OK guys. There has been much discussion about the issue of grid to ground and vice-versa. I have heard and read many different options to calc the position of a traverse from holding one point and applying the scale factor to the vector (or line shot by a gun or GPS RTK. no static work) to a point and others say you calc the scale factor at the point(s) in question and apply the scale factor for each and every point (assuming you are on state plane (florida east SPC). So....my question is, do you apply the scale factor to the coords to each point or do you hold a point and use the scale factor to calc the azimuth and distance? Or a combo of both....it's been a very long week and we have 2 &1/2 miles of ROW with a good amount of control that encompasses the site. please let me know your thoughts....
Hold One Point
and scale all coordinates horizontal (not vertical) from it.
Most programs give GPS vectors at the surface so the vectors do not get scaled.
Paul in PA
Whichever you do, you are making a project coordinate system, so 1) subtract off some millions and hundred thousands from the scaled coordinates so people don't confuse them with real SPC, which they are not, and 2) record exactly what your conversion process is so somebody else can understand it.
First of all, when converting a grid system to ground you use a project wide combined factor (Scale factor x Sealevel Factor). Choose a combined factor (CF) that best represent the area you are interested in modifying. This could be based on single point or an mean. Next, scale all points in 2D by the CF about 0,0. You can actually use any point but 0,0 makes the meta data a little simpler and if you choose to skip the next step you will cause even more problems. Next, as Kent points out, TRUNCATE the coordinates so no one thinks they are grid because they are not. I repeat, TRUNCATE. And remember that this is just an approximation.
Scaling each vector by its unique CF is how you compute ground distances between grid coordinates. In this case the coordinates are remain on the grid. This is also the reverse of how you convert ground distances derived for conventional measurements to grid values.
Just remember, friends don't let friends not truncate.
Opps, It was Bill's post with the 0,0 comment.
I did a quick scale factor for a point near Orlando and got a sf of 1.00004 or .4' in 10,000'.
The first question is this even something to deal with?
If it is; the DOT way to handle it is to multiply the coordinates by the scale factor. When you do that they don't allow you to truncate cause you lose the connection.
If you do use a scale factor only carry it out to 6 places say 1.000042, then you have a scale factor "accurate" to .01' in 10,000 and anything else is just noise, your elevation changes will swamp that.
Keep really good metadata on a spreadsheet and give it out to everyone whatever you do, if you use trimble you can multiply the coordinates, and subtract or add a number to the northing and easting, if you do that to state plane numbers you need to really, really, really track everything you do.
The important thing is that at the end of the day, each point, no matter what the x,y for the point is, has the actual NAD83 lat, long number attached to it.
>If it is; the DOT way to handle it is to multiply the coordinates by the scale factor. When you do that they don't allow you to truncate cause you lose the connection.
I don't understand this. The metadata supplies the connection between project coordinates and SPC grid coordinates.
They want to be able to divide the coordinates to get state plane, a simple batch file conversion, also if you import info into a drawing at state plane it's simple to just rescale say a quad sheet or photo to project coordinates.
It actually makes sense, and I've never had problems or heard of someone else having problems confusing working coordinates as state plane coordinates. There are statements all over the plan sets and spreadsheets for each project.
> I don't understand this. The metadata supplies the connection between project coordinates and SPC grid coordinates.
What MightyMoe describes was also DOT practice in Oregon before they came up with their Low Distortion Projections, and is still practiced on legacy projects. And for the same reason. I agree that it's not a very good reason. So it goes.
>a simple batch file conversion
The batch conversion in either direction could include the add/subtract as well as the multiply/divide operation. Making the coordinates look different puts everyone on notice to look for the conversion metadata.
I suppose if there are enough labels or notes and everyone on the project is aware of the method, it works, but we've read stories of people getting confused.
I've have taken over numerous projects where this was in fact a major problem. If the meta date is not included with the project, which it rarely is when passed to another surveyor, then you have no idea that it is not a true state plane coordinate. This is magnified when the previous surveyor scales about some random point.
If you don't want a grid coordinate, then don't make what you want look like one.
It's how they want it, I'm so used to it, it's just second nature, and have not ever had a problem with it.
It is funny you bring up a subtraction, back in the day everyone would lop off the million on a state plane coordinate, not to make it look different but because when you typed and hand wrote down coordinates it was just easier to leave off the 1 instead of typing it each time on the list, but we all knew it was 1 million something.
Never had that issue, all the highway plan sets show the scale factor, state that they are working coordinates and further explain how to get state plane numbers from them. They are printed on each sheet. This is going all the way back to the 70's.
The one that really caused a problem was a project that was done using international feet-that one took some figuring;-)
Here's a quick analysis that shows why distances computed from ground coordinates, calculated by dividing state plane coordinates by an average project combined factor, do not duplicate ground distances computed properly from state plane coordinates.
Suppose that you have two points, 
with combined factors f1 and f1, respectively. The following formula would calculate the ground distance between these points using state plane coordinates:
If the original two points are modified by dividing them by the average of n combined factors, the formula for the distance between the original two points becomes:
Note that the thing that changes is the denominator in the distance calculation. How significant the difference is depends upon the difference between the project combined factor and the average of the combined factors for all possible pairs of points in the project. And that will depend on the extent and shape of the project for the scale factor and the elevation changes for the elevation factor.
Back in the day, we used to run state plane control, calculating each state plane distance between control points, using the booklet to interpolate the grid factor and mean the two elevations for the elevation factor. Combine the two, apply to the measured distance and calc the next coordinate. This was time consuming rather than going through all that, combined scale factors were used, or project scale factors.
Get as close as possible to one factor for your project area and use it, in the old days you would ignore the distance adjustment using surface distances and adjust state plane control monuments using the project factor.
Now the computer calculates an adjustment using the given combined factor and creates an adjusted coordinate for each point, basically putting them on a new higher plane than sea level. So no the calcs are not exact. But they are coordinates that can be worked with in the field, road and bridge construction, that kind of thing without adjusting each measured distance.
So project factors are intended to "be close" to each distance factor across the project but not perfect.
"friends don't let friends not truncate."
Amen Brother !
:good:
Who in their right mind would want to work with coordinates that are 2-3' or so from SPC?
It's not always a question of "want" sometimes you have to based on project specs. Granted these may be written by someone who doesn't even understand this stuff. They'll want it on "state plane" but if cadastral ground distance don't match the coordinate inverse, which they won't, they will complain. I've been in that situation and have had to provide one location in state plane and then scaled out from that point to the other coordinates indicated by the scale factor at that first location. I tried to indicate that a scale factor was applied making the coordinate system no longer state plane, so at least the next surveyor could figure out what was going on.
Either way you decide to modify the coordinates, it's no longer state plane and you should state so on the plan.
Well then "they" are dummies. 🙂
Seriously, though, I have never ran into that problem with a client. All of my clients that specify that the project is in state plane actually provide me with Lat/Lon of important coordinates, and a CAD file of the linework, points, etc.
One time I did have to explain to a contractor that carries a total station why the distances didn't match when I was on a mile-long tangent section and he said "well, that makes sense".
> "friends don't let friends not truncate."
>
> Amen Brother !
I've worked on close to 100 DOT projects in WA and ID and have never once seen or used truncated values. Typically they are scaled to ground using an average elevation for the project, or a point close to the average elevation. Have never experienced any issues at all with people confusing grid for ground or vice versa.
If you're laying out a 5000' bridge, ground distances would be your best option. Otherwise, your bridge may be .5 short or long.