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Sub metric elevation surveying equipment for water systems in refugee camps

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Bill93
(@bill93)
5,000+ posts Member
Posted by: @geeoddmike

physically meaningful heights like orthometric heights

For a water project, the theoretical ideal is "dynamic heights" not "orthometric heights" but that probably is too fine a distinction for this project.

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Posted : August 3, 2022 8:34 am
Santiago
(@santiago)
FNG Member

@geeoddmike Thanks a lot it is very helpful. I don't know much about surveying, for example, the meaning of this:

Accuracy (per double km run) ±1.0mm

If I survey one km with let's say 10-20m drop what error can I expect?

S.

 

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Topic starter Posted : August 5, 2022 2:24 am
Lurker
(@lurker)
500+ posts Member

@santiago While the specs are much better. Someone new to leveling, without making any blunders, should be well within 0.10'. A myriad of small things would have to accumulate for this error to occur. Reading the wrong "foot" on the rod, not leveling the instrument, reading the wrong crosshair, (all blunders) would result in a much larger error. You should be very comfortably within  a tenth without blunders.

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Posted : August 5, 2022 5:52 am

GeeOddMike
(@geeoddmike)
1,000+ posts Member
Posted by: @santiago

@geeoddmike Thanks a lot it is very helpful. I don't know much about surveying, for example, the meaning of this:

Accuracy (per double km run) ±1.0mm

If I survey one km with let's say 10-20m drop what error can I expect?

S.

 

Recognizing that equipment and their users are fallible, reasonable expectations of achievable accuracy/precision have been developed. Standards and specifications have been developed to assist the assessment of whether the work performed was acceptable (meets the standard).

In the US the relevant document is: https://www.ngs.noaa.gov/FGCS/tech_pub/Fgcsvert.v41.specs.pdf  worldwide there are other standards and specifications. 

Note that the US document linked is indicated to be for digital bar code leveling. There are some differences when using optical levels.

The older US standards document is: https://www.ngs.noaa.gov/FGCS/tech_pub/1984-stds-specs-geodetic-control-networks.pdf   It covers horizontal, vertical and gravity networks.

As I understand your message, your work would be third-order. Reading the linked document you will find lots of good advice on things like sight lengths, agreement between forward and backward runs (leveling from A to B then B to A), loop closures (how well leveling performed from one point through others then back to the starting point) and more.

Rather than thinking of an “expected” error, the statistic used determines the maximum acceptable value  for leveling performed according to the standard following the specifications.

 

Looking at the loop closure line for third-order leveling we see that the maximum acceptable error (in mm) is 12mm * square root of the length of the loop in kilometers. In other words, a level loop of one kilometer will have a maximum acceptable error (disagreement) of 12mm.

The referenced document does not address the issue of large vertical changes directly. With large gradients the challenge will be to keep sight lengths nearly equal, Not observing either too high or too low on the rod, careful plumbing of the level rod (use rod levels), and stable instrument setups are important considerations.

If your loop closure on a one km level loop meets standards with a closure of 11mm, what do you do if you have some points inside the loop end points that you might use subsequently? You can adjust the preliminary values determined for these points by dividing your closure by the number of turns and apply the correction to the points.

If you end up with a large set of level lines that you wish to combine optimally, you should consider using the method of least squares. Dr Charles Ghilani developed some “free goodies” for his students at Penn State University-Wilkes Barre. He is now emeritus. His site is: https://personal.psu.edu/cdg3/free.htm

Hope this helps,

DMM 

 

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Posted : August 5, 2022 1:08 pm
Santiago
(@santiago)
FNG Member

@geeoddmike Thanks for the detailed image and the documents linked. I will work my way through them, but I have to recognize I struggle now with all the terminology.

What I wanted was a rough estimation of the error, an order of magnitude for these optical theodolites, i.e. less than 20 cm per km. Or in plain words, "is this instrument precise enough for the use I intent?

For me, if the error is less than say 0.1m per km that is good enough since head losses in pipes are in the order of 10m/km.

 

 

This post was modified 2 days ago by Santiago
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Topic starter Posted : August 6, 2022 9:05 am
GeeOddMike
(@geeoddmike)
1,000+ posts Member
Posted by: @santiago

@geeoddmike Thanks for the detailed image and the documents linked. I will work my way through them, but I have to recognize I struggle now with all the terminology.

What I wanted was a rough estimation of the error, an order of magnitude for these optical theodolites, i.e. less than 20 cm per km. Or in plain words, "is this instrument precise enough for the use I intent?

For me, if the error is less than say 0.1m per km that is good enough since head losses in pipes are in the order of 10m/km.

 

 

I see we continue to have communication problems.

1. You want a rough estimate of the error using "optical theodolites." An theodolite is a surveying instrument measuring horizonatal and vertical angles. One can use the vertical angle feature to determine height differences using trigonometry. 

Optical theodolites can use techniques to determine distances (e.g. stadia or subtense bar) or use survey tapes to measure distances. 

2. Modern optical instrumentation are combines the angle measuring with a built-in electronic distance measuring capability. These are referred to as "total stations." There are versions that require prism reflections and those that do not.

3. Levels are designed specifically to precisely determine differences in height. Some include the ability to measure rough horizontal angles.  They come in optical and digital versions. 

Optical theodolites (e.g. the venerable Wild T2), total stations (e.g. Leica TC-1205) and levels (see models and specs below). I provide these manufacturer/model numbers for you to review and gain a sense of their capabilities.

My answer to your question is two fold. You can use the manufacturer's specifications as a guide in the choice of instrument. You must also recognize that even the best equipment will not meet specifications if not operated correctly, not kept in adjustment, nor failing to follow specifications.

An optical level (like those in the screen capture) is the cheapest, easiest to use and most accurate.

Operating any survey equipment requires attention to detail and an understanding of possible error sources.

Hire a surveyor to "train the trainers" if possible.

You may find the materials linked from this site to be useful: https://geodesy.noaa.gov/pub/corbin/Training/Leveling/precise-leveling-workshop/   Obviously you do not intend to perform geodetic leveling. Nonetheless there is lots of useful information on leveling.

 

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Posted : August 7, 2022 12:50 am

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