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TOPEX

TOPEX, an index of topographic exposure, is a method for assessing the wind exposure of a location or site. It has up to now been used predominantly in fields of forestry and ecology as part of the assessment of suitability of a site for establishing trees and plants, or the the vulnerability of existing forest or woodland to wind. It was first described in 1972 by Douglas Malcolm of University of Edinburgh in describing the exposure and elevation that trees could tolerate in Scotland and be grown with the objective of timber production. It is a simple, objective characterisation of the topographic shelter of a site



Measurement
TOPEX is calculated simply by measuring the angle to the horizon from a chosen point, in eight cardinal directions; north, north-east, east, south-east, south, south-west, west, north-west. These eight angular measurements are summed to give the site's topex value. Topex assessments should be carried out in clear weather, using a compass to provide the sighting directions and an optical clinometer to measure skyline angles. Alternatively Topex can be calculated for one or multiple sites by computer using GIS with data from a digital elevation model.

Application
Topex has been used in forestry to consider exposure of potential sites for planting or woodland restoration, and in ecology to assess the suitability for restoration of ecological communities. In the UK it is used as an input variable for the Ecological Site Classification system that assesses site suitability for tree species, and provides changing suitability with projected climates. It has also been used in conjunction with elevation to calculate a windiness score (Detailed aspect method of scoring "DAMS") that is an input parameter for the ForestGALES model used to assess wind risk in British forestry. It has been used by foresters in various countries as part of wind risk assessment, including Canada and New Zealand.