We get a request for a survey. In that request are the parameters for control. This is in a local community where a Project factor was developed some time ago. That factor is 1.000235. The request for comes with a spreadsheet and control. The Project Factor is 1.000234984. They put together a calculation to come up with that number, it will make .016' difference in a coordinate of a million feet. At this location that will make it 0.03' smaller in the northing and 0.02' smaller in the eastings.
However, the coordinates are State Plane NAD83 International Feet instead of US Survey feet which is what the state agreed to use back when NAD83 was developed and community data including high resolution mapping is still on. So that shift wipes out the .03'. But, using the community scale factor developed during a water project 35 years ago would have made more sense, to me anyway.
Did anyone consider the significant figures when overthinking this? Just wondring....
Did anyone consider the significant figures when overthinking this? Just wondring....
It's an interesting question. To me holding the established factor would be the default position. I have quite a data base for this area so in theory that would be great for me. Practically, it's not that simple.
Much of my GPS data is older, 83(93) the new realization of 83(2011) is a shift. Usually it's between 0.10' and 0.20'. Minor but measurable. Now that they have added International Feet the shift grows to about 5' and it's actually easier to deal with.
It's amusing that they multiplied by x divided by y2 shoved the results into a mixer of Least Squares, called 3I/Atlas and came up with almost but not quite the community factor. At that point is more alchemy than rational math.
The sixth place is 1PPM or .01' in 10,000'. Also the sixth place represents 21' of elevation change so this project that has about 80' of elevation means that the scale factor shifts 4 numbers at the sixth place. Pointless to run the number out to 9 places. At 9 places the scale shifts with .02' of elevation change.
It seems to me that this is a case of lack of local knowledge. I’ll wager that the origin of this directive came from outside of your state.
It seems to me that this is a case of lack of local knowledge. I’ll wager that the origin of this directive came from outside of your state.
That's where it gets very curious. I finished two rather large projects for them, one running north of the city and one running south. Both were on the community factor, which I thought was pushing it too far. Now one shows up with this old style factor on the control sheet. My guess is that a couple of old hands that retired in late summer turned this over to new workers and....... here we are. Not a big deal, I'm more worried about what might show up next.
What they did was verify that the legacy scale factor is an extremely good one. The ratio x/y^2 is a sort of plane version of a/b^2, which is the curvature of an ellipse at the ends of its major axis. It's used as a weight in least squares derivation of a project scale factor.
Slick's scale factor rounded is identical to the legacy factor. Thus it verifies that factor using all available data, if they did it right. Their extra digits add nothing, and after the legacy value was verified, it should have been applied.
Slick had to go through some work to use International Feet.
All in all, it looks like teleprompter work, or for us older ones, use of a go-by sheet.
Bless you, Moe, for dealing with garbage like this.
You've got a handle on it mathteacher. I've been cautiously excited to have the new 2022 NGS coordinate systems get unveiled. Or is it 2026, or 2030, or 2035. Anyway, the new zones in this state are intended to bring up the surfaces so that 95% of the population will be covered with zones no more than 75PPM of error. Plus, the idea is to offer LDPs, still not sure how that will be structured.
The big problem with roll your own LPDs is that they aren't in main data-bases, such as Autocad, GIS, Trimble, ect.
It's difficult for some to use them if they can't click on them.
DOT came up with the idea to add a scale factor and multiply it to the control coordinates for a project. They limited a scale factor to 10 miles east-west, it can be extended north-south if elevation doesn't change much. In effect there are a bunch of LDPs. There are many scale factors the office and surveyors know by continued use. We usually call them out once the integer begins with the number, 235, 238, 304, 307, just a sample of nearby factors. Then there are the extended ones. I only recall one of them, 246821. That one covers a nearby almost mountain town and quickly falls apart as the road passing through climbs 3500'.
In the beginning DOT had regional scale factors the one covering this area was 300, or 1.0003. 300PPM, it worked OK but not good enough for the button pushers at headquarters, so as computer programs became more sophisticated, the scale factors multiplied and grew in number.
I wrote a letter to headquarters not to shift to international feet until the 2022 official systems are activated. Turns out my opinion doesn't carry much weight. LOL.
Then I called and asked if the new systems will end the scale factors. I was told no, they will still shift to surface. By the time 2022 gets into use I should be completely retired.
Did the change to International Feet require legislative approval or was it just done on a whim?
Did the change to International Feet require legislative approval or was it just done on a whim?
It was based on the idea that the 2022 will be IF, so they decided to shift newer NAD83 SP projects to IF from USF (don't blame me). There is no legislation establishing which feet to use in this state so NGS and maybe the board got together and picked USF for the NAD83 coordinates. All coordinates shown by NGS in the state are expressed as USF. It's worked out well cause drift from older coordinates are showing up. The IF ones move about 5' and are easier then the tiny shifts to deal with.
Then I called and asked if the new systems will end the scale factors. I was told no, they will still shift to surface. By the time 2022 gets into use I should be completely retired.
Oregon has 39 zones and scaling is unnecessary. Many states of similar size are getting only ±4 zones with '2022, which would still want scaling. It seems like a case of half measures availing us nothing.
Did the change to International Feet require legislative approval or was it just done on a whim?
I do find the term "International Feet' somewhat odd
After all, its only the US that uses Feet at all.
This does look to have the potential to be another long-term SNAFU in the making
After all, its only the US that uses Feet at all.
The alternative is US Survey Feet, so the "International" part is mainly for US consumption. When you buy lumber in the NZ big box home center how is it denominated?
The alternative is US Survey Feet, so the "International" part is mainly for US consumption. When you buy lumber in the NZ big box home center how is it denominated?
meters and millimeters - I'll have 3 x 3.6m sticks of 100 by 50, please
@jimcox I can't even figure out what the heck you bought, except I know it is some type of lumber. 🙂
I'll have 3 x 3.6m sticks of 100 by 50, please
Is that he actual size you get or is it closer to 38mm x 90mm x 3.65m?
I do find the term "International Feet' somewhat odd after all, its only the US that uses Feet at all.This does look to have the potential to be another long-term SNAFU in the making
Some long time ago the inch was defined as 2.54 cm for international industrial purposes. That's the connection for "international." That was 2 ppm different from a previous definition that had been used for feet.
The only place the 2 ppm difference is of practical significance is for huge distances like big coordinate systems.
It was a mish-mash with some state plane coordinate systems on each standard, and a somewhat frequent cause of problems.
New state plane and regional low distortion systems have been defined on international feet. That "should" not add much confusion once everyone remembers the new systems are all the same.
From Google AI:
- The international yard was defined as exactly 0.9144 meters.
- Consequently, because one foot is exactly one-third of a yard, the international foot was defined as exactly 0.3048 meters, which equates to exactly 30.48 centimeters, or 12 inches of exactly 2.54 cm each.
Is that he actual size you get or is it closer to 38mm x 90mm x 3.65m?
Depends on if its rough sawn or dressed and whether it was milled wet or dry and on any subsequent shrinkage
Depends on if its rough sawn or dressed
Paraphrasing Shakespeare, a 12 foot "2 by 4" by any other name would smell as sweet.