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Sub metric elevation surveying equipment for water systems in refugee camps
Sub metric elevation surveying equipment for water systems in refugee camps
Hello,
I am Santiago and I work as a water systems expert for the UNHCR, the UN refugee agency. I am not a surveyor myself.
I need advice on what kind of equipment to use to get elevations within a +- 0.5 meter error bracket to design water systems in remote locations worldwide (no RTK correction for example). Ideally, besides the precision, it would be:
- Fast to operate and learn.
- Lightweight.
- Cross borders easily and quickly without sensibilities.
- Affordable
Any information on technology or even particular equipment is greatly appreciated.
Thank you,
Santiago
- 21 Replies
https://positioningservices.trimble.com/services/rtx/fieldpoint-rtx/
I have used the Centerpoint RTX service with Trimble R10 receivers for water supply projects in Ecuador, but it is higher accuracy (and more $$$) than you require.
https://www.waterworld.com/drinking-water/distribution/article/14276477/mapping-in-the-mountains
You could use SBAS corrections, if they were available in the region your working in, which I don’t believe includes Africa or the Middle East. I could be wrong.
https://en.wikipedia.org/wiki/GNSS_augmentation
Honestly? Nobody doing the work is a surveyor? Conventional level runs. Decidedly low tech, easy to learn, no whiz-bang flashy light electronics to raise suspicion. It’s been done this way for a long time.
The other option involves resource grade GPS and post processing and some body familiar with processing to apply the corrections. One unit collects GPS data as a base station and a second unit acts as a rover storing observation along the route. The roving data is combined with the base data to apply the necessary corrections to achieve desired accuracy.
Good luck!
Willy
An X-90 OPUS static receiver and a used total station for trig levels based on the OPUS points.
I doubt they care about actual elevation – only differences across the project, so trig levels would be sufficient.
.An X-90 OPUS static receiver and a used total station for trig levels based on the OPUS points.
AUSPOS or Trimble RTX post processing would be the way to go rather than OPUS.
@bill93 That is true. We used ITRF2014 positions at the current epoch, and EGM08 geoid model.
The nice thing about using RTX is that the elevations are displayed right away, no comps necessary. I would assume that those decimeter solutions can be obtained using something less than a fully capable data collector, maybe even a cell phone, and be able to apply EGM08. But, I am not familiar with the non-survey grade Trimble receivers.
Could be that something like the Lidar function on an Iphone 13 would answer the low cost / easily transportable portion of your needs. Not so sure about the easy to learn part, at least for the data resolution portion. Check out this Youtube on the process.
Honestly? Nobody doing the work is a surveyor? Conventional level runs. Decidedly low tech, easy to learn, no whiz-bang flashy light electronics to raise suspicion. It’s been done this way for a long time.
The other option involves resource grade GPS and post processing and some body familiar with processing to apply the corrections. One unit collects GPS data as a base station and a second unit acts as a rover storing observation along the route. The roving data is combined with the base data to apply the necessary corrections to achieve desired accuracy.
Good luck!
@ Williwaw Sorry for the language barrier and the tech barrier.
What would be conventional level runs? These with an optical theodolite?
I am certainly hoping I am not the one doing these surveys :), but find a solution that can be rolled out. Refugee camps are usually in remote locations where contracting is complicated.
@john-hamilton What would be the rough cost of equipment and subscription?
@bill93 Yes, we only need relative elevations but speed and ease of training would be nice to have.
Any particular equipment you could point to/recommend?
Topo surveying by stadia methods, which was discussed just last week in another thread, might be a low cost/low tech answer to your problem.
My understanding of your messages is that you want to determine heights of points in a water distribution system at refugee camps. If that is the case, you should be able to determine relative heights with a basic level, tripod and rod like this set:
BTW, the equipment shown above is illustrative of a low-cost option. I have not used them.
I assume that you are talking about remote sites where equipment would be transported over bad roads and the like. This requires periodic checks to insure the level remains in adjustment.
Look on the web for ??leveling peg test? or ??Forster method? or ??Kukkamaki method.?
Setting or choosing stable and recoverable points at or near the sites of interest is important.
I do not know how much you know about leveling. I recommend that all leveling begin and end on stable and recoverable points.
Establishing ??permanent? monuments will expedite future expansion to new sites.
All leveling should be done in closed loops meaning that the summation of rod readings should equal zero with the difference being errors in your observations. Distances between the level instrument and rod should be consistent and with these distances limited by the ability of the instrument person to clearly read the rod. Intermediate sites observed between monumented points (turning points) should be use clear and distinct points (like a screwdriver driven into soil).
Note that my comments assume that you want to establish relative heights with respect a point on a relatively compact site and are not attempting to tie your work to a national datum
Make sure the tripod is firmly set and stable (check clamps and the tripod head and feet for any play. Keep the base of the rod (and face) clear of anything that would alter the rod??s direct contact with the point of interest.
There are a number of samples of leveling notes on the web, I would look at examples from surveying programs at universities. Clearing labeling points and keeping track of them certainly helps.
With respect to GNSS, remember that physically meaningful heights like orthometric heights (H) are approximated by the combination of GPS-derived ellipsoid heights (h) and a geoid model (N). Their relationship is h – H – N = 0 (ideally). While both the UNAVCO and NGA sites have on-line tools reporting N they provide different numeric results. These sites require geographic coordinates for the point of interest.
The NGA tool is available from this link: https://earth-info.nga.mil/#tab_data
Good luck,
DMM
GNSS technology is the appropriate tool to connect distant sites together. Long leveling runs accumulate error and take a lot of time.
Thank you everybody for the replies, it was very helpful ???? .
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.
.@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) ?ñ1.0mm If I survey one km with let’s say 10-20m drop what error can I expect?
S.
@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.
@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) ?ñ1.0mm 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
@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.
@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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