That's pretty much it.
To figure out a DAF I never even look at scale factors. My only interest is to match Ground and Grid distances as closely as possible.
With modern software all that's needed is to inverse and compare the two since the software will display them. If you want to find a DAF for a site then figure out the limits of the project, plot it on a quad sheet, a county GIS, Google, any data source that will match lat, long with elevation.
Then put some random points that cover the site into the software and inverse, you will soon find out the DAF. It's a simple ground/grid calculation and the mean or close to the mean normally jumps out at 1ppm.
That's the DAF.
Enter it into the software and it will do the rest.
Print out a data sheet, put it into the file and your metadata is covered, it gets scanned into the job when it's finished.
Every time you survey, you're creating a surface. When a TS is set over a point and is used to locate points the surface is the Total Station height. As you move to different points, a new surface is created. This doesn't crop up as a calculation concern unless you're doing huge traverses. But it's always there. To close out a subdivision survey with a dozen set-ups you ignore the dozen surfaces and calculate essentially with a DAF of 1.
No one thinks about that unless you're an old surveyor that did big jobs with conventional traversing, then you needed to consider it.
You're describing process, not defining DAF. That's like saying that to create carbon dioxide, mix vinegar and baking soda. The process works but why it works is not mentioned.
However, you did define DAF when you said that ir's a grid/ground conversion factor. That makes it a form of a combined factor.
The reason that your process works is that state plane coordinates are determined by the rigorous mathematical relationship between an ellipsoid and a mapping plane and our ability to measure elevations. Those elements give us a combined factor for each point.
Those combined factors can be averaged to produce a site-wide combined factor.
Your DAF is the reciprocal of that site-wide combined factor. My guess is that the reciprocal is used because multiplication was much easier to perform in pre electronic computer days.
Suppose that you have state plane coordinates for two points. Suppose that you also have the ground distance between those points.
If you divide the grid distance between them by the ground distance, you get the average combined factor for the line that joins them. If you divide the ground distance by the grid distance, you get the average DAF.
But, you say, your process for DAF never uses distances. No matter. Just average the state plane combined factors and, if your ground measurement is as accurate as the state plane coordinates, you'll get the same answer as before. The methods are mathematical equivalents. Take the reciprocal of that number and you'll get the same DAF as before.
But, you say, you use more than two points. Fine. Just average as many combined factors as you want. It's the underlying math that controls, not the process nor the arithmetic.
The DAF is not mysticism applied to surveying. Instead, it's a number that converts grid to ground by multiplication rather than division. And it relies on the mathematics of Gauss, Lambert, Mercator and others, along with the insights of Oscar Adams in order to work.
All true, but the real world smacks you in the face, surveying in the Madison isn't useful when you're building or defining legal areas in the real world. Efficiently making the world work needs to be done.
Which is why this process is used.
I couldn't agree more, but there are at least a dozen equivalent processes that work, too. And the thing that makes them work, and also makes them equivalent, is that they all build on the mathematical principles that define the combined factor.
Knowing why a process works and what it is equivalent to is invaluable when things don't go according to plan. Otherwise, all we have is the excuse that we've always done it that way.
We have always done it that way is a saying that is very much heard in the surveying community. If I had a nickel for every time I heard someone say that I would be wealthier than Jeff Bezos or Bill Gates. lol. You are correct though having understanding is invaluable for sure.
Any attempts to use micrometers to reconcile work done first by axes and later by cross-cut saws will be mindwarpingly futile.
It is more vital to conquer the sloppiness that exists today. A section corner and the section's east quarter corner were monumented as being 2579.86 feet from each other many years ago. Section corner references reports were filed with the State and the County for those corners. Surveys were made for small tracts on either side of the section line they formed. Those surveys were on file with the County. A highway improvement project comes along straddling that line in 2020. The new monuments, in monument boxes, are now 2577 61 feet apart. It makes one wonder what happened every half mile over a 15 mile project.
Could the old monuments not be found. Why did they set new monuments.
A section many years ago we were retracing for ROW survey had a nice big disc all stamped and the RM’s all done easy to find nice well drawn monuments record. We we eating lunch a few of us had gathered and had completed a weeks worth of static surveys. As always in my younger days I was curious and always practicing setting up the instrument to become better and faster than the next guy during lunch. I bucked in on a fence line to see how it lined up with the monument. It did so nicely. As I walked around in awe of the beautiful landscape in Colorado being from Mississippi I noticed a nice hedge row of those tumble weds in the distance. I wiggled in on it and spun the bearing around to the monument and I was missing it so kept trying I walked down a hill and found a small patch of cotton wood trees and remembered some of the old notes and started peaking around. That grove was about 50’ ft off the line of the new fence line but the pristine was found that day and the reference stone beneath that. A little wiggling in and looking at how the old fence line caught all those tumble weeds like a net proved to have been built on that line. A nice subdivision was tied to the new mark not the original. All luck I found it but it was neat and the distance matched they had just drifted over the course. I use to love reading the GLO notes and such.
Learning the underlying math is relatively easy when one has a lot of free time, but applying this stuff in the field under time and terrain pressures is a whole different matter. Using abstract mathematical inventions like ellipsoids and the planes that approximate them to produce usable and valuable surveys is the enviable skill.
When I was in school in an engineering class road and bridges. The professor made a statement to us students. He was trying his best to attract more of us to the civil engineering side of the house.
Yall can do this in here out of the weather as engineers or you can become surveyors and do it on the hood of a truck in all weather and fighting mosquitoes ticks snakes and who knows what else and make less money. Your choice.
Well I joined the United States Marines because it was a challenge and I followed the surveying route. I reckon I will never learn LOL. I guess he motivated me in the other direction. Or just stubborn pride to do something that most would not even try.
There are many ways to "get on ground". When we first got GPS we calibrated to terrestrial control. That didn't last long as it tends to degrade the accuracy of the GPS putting it down to the level of the total station.
Then we started using LDP's for areas, which I prefer to do, since I can get near to ground, and my grid bearings are closer to "true".
A township survey with the CM of a TM projection near the center of the township will rotate about 2 minutes along the east west edges. For most townships that's well within the original survey's accuracy.
But the advantage with the DOT method is that you can easily import-export data into your programs. All you need to do is have the one scale factor and much of online data will sample in without any manipulation.
"But the advantage with the DOT method is that you can easily import-export data into your programs. All you need to do is have the one scale factor and much of online data will sample in without any manipulation."
Absolutely true and that is sufficient justification for using ground coordinates, given that distance calculations produce the same result regardless.
Here is a link to NGS Publication 5 where, on page 50, more insight to the site-wide combined factor is given: ManualNOSNGS5.pdf (noaa.gov) It will go to your search engine rather than to the site where navigation is pretty straight forward.
It's also helpful to read beginning on page 46. The discussion is centered on converting ground observations to state plane, but it's just the other side of converting state plane to ground.
Of course, reading the whole manual with pencil and paper in hand can be a very productive use of time.
Which is is why I will never get licensed! 😚
