[[111:uas_mapping:stereo#2. overlaying and navigating in the stereo tab|{{  :orbitgis:return_overview.png|Return to Overview - Orbit UAS}}]]

====== floating mark ======

Floating mark is a mark seen as occupying a position in the 3 dimensional space formed by the stereoscopic fusion of a pair of vertical photograph and used as a reference mark for examining or measuring the stereoscopic model. 
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{{111:uas_mapping:stereo:floating_marka.png?nolink&550 |}}
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([[http://clouds.eos.ubc.ca/~phil/courses/geog373/lectures/|Source]])
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====== Differential parallax ======
\\ The dP term is the differential parallax. We find it by measuring the 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. We can optically find the value of dP by using special devices, such as a stereometer, a parallax wedge, or, most commonly, a parallax bar. Each uses a variant of the "floating point" method in which reference marks–dots or crosses–are visible when we view the aligned photos under a mirror stereoscope. We place 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), if we reference the points to benchmarks.\\ \\ 
{{111:uas_mapping:stereo:differential_parallax.png?nolink |}} \\ \\ 
More information & source can be found at :     
  * [[http://faculty.bemidjistate.edu/pconklin/air%20photo/study%20outlines/study%20outline-differential%20parallax.html|differential parallax]]
  * [[http://rscc.umn.edu/rscc/v1m7.html|differential parallax2]]
  * [[http://geoinfo.amu.edu.pl/wpk/rst/rst/Sect11/Sect11_4.html|differential parallax3]]