Most of my surveying career had been with the State Department of Roads. We used modified SPC for every project.
Coordinates were always scaled from the origin and all plan sets included the scale factor used. I can't ever remember there ever being any kind of issues with using this system.
I see a ton of benefits to using modified SPC's but I don't really see the downside as long as all the metadata is included.
I'm always looking to find a better way of doing things. I've been reading up on LDP's and they do seem like a real improvement to SPC's. So other than using a LDP what is better than modified SPC's?
> So other than using a LDP what is better than modified SPC's?
You have answered your own question. LDP's.
Beyond that, what is the problem with just using SPC?
State Plane Coordinates Are Clearly Defined. Period!
Once a Scale Factor is applied it is a Grid System.
At best you have one (1) point that has the same value in the SPC and the Grid system.
As a professional you are misleading the public by including the words "State Plane Coodinates System" even if it is preceded by "Modified" because it is not SPC.
Paul in PA
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> Beyond that, what is the problem with just using SPC?
In my area the Grid vs. Ground difference is about 0.4' per thousand feet.
State Plane Coordinates Are Clearly Defined. Period!
I have seen several cases, construction projects costings hundreds of millions of dollars (light rail, and Pittsburgh airport), where the fact that it was a "modified" SPC somehow got lost over the years, and created a lot of problems when people were bringing in control from outside. They would misclose by 100 feet or so, and then they would go back and check their traverses, etc. So, anytime I hear that a point doesn't close by 100 feet or so, around Pittsburgh, I immediately suspect that someone has modified the system and didn't include that fact on all the plans.
That's what they have been called.
Usually the DOT will call them surface coordinates or working plane coordinates.
However, if you want to get really, really picky, state plane should never be used as a description for the coordinates, modified or not. They should be referred to as the state coordinate system.
Not sure how the Nebraska statute reads but a guess would be:
The Nebraska Coordinate System, NAD 1893, South Zone.
No longer are you to use State Plane in the description.
So:
Surface Coordinates based on the Nebraska Coordinate System, NAD 1983(EPOCH), South Zone. Then list out the metadata (scale factor and such that allows the surface coordinates to be calculated from Nebraska Coordinate System values).
I created several "modified" SPC systems for the PA Turnpike Commission. The difference was that I insisted (and they agreed) that we should remove digits from the left of the number (i.e. 1,432,567.234 became 432,567.234 or 32,567.234). That way they could not be confused with true SPC, but yet could still be transformed back to true SPC or lat/long or UTM or whatever as long as you had the secret formula hidden inside the JG decoder ring.
State Plane Coordinates Are Clearly Defined. Period!
They would misclose by 100 feet or so, and then they would go back and check their traverses, etc. So, anytime I hear that a point doesn't close by 100 feet or so, around Pittsburgh
So they just assume that they have SPC coordinates from years ago regardless of EPOCH changes or project control and then run some new traverse into the site and expect it to check? Is this SOP in that area? Those are some really dumb surveyors. Why wouldn't they use project control? This is similar to the idiots throwing new OPUS solutions over older control. Like they can't survey without OPUS.
Good Friday morning topic. Too me a modified SPCS and a LDP share many things. They attempt to match the plane of the projection to the existing terrain. Each is less accurate as the area grows in size and as the difference in the high and low elevation increases. Yes it is possible that some could mistake a modified state plane for a true state plane coordinate, especially in an area near the lines of equal scale and at a low enough elevation. Since they both have pretty much the same limitations, I think using a LDP is unnecessary. If you want to be sure no one confuses true state plane with modified just drop the first digit from the north and east.
BTW.. you could do a modified state plane, then drop a couple of first digits and call it a LDP... if that suited you.
Another thought is when the metadata goes missing on a LDP project you are really in a pickle. Not so much on a modified state plane project. But there are a couple of ways to go at "modifying" a state plane job too.
Bottom line... metadata matters.
Won't work, you lose the scale factor connection.
They will not do that out here.
Nor should they.
However, SPC values have almost never been used out here-just too much distance distortion. So I guess everyone is used to using the surface coordinates. No big deal.
Drop the digits after they are modified.
> In my area the Grid vs. Ground difference is about 0.4' per thousand feet.
You really need an LDP if the scale factors are that large.
through a little trial and error and with the help of Loyal, I was able to create an LDP that mimicked a "modified State Plane Coordinate System" here in the Texas Coordinate System of 1983, North Central Zone. Over an 8 mile corridor the differences were less than 0.01 foot. The benefits are that these parameters can be loaded in a data collector or office software and produce modified coordinates with no need for localizations. So your RTN can be sending you ECEF based corrections and the data collector can spit out coordinates on the modified system automatically.
I still don't like "modified SPC" but as has been said, many DOT's use them and will continue to use them. LDP's have a place in assisting with these systems as well.
subtracting an offset does not mess with the scale factor.
State Plane Coordinates Are Clearly Defined. Period!
:As a professional you are misleading the public by including the words "State Plane Coodinates System" even if it is preceded by "Modified" because it is not SPC."
Amen brother!
No molested/modified SPC's for me please. Just give me the metadata and let me work on the grid.
State Plane Coordinates Are Clearly Defined. Period!
You didn't exactly "take the words out of my mouth" but that's close to the same thoughts I was havin.
subtracting an offset does not mess with the scale factor.
Yes it does. The surface coordinates are obtained by multiplying the state plane coordinate by the scale factor. So the working plane needs all it's digits. It is very useful; usually (in this area) the working plane is about 600' or so NE of the state plane so if a quad, ortho, GCBD data, GIS property lines are imported into a cad drawing it's simple to shift it over to the working plane by a simple scale around 0,0. By truncating the numbers you would lose that connection. And that's why it's not allowed by the DOT.
Low Distortion Projections (LDP) are in no way similar to so called modified state plane coordinates. In fact they are basically the same thing as a state plane coordinate system. The only real difference is that the projection parameters of an LDP are designed to minimize the distortions over a given area. There is still a grid and height scale factor associated with each discrete LDP grid coordinate. The idea is that the difference between the grid and ground in a LDP system is negligible or at least minimized. The parameters can be loaded into any geodetic CAD, GIS or data collection software. The good old modified grid system takes the average combined scale factor and applies it to the entire project. This is great for a small, relatively flat area. They also may be problematic in terms of combining with other geodetic data.
I have been successfully utilizing both LDP's and modified grid systems my entire career. They both have there place. Which leads me to my pet peeve with modified grid systems. Scale about the origin and truncate. Yes modified grid systems can be Jerry-rigged to look like a true SCP by scaling about some point in the project but that just adds a great deal of probability that somewhere down the line someone will mistake it for a true SPC when the metedata is lost. When the same thing happens to the truncated version those following in your foot steps will not assume that they are a SPC.
Jerry-rigged to look like a true SCP by scaling about some point in the project but that just adds a great deal of probability that somewhere down the line someone will mistake it for a true SPC when the metedata is lost. When the same thing happens to the truncated version those following in your foot steps will not assume that they are a SPC.
This is always a bad idea to me.
And this is the not the way I would ever do a "modified" project. It will eventually cause trouble. But sometimes clients insist on it. I prefer the other way.... do it all on the grid the develop the sea level and mean scale to get the combined and transform all.