My bad, still it will work well to rotate onto grid if you or someone else can get him a Lat, Long in NAD83 on the ends. A few check points through the traverse will help confirm.?ÿ
This can easily be accomplished by using quad sheet numbers or a program such as Google
This is probably swinging for the fences, but...
Google products (Earth/Map) don't use a reference ellipsoid, they are using a reference sphere(oid).
So I'm sure you're accounting for that disctinction in your evaluation of the calculations?
@michigan-left They also work off a web mercator. ?ÿAnd I do not believe anywhere they state mapping standards or accuracy. ?ÿAnd if you truly want to see take some time to understand that google is updated continuously for different frames so it is good one day bad another depending on just where one clicks. ?ÿI have seen it off a half a mile around a school and then be right on weeks later. Nice point.
Transverse Mercator or Lambert are not different in the use of scale, elevation, and combined factors.
In NGS definitions, which it seems to me should be the holy grail, what you call the "grid factor" is the combined factor, whether it's for a point or the average for a line or a project.
Again, what you call the "project scale factor" is the NGS project combined factor. Using NGS terminology, which, since they derived the whole SPC system, seems advisable to use, a "project 'scale' factor" would be the ratio of grid distance to ellipsoid distance.
When I was learning the math and applications of LDPs, that use of scale factor, a term that, when properly used, does not include elevation, was a real stumbling block. Many hours of back and forth between Manual 5 and magazine writings until it became obvious that the magazine writers were misusing a rigorous mathematical term.
It's all really so simple. To reduce a ground distance to an ellipsoid distance, multiply the ground distance by the elevation factor. To reduce an ellipsoid distance to a grid distance, multiply the ellipsoid distance by the scale factor. In going from ground to grid, we've done two multiplications. If need only the grid distance, we can multiply the elevation factor by the scale factor, creating the combined factor, and multiply that by the ground distance.
As a little rearranging shows, you are absolutely correct that the ratio of a grid distance to a ground distance is a factor, but it's the combined factor, not the scale factor.
Regardless, the perversion of the terms is ubiquitous, confusion and misunderstanding abound, equipment manufacturers are on board, so, carry on.
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I have seen it off a half a mile around a school and then be right on weeks later.
You gonna need to show an example of that?ÿ
The OP has an ??open traverse ?? he has also ran through some usc&gs monuments. Which may help solve the whole problem if he can get the information on them and they are lying in the existing traverse in a way to have confidence on adjusting to them. ?ÿNow He will still need to either use software or do the calculations to get on what he stated ??SPC?. The traverse ran ??.8 miles?. That??s 4224 feet. Depending on elevation differences on the site and direction north south east west. And if it is a Lambert or Mercator. Where the site lies from the central meridian etc. ?ÿthis will all go into how good is good enough. In GA on average a rule of thumb is .10 ft per 1000 ft between grid and ground. ?ÿThats a rough number to aid crews and managers when making decisions of how much it will affect the survey to disregard or perform the corrections. ?ÿOne thing besides grid to ground is will he also need to apply the convergence to stay on State Plane grid North etc. This all is a simple process and it truly is not difficult. It just takes a little time. The link I provided from Mr. G. Has addressed almost everything I have stated. It is also very close to the same distance that was ran here by the OP. ?ÿDirection is something i have no idea about. But If he gets the info from NGS on the usc&gs monuments. He might have all the starting scale elev csf to do the calculations in between and extend from within his traverse. Or if he has a software package that allows him he could do it by fixing those monuments and allowing the software to generate the rest of the coordinates and corrections for him. Maybe he still does a opus or rtk observation on the extents. As a sanity check.?ÿ
@larry-scott I wish I could. This error was many years ago on a school in close proximity to a place that also was not correct for lat and longs. I believe congress actually addressed this at some point for National Security Reasons. ?ÿBut no where I have found recently as of a hour long search that google states it??s accuracy either. Now we can use it but unlike any other Topo quad or other maps that we can derive an accuracy for it. ?ÿAnd I probably should have worded the statement better. The imagery falls correctly just the lat and longs did not. ?ÿIf you measured a distance on say google maps across a parking lot it was good relative.
The process I go through is fairly simple.?ÿ
For instance I have an isolated section to survey not near any established control.?ÿ
I want it to be on ground distances but I want to use State Plane bearings.?ÿ
So I set up a State Coordinate file in Trimble. The NW corner is high and about 5200' in elevation, the SE section corner is the low point at about 5000'. I hover over the NW corner in Google getting a Lat, Long, type that into Trimble and use 5155' for the height which is a rough Geoid height subtracted from the elevation, then repeat for the SE corner. A simple inverse in Trimble will show the ground, grid, ellipsoid distances between the two points.?ÿ
Divide the ground by the grid and you get say 1.000265 and that's the project scale factor. It can be applied in trimble under the local site settings. For other programs such as Autocad the compliment of 1.000265 is needed to make it work or .999735070206.?ÿ
The extended value is necessary when the compliment is calculated.?ÿ
Of course larger areas with complicated topography will need more detail and more points to be entered to get the best number. I like to keep small jobs under 5ppm. Some jobs will creep over 10ppm near the high or low points.?ÿ
DOT will restrict a project scale factor to 10 miles east west they will allow you to extend out to 20 miles for a flat north south project in a Transverse Mercator projection.?ÿ
I followed a pipeline project in Montana heading north-south and the pipeline surveyor changed project factors as he crossed township lines, I thought that was an elegant solution to the large distortions in the Montana Coordinate system.?ÿ
I've not calculated a combined factor for any point in many years, it used to be the way to get a project scale factor but I'd rather compare actual ground vs grid for the site.?ÿ
I always express a project factor as larger than 1 and a combined scale factor as less than one.?ÿ
There is little reason to extend the project scale factor beyond the sixth decimal place since that represents 1ppm. The old state plane books didn't publish grid scale factors beyond the sixth place so the calculations started there.?ÿ
The geoid height difference is about 45' in this area, that's 2ppm and not all that relevant, changing the PSF from 1.000265 to 1.000267 will not effect much of anything it's .01' per mile.?ÿ
I have seen it off a half a mile around a school and then be right on weeks later.
You gonna need to show an example of that?ÿ
I've seen the same thing, it was south of Baker Montana along a county road, the shift was about 1/4 mile, now it's not there, I tried to go back into older photos to reproduce it and couldn't find it again, but it was a chuckle as the engineer I was working with noticed it too, made it difficult to find the site I was going to using Google. I also have pinned an area with a small shift that really shows up where it crosses a railroad.
Honest question here: Where did you learn these techniques and procedures you are using?
@mightymoe True. On the shifts as well. There are several academic papers written on point accuracy vs the point chosen in google. Like gps derived vs google derived. ?ÿNow many other factors go into this. One the datum google uses itself. The source of the images and its resolution vs accuracy. Blah blah blah. Google uses many sources from many reputable agencies for a lot of the data. And having personal experience in using many data sources and it??s intended use for google or other maps etc like them. I would not use it for survey quality data. I do use it to aid in explaining or getting close but I understand how all that data is mixed together and why it is that I would not use it as a surveying standard. To many more sources that google uses I could use independent of google for survey quality work.?ÿ
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@mightymoe I am not following this maybe I am missing something here. But you are using google lat long and keying into Trimble to inverse. Those lat longs may or may not be on NAD83 which is what State Plane coordinates are usually on. I understand that grs80 ellipsoid and wgs84 ellipsoid in size and shape are so close it doesn??t matter. However the origin is at best 2m x2m roughly. Hence why itrf and wgs84 datums are becoming closer together. And why new datums are being created as well. I believe google uses wgs84 datum in a way. A web mercator projection. I use Trimble as well and understand how the local site scale factor??s (not the scale factor) are used. Now I understand that on a site we can derive a factor so that the whole project uses one factor. This is essentially how TBC uses the ground scale factor from one point. Now that does work a lot of the time but it is not perfect and in certain cases has bitten folks in the rear. When I am asked to scale spc to ground my first reaction is to cringe the geodetic surveyor in me does not like scaling coordinates. And leaving them looking like spc values. But in this OP situation I believe if he simply takes the ground distance and applies the calculations as per ngs pub or the article i posted he will have no worries.?ÿ
now you state ground distance but grid bearings in the above and move right into the distance scale process you utilizing. ?ÿWhat about convergence. I am trying to understand where and how all of this flows. I am sure it is my pea brain and just missing the obvious. Math teacher had the verbiage down in a recent post that is spot on. I am not one to be well with communicating in writing well But your terminology has me a bit confused. ?ÿ?ÿ
