Been there, not fun, you either have two slightly different DAF numbers or US Survey feet vs. International feet.
Sounds like the mapping is finished, the design work completed, right of ways purchased so you can't go backwards and start over. Imagine the meeting, you sit at the table and tell them that everything has to be redone............
Not gonna happen.....
Yes, there is a difference, a minor one, but important.
When you measure a distance in the field the process to reduce it to a state coordinate distance is to combine the sea level factor and the grid factor, then multiply the distance by the combined factor and get the state coordinate distance. This number is usually less than 1 causing the plane distance to be shorter.
However, now you have a set of control points in state coordinates and you want to expand them to a "working plane" or "surface coordinates". Inversing working plane coordinates of course doesn't really give you the actual ground distance, but the DAF should be calculated so the inversed distances are close to the real ground distance, how close depends on the geometry of the site the factor is applied to.
This factor (sometimes called a Datum Adjustment Factor) is expressed as a number larger than 1 (at least that's how I've always seen it) partly to differentiate it from a combined scale factor. The coordinates are multiplied by the DAF to get working plane coordinates, this in effect scales them around 0,0 using a number picked for the site.
I usually inverse a number of points across the site and compare ground distances to plane distances and develop a DAF that way (1.000XXX), I don't even look at the elevation or grid factor, because I'm not really doing that anymore. What I want is to have the surface distance be as close to a real ground distance for any two points across the site as I can get it.
I still say use my method and you could even totally ignore the scale factor. Just move the stuff onto state plane. 0.9999979631 at 1 1/2 miles means you're talking about 7,920 ft vs 7,919.983 ft. Why bother messing with the scale factor? Just get the stuff into state plane.
That's the way I remember it. Multiplying by the DAF is the same as dividing by the CF. When converting, you have to use the operation that matches the direction of the conversion for the given factor. Two places to get it wrong.
0.9999979631, that is a number developed by an engineer, lol.
If you think it through there is no reason to express it to ten places for what they are trying to do, each 20' of elevation changes the sixth place anyway.
Eddy can't change it to state coordinates now, he's stuck with what has been done, there is no going back, changing the coordinates would be impossible, he has to stake everything to the existing coordinate system, he just has to figure out what it is.
But I have to agree, why in the heck wouldn't they use state coordinates with a number like that for a factor?
40' of elevation and you are at 1.
"But I have to agree, why in the heck wouldn't they use state coordinates with a number like that for a factor?"
How about "Because they don't have the faintest idea of what they are doing, and no idea at all why they are trying to do it"?
I have seen it done both ways by holding a central point and having grid and ground coordinates for that one point to be the same, but that caused the coordinates on the edge of the project to be about 4-5 feet different and still looked like grid.
In our area our combined grid factor is around 0.99985 and what I have found in doing large subdivision projects was to divied all the grid control points by the combined grid factor and have ground coordinates for mapping and layout. This changes the grid control coordinates by about 300 feet in our area, that is big enough to see, but to get to grid on any point in our project all we had to do was multiply the ground coordinate for said point by the same combined grid factor that was used to create it. Anybody that used our data was given the meta data so that what appears to be grid coordinates was clearly stated how to obtain grid coordinates. In the old days when we had to hand enter coordinates we also would truncate the ground coordinates by a certain amount to make it easier to hand enter and to also remove the grid coordinate look.
This is old school from 20 to 30 years ago, have not seen any large subdivisions lately. We had several that lasted for 20 years. I know that there is modern software that can handle this now but holding a central point means that to get grid on a point you have to inverse from the held point and apply the combined grid factor to the inversed distance to get grid coordinates and on long projects it would be easy to loose tract of the held point.
I think you are correct;-)
If you have a CAD file of the design you could move the design to state plane. If that freaks you out because others may be using it, then you'd have to do a site calibration (localization) with your GPS to the local system. Sounds like the coordinates are crap anyway though.....glad I'm not in this particular pickle. (I'm in about 6 others at the moment anyway)
Until we see some standards Metadata needs to carry the extra digits. Half of our software uses CAF and the other half uses 1/CAF. A few trips through the machine and the points and line work are disconnected.
On the ground there is no difference at the 10th decimal (or 7th for that matter). In the digital world 0.00001 feet can dog you trying to make ACAD routines work...
well, if anyone can Make It Work, it's you, Randy! good luck!
Exactly. Design is what it is and probably fits whats on the ground. Just need to find or create some control that agrees with it for layout. Hopefully any thing done up to this point is not too out of whack.
I was involved in a project with LOTT (Lacy-Olympia-Tumwater-Thurston County) It started with 3 miles of control and topo for a sewer force main/water line project. We were just about to start the construction phase of this project when the 2001 Nisqually earthquake hit.
Someone smarter than me decided it wasn't going to be "cost effective" to run new control and tighten things up; so we were forced to stake from existing control.....
They laid 2 waterlines and the sewer force main in one trench; 4' deep and 15' wide, so it wasn't a real big deal. But it was a pain in the ..... the best back sight check was about a half foot.
Everyone new what was going on; but it was still tough getting things to fit...
Dougie
As he said a central point was picked and that was used for the DAF, but the DAF is fixed across the site, you chose it and assign it.
So it doesn't mean anything to carry it out past the 6 place I suppose you could express it as 1.0002350000 but why?
No doubt an engineer picked the number without understanding that beyond a certain value it becomes pointless.
Each point across the sight and each line has a different scale factor the trick is to apply a DAF that means all those numbers as close as possible.
And there is a good reason to not express these numbers the way they do, as you said little changes at the end of the number can cause issues, for most DAFs they can be thought of as two or three numbers, the one above is called 235 for just about everyone who uses it. easy to remember, another one in the area is 250 or 1.00025 much easier to remember than 1.0002352167 or some number like that.
In the design stage for about 25 years Well...That gonna be real fun. You are going to have to field-fit everything.
I agree. Whatever coordinate mishap occurred sounds irrelevant at this point unless you are doing boundary surveying and private property rights have been impacted by a blunder. Otherwise you're at the mercy of what's on the ground now.
I'm appreciating the use of ppm for scale factors more and more for that reason. One, it's more abbreviated. Two, it keeps the precision in mind.
This is a good example ...
exactly, multiplying coordinates by a scale factor may be expedient but in the long run, it causes more problems than it is worth. You scale distances, never coordinates.
> Your original post still leaves a lingering question. The CAF you posted is less than 1 and you stated you divide ground by the CAF to get Grid. Later I see you are in a 'ground is smaller than grid' location. Are you using 1/CAF? That would make sense when its all put together, otherwise I'm not following you...
If project coordinate is 1 and you divide 1 by .9, you get 1.11 SPC.
It's not the math that has me confused. The term CAF or CSF as I've seen it used works the opposite of what you are saying. When Magnet came out they used the term scale factor to describe 1/CAF. Until then I'd never seen that done...
