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That's OPUS.jpg, they call that a J-PEG (address ends with ".jpg").
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J-peg image technology has been a standard for over 25 years now. They call it "old school."
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Sorry, was trying to be funny... I knew about "J-PEG," but was not familiar with "OPUS."

Sorry, was trying to be funny... I knew about "J-PEG," but was not familiar with "OPUS."

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Okay, ha-ha-ha.  
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I found the earlier spot where they used the same j-peg before. It's the second picture in this post, 3h:
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https://www.cathinfo.com/fighting-errors-in-the-modern-world/global-navigation-satellite-systems-tutorial/msg569789/#msg569789
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Maybe you can't see the picture there too? That's where they explained OPUS: Online Position User Service, or OPUS
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Doesn't show up there either... will try viewing on the laptop tomorrow to see if any of the pictures show up.  Just not having having any luck on this device. Thanks though for your efforts.
Time for me to put the kids in bed... chow.

So, Matthew's file share is a no-go?

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This is the end of unit 4, Reducing errors in GNSS processing. They're using surveying principles in obtaining coordinates of land located positions, but since they're working with moving satellites, they have to rely on iteration programming to automatically identify and remove errors (or inaccuracies) in the computations. That's why this unit is important. Most error compensation in surveying is done deliberately and one at a time. Not here.
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In this unit, we explored how multiple sources of error enter into the equations that affect our estimated distances between the GNSS satellites and the GNSS receivers on Earth. Interactions between the satellite signals and the charged ions in the ionosphere or the atmospheric conditions in the troposphere can cause unexpected delays in the signals as they travel from the satellites to the receiver. We discussed how clock errors—at both the satellites and the receiver—cause errors in our estimate of the distances. We explored various methods of removing or reducing these errors, either using physical properties of radio waves or multiple receivers receiving signals from the same satellites at the same time. We looked at the important role of observation time on the accuracy of positioning results. We also discussed the role of post-processing GNSS data in the reduction or removal of many of these sources of error.
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Foundations of Global Navigation Satellite Systems (GNSS)
Converting GNSS raw data into positions (unit 5) 

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• 5. Unit description and objectives
• 5a. The limitations of GPS positions in an X, Y, Z coordinate system
• 5b. Expressing positions in an ellipsoid model
• 5c. Expressing positions in an orthometric reference surface
• 5d. Summary
• 5e. Review questions

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The precise orbits of GPS satellites (accurate to ± 2 cm) are referenced to an international geometric reference frame, called the International Terrestrial Reference Frame, or ITRF. The ITRF has its origin at the center of mass of Earth, with its three axes oriented at 90⁰ from each other.

So now we have precise positions from our GPS receiver but they are in a Cartesian coordinate system. Although this system may be easy to work with mathematically, it’s very difficult for us to conceptualize the corresponding locations on Earth’s surface. It would be more useful for most applications if we had them referenced to a geodetic coordinate system such as latitude, longitude, and height on Earth’s surface.

Geodetic vs. geometric systems
The word “geodetic”, derived from “geodesy” is used to indicate that we are relating positions to the precise size and shape of the earth. This is distinct from a purely “geometric” system which is referenced to single point and orthogonal axes.


For example, the peak of the Washington Monument expressed in Cartesian (geometric) coordinates is: (1115287.503, -4844432.918, 3982867.096). These are actually distances measured in meters from the center of mass of Earth along the x, y, z axes of the ITRF frame.

Not the most useful piece of information, is it? These numbers are unwieldy and not commonly used. To be more useful, we’d like to transform them into a more familiar geodetic coordinate system expressed in latitude, longitude and height.

So how do we translate our x, y, and z into latitude, longitude, and height? We need a model of the surface of Earth and its relationship to its center of mass.
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I went ahead and uploaded these two images which should be visible below. Their order is reversed, though..........
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I remember someone asking about elevations in this tutorial, and at the time I wasn't sure where they would be, but now that we're covering ellipsoids, this is the part to see for elevations.
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In engineering traditionally a datum line is used for a construction project for elevations. When building a dam or a bridge or a high rise building, some depth level is selected which will be a convenient level from which to reference all elevations in the project. Quite often it's a plane about 100 feet below the earth's surface. This is a theoretical plane and nobody ever has to go digging down to "find it" because that would accomplish nothing. They wouldn't be able to "see" anything there anyway. It's just a number on a piece of paper like a plan map.
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In this tutorial, the ellipsoids described are theoretical spheroid constructions that closely imitate the "smoothed out" shape of the earth, and as such they are located in some places above the surface of the earth, and in some places cutting through hills or mountains or even through flat plains or lakes.
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For geodetic surveying then, ellipsoids are closely related to datum lines since vertical measurements to describe elevations are measured as being above or below the ellipsoid by a particular number of meters.
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That last section, above, that came out in italics was not supposed to. Here is a copy of it without the italics:
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For example, along much of the coastline of the continental United States, the ellipsoid is above the land surface...so low-lying coastal land may have a negative ellipsoid height! In the mountains, the ellipsoid is typically below the surface. One problem with ellipsoid-based heights is that they do not accurately predict the direction of water flow. To do this, we need to convert to orthometric heights which are more closely related to gravity.

The surface of Earth is most closely described as an ellipsoid, a sphere somewhat flattened at the poles. We can convert the geometric coordinates derived from GPS satellites to points on this ellipsoid surface. This lets us express positions in terms of defined geodetic latitude, longitude, and height.
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These two paragraphs are important for this thread, and we'll probably be referring back to them later.
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