What is it, then?
The grid scale factor will be easting dependent in a Transverse Mercator projection. The NGS published books with eastings and a grid scale. To get the factor for your easting along a line between two points the mean easting would be calculated and then the grid scale factor would be interpolated between the chart numbers.
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See, Moe, that's perversion of terms that I'm talking about. When you divide a ground distance by a grid distance, you are calculating the reciprocal of the average combined factor for that line, regardless of what Trimble calls it. Don't get hung up on combined factors only for points. Both a line and a project have an average scale factor, an average elevation factor, and an average combined factor.
The one that converts directly between ground and grid is the combined factor in NGS terminology. Why would anyone use anything else?
But look at what you wrote about Autocad and other programs. You have to take the reciprocal of the Trimble "project scale factor" to "make them work," even though I would bet that it's also called scale factor in those other programs. Of course, it's really a combined factor there, too, but at least it's right side up.
Now, Google Earth is a whole 'nother ball game. It is not the GRS80 ellipsoid, it's a sphere derived from the WGS84 ellipsoiid. So you're using coordinates from a sphere to calculate a distance on an ellipsoid. Now that can work quite well if the instantaneous radius of the ellipsoid at those coordinates is the same as the radius of the sphere. You're only carrying 3 or 4 significant digits, so you're likely ok, but when you take the reciprocal, the error potential is bigger.
In any event, how would you teach this to a new surveyor? Would you talk about scale, elevation and combined factors, or would you just say scale factor but divide it into one for Autocad? And never mind that stuff on Data Sheets, that stuff and where it came from is irrelevant today.
No, the perversion of what was once clear terminology is complete, the conflation of approximate mapping with precise surveying is complete, and all is well with the world.
Until 2022 gets done and the survey foot is history.
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Honest question here: Where did you learn these techniques and procedures you are using?
Well for years I ran hundreds miles of state plane traverses, for DOT, for MX missiles, for mines, ranch boundaries, oil and gas projects.
Using State plane was a daily occurrence. The proper way to calculate is to geodetically survey forward points and reduce the Lat, Long to a State Plane number. We learned to do that, in the late 70's a poster that sometimes posts here developed programs to geodetically survey using HP calculators. The programs even converted to SPC from Lat, Long.
However, the more often way was to reduce each distance using the mean combine factor of the line between, then apply cos, sine to the horizontal distance from the bearing and calculate the difference in northing and easting. That was time consuming and usually a Project scale factor was applied to a closing NGS monument from the starting monument and the traverse was done using ground distances.?ÿ
So when I got Trimble and a simple inverse gave me all that data; calculating combined factors became a buggy whip thing. Haven't looked at one in years.?ÿ
"The grid scale factor will be easting dependent in a Transverse Mercator projection. The NGS published books with eastings and a grid scale. To get the factor for your easting along a line between two points the mean easting would be calculated and then the grid scale factor would be interpolated between the chart numbers."
OK, but what is the grid scale factor?
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If you had to guess, how many pairs of boots do you think you wore out over all those miles of foresight/backsight? I prefer Red Wing brand boots. What is your go to brand of boots?
I'm just sitting here counting my blessings for working in a place that has a defined system of low distortion projections. Thank you, Ron Singh and Michael Dennis. Well done.?ÿ
If you had to guess, how many pairs of boots do you think you wore out over all those miles of foresight/backsight? I prefer Red Wing brand boots. What is your go to brand of boots?
I wear Whites boots, but I drove everywhere, some hikes up small mountains lugging gear, some helicopters flights.
We often set one gallon ice cream buckets over a 4' high galvanized pipe, held up using wires. You can easily see them 5 miles away when painted orange. And you can stack 9 mirrors in them for distance measurement, the top of the bucket makes a nice line for trig leveling.?ÿ
Usually the hiking happens during boundary work.?ÿ
Here are a few Combined Factors in NY State, where I have been recently.
STATE PLANE COORDINATES
SPC (3101 NY E)
Combined Factor?ÿ ?ÿ0.99986421
5000ft(ground) x 0.99986421 = 4999.32ft(grid)
0.68ft shorter distance
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SPC (3102 NY C)
Combined Factor?ÿ 1.00005638
5000ft(ground) x 1.00005638 = 5000.28ft(grid)
0.28ft longer distance
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A lookup or calculation (CORPSCON6 must use NAVD88) from longitude (latitude for Lambert) to get the scale factor makes good sense, and the accuracy needed is probably obtainable from MOST Google Earth views. Also from quad maps, where ideally you would correct from NAD27 to NAD83 first, but again the difference would fall well within most projects.
Then the elevation factor can be likewise obtained to sufficient accuracy by calculation from those elevation sources.
You need both factors, i.e. the combined factor, to convert between ground and SPC grid distances.
Here are a few Combined Factors in NY State, where I have been recently.
STATE PLANE COORDINATES
SPC (3101 NY E)
Combined Factor?ÿ ?ÿ0.99986421
5000ft(ground) x 0.99986421 = 4999.32ft(grid)
0.68ft shorter distance
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SPC (3102 NY C)
Combined Factor?ÿ 1.00005638
5000ft(ground) x 1.00005638 = 5000.28ft(grid)
0.28ft longer distance
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Were you at low elevation and near the edge of the zone for the second example?
I start getting close to 1 along the edges of the zone for the grid scale, but since I'm at elevation I never seem to get the combined factor anywhere near 1; 0.99985 is about as close as I ever get.?ÿ
Yes, near edge with elevation. Less than a mile from the edge of East and Central zones at elevation of ~1300ft.
Yes, near edge with elevation. Less than a mile from the edge of East and Central zones at elevation of ~1300ft.
You're up high and still getting over 1. I should look at those NY zones. ????
Can you please help me understand the difference between 'factor' and 'scale factor'.?ÿ
From Oxford Languages:?ÿ
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MATHEMATICSa number or algebraic expression by which another is exactly divisible.
Both above definitions to me imply that a factor is a ratio.?ÿ
And math-dictionary.com:?ÿ
A?ÿscale factor?ÿis the ratio of sizes of two similar figures.
Thank you!
Melita
I'm near the foothills of the Adirondacks (~5000' elev at the peak) in NY. The OP mentioned he was down state NY. That could be near the Hudson River with elevation close to zero or in Catskill Mountains with elevations near 2000 ft. or more. I do alot of construction layout and setup for Machine Guidance systems all over the US. I follow a many surveyors that state their project's are on SPC, while they are off by feet.
I'm just sitting here counting my blessings for working in a place that has a defined system of low distortion projections.
I'm always jealous when I get to work-share on Oregon projects.
Recently we've had a lot of success with designing project-specific LDPs. One large energy project was split between two states, and two UTM zones to boot. We had to produce an ALTA plus do targets for aerial mapping, as well as deliver data to be used in a GIS.
It would have gotten super messy trying to scale state plane values across a state border, or trying to pass the buck and just give UTM values.?ÿThe ALTAs would never have worked in UTM anyways, and the variation in combined factors across the project would have made it unworkable.
There are a few states that are getting with the program and doing LDPs for NATRF2022, but by and large we dropped the ball and as a result I suspect this grid/ground SNAFU will continue.
We'll just have the added fun of ditching the USFt and moving to a global reference frame too.
I'm not sweating it, I see a lot of job security in my future fixing screwups after the switch. Maybe that's a poor attitude?
