12-14-2016 12:14 AM
I wonder what is the effect of setting the reference system as “Unknown” when building stereo block.
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12-15-2016 10:26 AM
By selecting "Unknown" you are setting a Local Spatial Reference (LSR) which is a cartesian coordinate system. You must specify horizontal and vertical units only.
If your block is covering large areas of the earth and small scale photography you might not prefer LSR. Because if you want to consider earth curvature and define topocenter during triangulation the option "consider earth curvature in calculation" will be grayed out for LSR coordinate system on Aeiral Triangulation dialoge box.
12-16-2016 04:05 AM
I would like to thank you for your input, as I understand that using LSR can be done in relatively small projects when the earth curvature can be ignored. Are there any recommendations from Erdas about the maximum size of project that we can use LSR without affecting the results accuracy?
Besides is there any advantage of using LSR or not, for me I would like to define the coordinate system that I am working with rather than setting LSR unless there is an advantage of using it.
Thanks again for your contribution.
01-18-2017 09:01 AM
It depends on a lot of variables, like focal length, film format/size, flying height, area covered, digital vs. film sensor. Back in the film days with 6” focal length, 9”x9” format, it was generally considered if photo scale was 1:10,000 or smaller, but it can also be an issue at lower flying levels (larger photo scales) if covering large areas. In geodetic land surveying they typically apply earth curvature corrections for traverses that exceed 10 miles in any direction. Today’s digital sensor properties vary widely so there is no simple rule of thumb.