BARS PARALLAX TYPE PB.2 in Box (15) (ET) *
£95.00
In stock
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BARS PARALLAX TYPE PB.2 in Box with its record of inspections which dates from 1975 to 1990, a very nice set.
Stereo parallax is a condition that refers to the apparent displacement of a point or a feature of some height in an photo or image caused by a shift in the position of observation: This will occur in aerial or space photos that have been taken in succession (such that a stereo pair has either overlap (same flight line) or sidelap (parallel flight lines). It is possible to calculate height differences (relief) between points of different elevation along surfaces from stereo pairs by using a variant of the parallax method. This formula applies: h = (H) dP/(P + dP). P is the absolute stereo parallax. The dP term is the differential parallax, which is the difference between the absolute stereo parallax at the top and at the bottom (base) of the feature or point being measured in the pair. One can find the differential parallax by measuring ths distance between the base and the top of the feature, or between two proximate terrain points at different elevations on a slope, that we locate in each of the two photos when the pair is in optimum alignment for stereo viewing. Then, dP is the numerical difference between the two distance values (x2 - x1), which is different in each of the photo pairs. This type of device was commonly used by photogrammetrists in conjunction with a stereometer to find the values of P and dP by a variant of the "floating point" method in which reference marks-dots or crosses-are visible when the aligned photos are viewed under a mirror stereoscope. One places the parallax bar on the photo plane so that a fixed mark over one photo coincides with a movable mark (fused visibly in the stereo view) by turning a screw drive that translates that mark into coincidence. We then read the value of dP from a graduated scale. When we do this for a number of points, then, we can calculate heights and relief values for parts of the map. These become absolute values (elevations),
