To be completely honest, this is a survey nobody enjoys doing. If the National Weather Service is heading out to survey if a tornado impacted an area the one thing they are looking for is damage. Trees, roofs, "Large, Isolated Retail Buildings" or even transmission line towers all have damage signatures when exposed to strong winds. When damage in the field matches research damage found in the labs, the NWS will assign a tornado a rating as well as specify the strength of the winds.
While a 96 page PDF helps survey teams in the field, the eventual rating ultimately comes down to the observations and interpretations of the NWS individuals. The rating is based on the worst damage that the team has been able to identify.
Click through the various images to get an idea of how this process is instituted after such an event. This particular example is taken from the Pantego/Belhaven tornado that impacted Beaufort County April 7th.
After a piece of damage is rated, in our example it was the trees, a dot is placed using GPS coordinates. By stringing together multiple "damage dots", the path of the tornado can be seen and studied.
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