Speaking of Power Squadron...
This post earlier this year in a [msg=131006]thread[/msg]gave a good answer.
Thanks, Deral. I'll look into what the cost will be to rent dual-freq equipment. The project has clusters of wells (these are water wells) scattered around the county- about thirty individual wells total. The county is pretty much a perfect rectangle, approx. 48 mi N-S x 30 mi E-W. Again, about the same size as Rhode Island.
There seems to be an assumption that it has to be referred to HARN stations or long OPUS sessions. Isn't it true that these are not as desirable for vertical control as first order bench marks that are closer to the wells being measured?
There are a lot of first order bench marks near the destroyed HARN station discussed above, and he should check the NGS list for ones close to the wells.
Bill is right. OPUS height solutions out there could be half a foot off local published heights. OPUS RS isn't even available there which means OPUS static height error could be about 10 cm or higher relative to the national network. Who knows on the local network? I think its a digital leveling job for the accuracy they seek.
Generally, HARN points are good vertical bench marks. In fact all the HARN points I use are first order bench marks with good stability.
Using the HARN control is the way to go if what you want is good horizontal control and vertical control that matches elevations in the area. I would also include bench marks that are near to the wells. If you are doing 30 wells over a 1400 sq mile area I would consider the HARN control as primary. You just need to consider the density of the control and how many receivers you can put into the network. 30 wells and bench marks will get very time consuming if you occupy each one for 4 hours for OPUS solutions and I think it would too costly and counter-productive.
OPUS and the new GEOID might well shift the elevations quite a bit, but if HARN control is occupied and the elevations for them are fixed you will have better elevations results that are in agreement with the local elevation control system. Which is what these well surveys are usually trying to achieve.
Why would you not fully constrain the vertical component of the network to first order marks nearer the wells as Bill describes? I agree you would want to constrain to HARN for horizontal.
Getting error estimates of 2 cm or less would be a trick even for an experienced geodetic surveyor let alone a person learning.
I can see pumping a lot of effort into this and end up leveling anyway if the 2cm is a requirement.
Maybe somebody else can find different, but so far what I found was that all the other benchmarks, first order vertical or otherwise, are generally clustered around the two main highways which cut the county into quadrants. None of the wells are on those highways and in fact all of them range from 5-20+ km away from the main roads. And since I only have L1, OPUS isn't an option either. Given that, I think I would have to do some leveling to create secondary base stations if I wanted to stay within NGS-58 guidelines. I would like to stick to the guidelines as much as possible with my limited equipment and skills.
Oh, I would. But he needs horizontal and vertical. If the HARN control points are also bench marks, I would set on them and tie bench marks near the wells-if there are any.
If it's like many of these projects I've done then it's a long hike to a bench mark-sometimes 10-20 miles.
> I can see pumping a lot of effort into this and end up leveling anyway if the 2cm is a requirement.
I'm just guessing that some salesman told dliviskie's boss, or whoever bought the equipment, that with postprocessing they could get vertical accuracies to 2 cm or less. The boss decided to have dliviskie get elevations to 2 cm accuracy on every well.
You have to ask yourself two questions:
2 cm compared to what?
Why is the heck does anybody on the face of the earth need to know the elevation of a well to within 2 cm?
OK, maybe 3 questions.
What is more important? The elevation of each well or the difference in elevation between any two wells.
Personally I would tell the boss, that 2 cm you wanted ain't happening.
James
You've got it basically right. The conditions under which we could get 2 cm accuracy weren't as clearly explained as the fact that we *could* get 2 cm vertical accuracy. However, they understand the limitations of the equipment and my own limitations of skill. I've said it a couple times, this is a test to see what we can achieve with the equipment we have. Ideally we would want to be within 2 cm of the true elevation. In reality that probably means tied to the local HARNs or whatever other local vertical control we can use. The 2 cm accuracy is necessary because the wells are in part meant to measure groundwater flow. Water level is measured from the top of the well casing, and an error of a few centimeters can make the difference in which direction the gradient is going. If we can't get 2 cm, we can't get 2 cm, but I'm taking it as a challenge. These wells won't be measured again until spring at least, so there is time to do it right and get the best results possible.
So that answers two questions.
The difference in elevation between any two wells is more important than the absolute elevation of either (as if an absolute elevation was even possible). That being the answer to both.
So that only leaves the question of 2 cm compared to what?
Or, What do we want to use for control or to hold as a basis of elevation?
I don't see where it really matters that much other than being able to repeat it 20 years from now. If any neighboring counties are doing the same thing then it would be great if your elevations matched theirs to increase the size of your study. It may not happen now but who knows 10 to 20 years down the road. If this information is ever released to neighboring counties, the basis of your elevations should be included with it.
James
Dynamic heights are preferred over geometric heights in oceanography and meteorology because energy is generally lost or gained when a parcel of fluid moves along a surface of equal geometric height but not when it moves along a surface of equal dynamic height.
Are you sure you are needing an orthometric height difference (NAVD88) ?
So relative height between wells is what is critically important, and absolute height, or height relative to any other features is secondary. That's an important piece of information.
How far apart are the furthest wells? If it isn't too many miles, the idea of a digital level and a textbook on geodesy sound like the way to go.
If it is over a whole county or several counties, then I see the appeal of GPS, but question if you will ever get the desired accuracy for what really matters - slope of the water table relative to something. It isn't just the accuracy of GPS (ellipsoidal heights), it is also the conversion to orthometric heights at the well heads, or possibly dynamic heights at the water table. The saving grace is that you probably don't have huge changes in elevation in your area.
The textbook is to understand what it is you are measuring. I suspect that proper interpretation of your results for underground aquifers may call for understanding the orthometric correction, Bouguer plate theory, and dynamic height.
I've been casually reading on this stuff, it is a lot to digest, and I'm not there yet. The meaning of height is far more complicated than most people (even most surveyors) will ever comprehend.
What would be the approximate minimum and maximum distances below the well top down to the water?
Spreadsheet shows need for dynamic heights?
I'm not sure I know what I'm doing, so would appreciate anyone's critique of the spreadsheet.
www.hartpierce.net/OrthoDynamicExample.xls
If the link doesn't work, then just paste this line in your browser
[inlinecode]www.hartpierce.net/OrthoDynamicExample.xls[/inlinecode]
It assumes water standing at a constant dynamic height of your choice in hypothetical wells next to each of several bench marks in the general neighborhood (20 or 30-mile spread) of the one discussed above, and attempts to calculate the orthometric height of the water.
If I'm right, there is a 2 or 3 cm variation in ortho height for the identical dynamic height. The strongest dependence is on the local gravity and the secondary dependence is on the height of the local terrain.
That says if the project is able to get into the 2 cm accuracy range targeted, they need to consider the effects of gravity in order to get the most accurate measure of tilt in the water table.
Comments, please. Am I lost or on the right track?
