Ellipsoid Height

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# Ellipsoid Height

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Posts: 2
Member
(@interwebs)
FNG
Joined: 1 year ago

In Survey Pro, I need to calculate the scale factor from the current grid to ground. After I select a point to scale from (approx. center of the project), it is asking for the Ellipsoid Height.  Is the value shown actually calculated by Survey Pro from my point selected and I simply press enter and continue?  If it is not calculated, how do I best calc this Ellipsoid Height?

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Member
(@bill93)
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What height information do you have to work from?  Does the program already have an ellipsoidal height, orthometric height, or none of the above?

Member
(@interwebs)
Joined: 1 year ago

FNG
Posts: 2

A GPS derived ground elevation.

Posts: 5717
Member
(@bill93)
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Joined: 9 years ago

The GPS measures ellipsoidal height.  If it also gives you orthometric (NAVD88), that is done by applying a geoid model.  So you should have ellipsoidal available to you.  I don't know what your program does with the numbers.

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(@thebionicman)
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RTK GPS initially computes a cartesian x,y,z relative to the equatorial plane. It then transforms those coordinates to geaographic on whatever ellipsoid you choose. At that stage heights will be ellipsoid, relative to the control used. That of course assumes you used the correct height.

Next, the coordinates are projected by math or transformed by math or localization. Ellipsoid heights are converted to elevations by applying the geoid separation or again by some localization or mathematical violence.

Bottom line, GPS spits out what you tell it to, nothing more, nothing less...

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Member
(@mightymoe)
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Joined: 9 years ago

An easy way to get a combined scale factor is using the free software corpscon. I don't have Survey Pro so I don't have any idea how it works. Corpscon can be googled and then downloaded, it is simple to use. Pay attention to the zone you want and the Geoid model however any Conus Geoid flavor will give you a combined scale factor within a ppm if you are in the US. I would use it even if you figure out the Survey Pro program as a check against what you are doing.

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