Grid/Ground coordinates SOPPosted by jmh4825 on July 2, 2019 at 6:23 pm
After setting pairs of control points with GPS, I have been scaling them to ground in order to make this data match with a TS traverse. I don’t track elevations that often so scaling the TS data to grid is not an option. If I pick this job up in the future, without looking through the raw data, I would like to be able to simply tell which is which. I have been making a note of which point is held grid & ground along with the CF. Just curious as to what others are doing to help keep up with which coordinates are grid vs localized coordinates in CAD or DC without making a big mess? Scale everything one way or the other? Different layers? Field codes?
- 13 Replies
- MemberJuly 2, 2019 at 6:40 pm
Subtract off the millions and hundred thousands when you convert to ground. Then it is obvious they are not SPC and someone needs to look at your metadata notes to see how the conversion is made. If they look like SPC someone will try to use them as SPC. I know some DOT people refuse to do this, but it seems sensible to me..
- MemberJuly 2, 2019 at 7:01 pm
As Bill said, truncate those coordinates after scaling. Depending on the parameters of your zone, drop off a couple hundred thousand or million for both northing and easting.
Some like to scale from a specific physical monument instead of the origin and then truncate such that the point has even values such as 20000N, 50000E. YMMV.
I used to create script files for C3D to convert every entity in a drawing file from local to state plane and vice versa. The design and environmental folks liked it because they frequently referenced GIS drawings or aerials that were in state plane. All they had to do was run our script to bring it to local and then reference it into the design drawing.
Whatever you do, document it. Metadata is critical.
I think I heard the saying here: “Friends don’t let friends scale state plane coordinates without truncating.”“…people will come to love their oppression, to adore the technologies that undo their capacities to think.” -Neil Postman
- MemberJuly 2, 2019 at 7:11 pm
We convert all our data to ground values within our processing software(with a few exceptions). We do not truncate values at all. With our method the cad files exist correctly in the real world when applying the appropriate scale factor to the data being used such as aerial imagery, GIS data, etc…. We found this to be the most efficient method for us. Once we setup proper database protocols we have never had a problem with this setup.
- MemberJuly 2, 2019 at 9:37 pm
If the combined scale factor (at the points which have the greatest error) exceeds (or even comes close too) our project error budget then we find or build an LDP (or LDPs on large projects with significant SF change). It’s the way to go….
- MemberJuly 2, 2019 at 9:41 pm
In the days of NAD27 State plane and hand calculations we used to truncate the million number from state plane so that all the coordinates were expressed in 6 figures. That was before computer programs, since then no truncating, it ruins the relationship between SPC and GC. I figure 40 years since I was removing the million place holder.
The good old days, it was never about local coordinates, more about not having to physically type the numbers, everyone knew what the real numbers were.
- MemberJuly 2, 2019 at 10:04 pmPosted by: MightyMoe
no truncating, it ruins the relationship between SPC and GC.
It gives a slightly different relationship which is just as easy to work with (to reverse, you just have to add before applying the factor), which doesn’t “ruin” it, and is a whole lot less likely to confuse someone outside your organization..
- MemberJuly 2, 2019 at 10:15 pm
No one should ever get confused, I work with all kinds of engineers and surveyors outside my organization, for decades, they don’t get confused by Surface Coordinates. I don’t know why it should be confusing.
Divide the Surface Coordinate by the PAF gives the SPC, multiply the SPC by the PAF and you get the SC. Works well with any program, Timble is set up to do it, same with autocad, makes putting photos, and quads into a drawing beyond simple, otherwise is almost impossible. Like I say, been done this way for 40 plus years. I would think most DOT’s do this.
- MemberJuly 2, 2019 at 11:27 pm
“No one should ever get confused…” I agree, but I see it happen over and over and over and over again. I have recounted on this site in the past an encounter with a Nationally recognized firm during which they gave me SPC and ground coords for the same points. I pointed out that they had them switched, but they insisted that they were correct. I objected, and even prepared a drawing showing why. After a ±6-week review they called me and told me that I was correct and they were wrong. They told me that they thought that they had done every job they had over the previous 15 years incorrectly! So while “no one should ever get confused” it happens every day…
- MemberJuly 2, 2019 at 11:54 pm
With Oregon’s low distortion projections I do the job on the LDP grid, and keep it on the LDP grid. The difference between grid distances and ground is so slight that the grid coordinates can be used as local coordinates without the user noticing anything amiss. It truly is the way to go.
In Washington I frequently just use the state plane grid coordinates. I set the data collector up to use grid coordinates and everything comes out fine.
- MemberJuly 3, 2019 at 12:53 am
They reversed the scale, multiplied instead of dividing, or divided instead of multiplying. They forgot about first principles, look at the results.
Of course anyone can get it wrong.
- MemberJuly 17, 2019 at 3:31 am
- MemberJuly 17, 2019 at 1:30 pm
I have left coordinates in files for years, decades even. I have yet to see a case where coordinates migrated from my SPC files to the Ground files all by thier lonesome…
- MemberJuly 17, 2019 at 2:37 pm
Friends don’t let friends drive when drinking or scale to ground. Use a plane coordinate system where grid is ground for all intents and purposes. You can’t take plane measurements on a globe without fibbing a little no matter what survey equipment you use.
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