuous forest inventory plots.  Methods for establishing permanent plots for monitoring growth, regeneration, and death have recently been described in detail by Alder and Synnott (1992), and Condit (1998).  Here only a few common mistakes will be mentioned:

• mark the height of dbh measurement with a continuous painted line around the bole;
• do not insert nails <20 cm from the measurement point;
• select plot size and shape on the basis of tree sizes, densities, and spatial arrangements.