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Radio Show & Podcast Feedback / Triangulation needs three reference points
« on: 08/05/2010 17:11:32 »
Using the term "exactly" is a little bit dangerous here... if one were only using grid bearings, a great degree of accuracy could be expected, however we're talking about taking bearings in the field which would be dependent upon magnetic and not grid bearings (and the previous comment by Bored Chemist is actually using the third "type" of north, true north, which varies from both grid and magnetic!)- depending on the age of your map, the magnetic declination could be inaccurately assumed and the adjustments normally estimated on ordinance survey maps are just that, estimates- the inaccuracies involved concern whether you are even using the current magnetic variation for that year and location (magnetic declination changes every year and the degree to which it changes is also not constant), and whether you are actually able to accurately measure the bearing using something like a prismatic compass where weather, technique, compass accuracy all become variables- my point essentially is that one will usually be less accurate taking field bearings, and when only using a bisecting technique, your location could, as exemplified by Matthew, be considerably off and rescue crews or fellow travelers, etc. would have to search a relatively unknown area to find you, whereas using three back-bearings, even if your back-bearings were slightly off, any searching team would know that you must be within the triangle created by slightly inaccurately measured bearings and therefore know exactly where to search...