Scale Factor
We have a number of scale factor areas to get up to surface. One is a higher elevation area that has a number of DOT projects to merge into, so we used one of the projects to set up a control network. The scale factor is 1.0003077650 per DOT.
So I'm finishing a BLA and in the basis statement I say that Bearings are based on the State Control System. NAD 1983, East Central Zone. Distances are multiplied to surface by the Project Adjustment Factor of 1.000308. During our review I got a redline saying the scale factor is 1.0003077650, not 1.000308.
I said think about it, it'll be fine. The only coordinates given on the ROS is a Lat, Long pair at a Section Corner.
Scale factor or combined factor or project factor. Also what is your false northing and easting. I usually carry out to 8 or 9 places. On combined factor for coordinates .
As an experiment; take a job you've done with a limit of 1/4 of a section that uses one of your 9 place project factors applied to state plane distances to get ground distances between points. Put a state plane coordinate on the NE corner of the area. Now take the state plane coordinate on the NE corner and recalculate through the job but only using the scale factor applied to the distances rounded to 6 places. Tell me the differences to .01' of the resulting coordinate values. For this project it would be necessary to (off the top of my head) extend out over 7 miles before coordinate differences of .01' would begin to show up.
DOT is funny. Back in the good old days, 1960s-1970s the old timers would run state plane coordinates all over the ROWs and they developed regional project factors. The one locally was called Pryor and it was 1.0003. Then they got nerdy and decided to get "real" project factors and started to restrict the area of each factor and publish them to 9 places. How they come up with one of those is ,,,,,,,,,,,,amusing.
To us old guys that calculated mountains of state plane coordinates by hand I believe we had way better handle on how the math actually works (the grid factors in the state plane books were only published to 6 places). 1.0003 was too sloppy, but 9 places is 1ppb and is unnecessary for any work. 6 places is 1ppm and is impossible to measure using standard field techniques. But 6 places is the "sweet spot" that can make you feel warm and fuzzy. So we usually use 6 place project factors.
A project factor is not a combined factor, it's used to convert (as a reverse) grid distances to surface distances moved up above the state plane surface to simulate ground distances. If I inverse across my site I get a surface grid distance of 3713.06' and a ground distance of 3713.04'. State plane distance would be 3711.92' using 1.000308 or using 1.0003077650 it would be 3711.92'.
Ain't reality fun?
More people need to get a grip on reality. Saw a subdivision earlier this week that had lot lines like 209.7682 feet.
That's a good one.
Our published subdivision labels of 1 second and .01' for everything is a fantasy already; there's no need to make up imaginary numbers.
Saw a subdivision earlier this week that had lot lines like 209.7682 feet.
That's probably just a sloppy precision setting in cad and they're not actually claiming to measure that; I think the default setting is 4 decimal places.
When you publish your work i.e. a subdivision plat, your are claiming that information shown to be true. If your are not going to claim 267.2589 then don't publish to the 4th decimal place.
The great Paul Reid advised me many years ago that the words true, correct, accurate should never make it into a surveyor's statement.
When you publish your work i.e. a subdivision plat, your are claiming that information shown to be true. If your are not going to claim 267.2589 then don’t publish to the 4th decimal place.
I hope not. If that is the case then every surveyor I have ever known is a liar. We report to a certain precision by convention, not by significant digits. Back in the day, perhaps some did use significant digits, but it is no longer allowed.
I remember high school math, and I can assure you no one uses significant digits on their surveys, at least not on plats that are reviewed by planners. Try putting distances to the nearest foot and 10 feet on a plat, and see what happens.
When look on a survey and you see 89.98 feet for measurement, that it is not making an an accuracy statement.
Should it? That is a discussion we can have, but I can promise you that it does not mean it at this point in our history.
Our published subdivision labels of 1 second and .01′ for everything is a fantasy already; there’s no need to make up imaginary numbers.
Had a bearing on a line to like 3 decimal places of a second on a survey I was looking at. It made no sense, until I realized it was far enough away that the relative accuracy of my GNSS observations would notice anything less. No other measurement on the map had that precision, and when I got through, I realized that by doing so when I searched for the points I hit them with the expected distances. It was the one time I did not think it was useless.
