Picture a mountain. At the bottom of the mountain is an existing section corner. At the top of the mountain is the next section corner north. It lies 2,500' higher in elevation than the section corner at the bottom of the hill.
Surveyor A breaks out his trusty GPS and ties in both corners. Finds that the 1/4 corner is "lost" and proportions in a new one set at the mid-point between the two SPC coordinates he has created. He prepares a record of survey showing all work in SPC and grid brg & distance. The measurements for the two half miles are shown to be equal on this grid depiction.
Surveyor B breaks out his trusty total station and surveys between the 2 section corners. He finds surveyor A's 1/4 corner, but notes that the ground horizontal distance between the lower section corner and surveyor A's 1/4 corner is considerably shorter than the ground horizontal distance between the higher section corner and Surveyor A's 1/4 corner, so he sets a new 1/4 corner at the midpoint of the horizontal ground distance between the 2 section corners.
In short, the grid to ground conversion creates a difference between the 2 half miles due to the difference in elevation between the 2 section corners and the corresponding projection distortions.
Question:
If you were surveyor A, would you adjust your proportioned location for the 1/4 corner and show a difference between the grid distances for the lower half mile and the upper grid half mile, so that your proportioned 1/4 corner falls at the mid-point of the horizontal ground measurement?
Thanks clearcut...this ought to be GOOOOD!
🙂
Loyal
If it was a Federal Authority survey, the BLM Manual would guide you to use distances reported as horizontal measurement at the mean ground elevation as historically GLO/BLM distances have been measured on the ground surface, either horizontally or on the slope with vertical angle reduction. Reference Section 2-2 of the BLM Manual, 2009. Also reference Sections 2-33 through 2-34 that addresses Grid System Coordinates and relationship to the PLSS datum, and also 2-40 which addresses computations in the PLSS datum.
Northern,
Yep, but unfortunately that's not how it is always done. Many grid out there showing proportioning based on grid distances and not ground.
Even know of the BLM doing such a thing. When they started using their CMM software, the proportioning was done often using an average elevation for the project. Thus, even though many of the surveys were still being done by total station traverse, the surveys were being adjusted to a SPC coordinate system where the proportioning took place at that mean elevation without taking account the often large elevational changes along some boundaries.
The ground reported distances all came out looking OK because the grid was projected to a mean elevation for the ground distances reported.
What is the distance between the two 1/4 corner monuments?
Keith
I think I would use the application of "Zen Surveying" as defined by Wiki as
Zen emphasizes experiential Wisdom in the attainment of enlightenment. As such, it de-emphasizes theoretical knowledge in favor of direct realization through meditation and dharma practice
i.e. it is where it is and even if it isn't it doesn't matter anyway and I just need to think about it somemore.
Wendell, Angel
Can you move this discussion to survey related.
thanks
short answer is "Yes."
That is assuming that the 1/4 is actually "lost" and just not fond where unexpected, or not found where it was expected to be...
> Picture a mountain. At the bottom of the mountain is an existing section corner. At the top of the mountain is the next section corner north. It lies 2,500' higher in elevation than the section corner at the bottom of the hill.
> In short, the grid to ground conversion creates a difference between the 2 half miles due to the difference in elevation between the 2 section corners and the corresponding projection distortions.
So, the Combined Grid Factor changes by about 120ppm over the mile as the line rises 2500 ft. in elevation?
If you use a mean value, as most surveyors would for convenience, the surface distance computed for the first half mile would be in error by about 30ppm if the line rises more or less uniformly.
2640 ft. x 0.000030 = 0.08 ft.
My 2 quests:
Are you surveyor A?
Is this on twp boundary or interior section line?
You need to be careful if you want to use State Plane in that situation. By projecting the coordinates to sea level and then calculating the results in State Plane you distort the results. The lower ground distance will actually end up shorter than the higher one.
But the difference shouldn't be much-.2' or so. If that bothers a surveyor I suppose he could play with the ground distances until they match. But then your state plane distances will not have be the same each direction on the plat. Imagine the plat might show 2640.4 one direction and 2640.6 the other.
Also conventional surveying can distort the results if the surveyor isn't careful. If a surveyor measures from the high section corner to the low one he should end up with a different horizontal distance (longer) that if he measures from the low one to the high one. So each system has issues with distortion.
I would accept the first monument anyway.
I am with Moe on this one....
Say, which planet is this? (Just checking!)
~ N
CMM can be operated to use elevations in all of it's proportioning programs. Variation of elevation is an issue which has to be dealt with even when using EDM and plane cogo and is not unique to CMM.
As a general guideline provided in CMM training, variation of elevation over a line of over 1000 feet should probably trigger an examinition of the effects. In extreme cases it can still be difficult to handle. I have seen examples over 4 or so miles of an E-W township line in UT where some corners could shift several links when the elevation was properly dealt with. On the other hand the consequences are often insignificant, but it is your burden to know how to use the tools we have.
CMM or any method or tool has to be used with proper consideration of the assumptions being made and can be misused like any tool. It is pretty easy to quantify the potential errors using methods like Kent has done or the actual values for the particular area.
I'd ignore both of them and look more for the original 1/4 corner.
But then, I just want an excuse to spend more time in the woods.
Is this a joke?
Picture it, Sicily, 1912.
I agree with eapls.
If the north and south section corners are found, the quarter post is out there somewhere.....
>
> Surveyor B breaks out his trusty total station and surveys between the 2 section corners. He finds surveyor A's 1/4 corner, but notes that the ground horizontal distance between the lower section corner and surveyor A's 1/4 corner is considerably shorter than the ground horizontal distance between the higher section corner and Surveyor A's 1/4 corner, so he sets a new 1/4 corner at the midpoint of the horizontal ground distance between the 2 section corners.
>
> In short, the grid to ground conversion creates a difference between the 2 half miles due to the difference in elevation between the 2 section corners and the corresponding projection distortions.
>
> Question:
>
> If you were surveyor A, would you adjust your proportioned location for the 1/4 corner and show a difference between the grid distances for the lower half mile and the upper grid half mile, so that your proportioned 1/4 corner falls at the mid-point of the horizontal ground measurement?
Your going a mile by the 'horizontal' distance by ground measurements, meaning, to me, that you would technically measure along the ground to get half-way. The distance that you would technically use is the measured distances by the traditional methods. You don't want a "geoid" halfway, because the landowner gets his land based on the elevation of the points or line. That is to say, that a mile in Colorado might measure 2 or 3 feet less than a mile at sea level.....but the landowner doesn't get extra land because he/she is higher.
By the same theory, the ground measurement would apply in a section breakdown. However, as many others have pointed out, I am not going to take 0.2' distance as enough to worry about any more than I am going to correct a monument set on a 'straight' line as opposed to a curved latitudinal line when going on an east-west division.
In short:
Surveyor A should have applied ground distances and adjusted his proportioned location, but if he didn't Surveyor B shouldn't pincushion it for that reason alone.
However, I don't even think the difference in distances are even 0.2 or 0.3. I think it might be around half that. The mean distances as you work/traverse your way up hill are slowly changing at every point. Or if you could see all the way to half-way between, you would shoot forward and backward and average out that difference. Same with the upper half. (or same with level-chaining in short segments.)
I hope that made sense. It is hard to explain my thinking in a few sentences.
> However, I don't even think the difference in distances are even 0.2 or 0.3. I think it might be around half that. The mean distances as you work/traverse your way up hill are slowly changing at every point.
The main effect is the change in the height scale factor. The change in that, to a good approximation, in a rise of 2500 ft. is going to be:
[(2500 + 20,905,000)/20,905,000] - 1 = 0.000120 = 120ppm
If you compute an average Combined Grid Factor for the line based upon the mean elevation, then that height scale factor will differ by about 60ppm from that at either end.
If the midpoint is 1250 ft. higher than the lower end, then the average height scale factor (averaging the scale factors at the endpoints) for each half mile will be 30ppm smaller or 30ppm larger than the mean for the mile, depending upon whether the average elevation of the half-miles is less or greater than the average for the mile.
I calculate a 0.63' difference in one mile by projecting to sea level from an elevation of 4000' compared to an elevation of 6500' (which would be typical for me). 5278.99' at 4000' compared to 5278.36 at 6500'. Or as Kent shows 120ppm.