An out-of-state consulting firm is preparing construction plans for a 3 mile water discharge line from a generating plant and I have prepared the topo for the off-site run. Our control was City Monuments using NAD83/CORS96 NSRS2007 and vertical is reported to be NAVD88/GEOID03.
I didn't run into any trouble with the control. My survey was actually a local plane centered on these (3) control points and they all fit exceptionally well. I had several conversations with the engineer about using SPC. Since my portion of the contract include R/W descriptions and prep we decided to stick to a project grid so there would not be any confusion.
There are two other surveyor companies involved in the project. One is preparing a site topo for the some treatment ponds and another is working within the plant itself for some HP gas line rearrangements. I have received an email from the consultant's CAD guy venting his frustration that the three surveys "don't fit together real well". Only one of the other surveys was included in his email.
From what I can tell from some common control that was located on this other survey, there is about a 5" rotation, some 0.05' to 0.10' horizontal differences (in places, depending how you "glue" it into a combined drawing)...and maybe 0.07' to 0.10' vertical differences. It's hard to tell on the vertical because a great deal of the data is merely ground shots, one quarter corner we have in common shows only 0.01' difference in these two surveys.
When I worked for consulting firms and had several different surveys we were "combining" these differences were not only common, but almost expected. Although I don't see a problem with twisting and gluing surveys together a minor amount; apparently this fella thinks it's a problem. He keeps using this term "geo-referenced".
My question is: Am I just a sloppy old guy that is missing something or is this young man making a mountain out of a mole hill? I really hesitate to "move" my survey...not because I have any problem with that...it's just that I think he needs to be the one to "adjust" three separate surveys to be congruent with one another.
signed..old and confused..:-S
Should of had you survey the entire project.
0.1 h and 0.1 v between two or three different surveys on a 3 mile water line come on man, make it work. I mean is it really going to be built that close. I don't think so. We are not building a piano.
Ron Lang, post: 375386, member: 6445 wrote: Should of had you survey the entire project.
0.1 h and 0.1 v between two or three different surveys on a 3 mile water line come on man, make it work. I mean is it really going to be built that close. I don't think so. We are not building a piano.
Those are my sentiments also. I just want to be aware of all the factors before I tell him my survey "is what it is..".
I'm with the prevailing winds, so far.
And, I add: 3 miles. How much relief is that? What's a tenth going to matter in the gravity scheme for the outfall? Squat?
The ponds being designed aren't going to flood if your horizontal floats a tenth either. And, the gas is pressure. It will go wherever it needs to go. The planimetrics are more important, there, and the construction crews can make it wiggle a bit.
All that said, I used to worry about my hair in the morning, too. Then I grew up and it all fell out.
the question "How many surveyors do you need to screw up a project? " comes to mind.
Seriously though, you can't have a project from 3 surveyors using 3 different reference points at 3 different time frames
coincide to sub-centimeter accuracy.
The only remedy I can think of if to locate 3 common points found within the 3 surveys and "georeference" the 3
survey drawings to fit. This means that you measure these 3 points today on the ground using whatever reference
datum you want & move/rotate the 3 previous drawings to fit to these 3 points.
Tell him the surveys are all geo-referenced to the same datum, that it's just survey tolerances that are showing up, and the 3 surveys just have to be finessed together. I would expect that if I did those three surveys as separate jobs at different times I might find the same kind of variations in my own work, as you suggested. Tell him to get his in-house surveyors involved, and if he has none, to send you the files and a contract for the "finessing".
paden cash, post: 375384, member: 20 wrote: "geo-referenced".
If I'm not mistaken, that's more of a GIS term than a surveying or engineering term.
If you were participating in the construction of a nuclear particle accelerator, I'd say yea, he might have a valid point, but as long as the stinky stuff flows down hill and not up, it's all good, right?
Just because I'm paranoid, doesn't mean they aren't out to get me.
Well, without knowing the exact reference frames the other surveyors used to establish control, this is a difficult question to answer. Geoid03 is two (three?) releases ago. CORS96, again, a slight, yet significant difference than 2011.
So, this likely lies with the client being ambiguous. They likely mandated NAD83 and NAVD88 without specifying the epoch or Geoid. Will the difference matter? Not likely. It could, if problems/delays arose and firms started pointing fingers.
I'd ask a couple quick questions on how control was established on each. It could be any number of other issues.
Good luck!
~Raybies
paden cash, post: 375384, member: 20 wrote: An out-of-state consulting firm is preparing construction plans for a 3 mile water discharge line from a generating plant and I have prepared the topo for the off-site run. Our control was City Monuments using NAD83/CORS96 NSRS2007 and vertical is reported to be NAVD88/GEOID03.
I didn't run into any trouble with the control. My survey was actually a local plane centered on these (3) control points and they all fit exceptionally well. I had several conversations with the engineer about using SPC. Since my portion of the contract include R/W descriptions and prep we decided to stick to a project grid so there would not be any confusion.
There are two other surveyor companies involved in the project. One is preparing a site topo for the some treatment ponds and another is working within the plant itself for some HP gas line rearrangements. I have received an email from the consultant's CAD guy venting his frustration that the three surveys "don't fit together real well". Only one of the other surveys was included in his email.
From what I can tell from some common control that was located on this other survey, there is about a 5" rotation, some 0.05' to 0.10' horizontal differences (in places, depending how you "glue" it into a combined drawing)...and maybe 0.07' to 0.10' vertical differences. It's hard to tell on the vertical because a great deal of the data is merely ground shots, one quarter corner we have in common shows only 0.01' difference in these two surveys.
When I worked for consulting firms and had several different surveys we were "combining" these differences were not only common, but almost expected. Although I don't see a problem with twisting and gluing surveys together a minor amount; apparently this fella thinks it's a problem. He keeps using this term "geo-referenced".
My question is: Am I just a sloppy old guy that is missing something or is this young man making a mountain out of a mole hill? I really hesitate to "move" my survey...not because I have any problem with that...it's just that I think he needs to be the one to "adjust" three separate surveys to be congruent with one another.
signed..old and confused..:-S
Now that's funny,,,,,give a guy perfect data and he can't handle it,,,,,,,
There is no comparing dirt grade and expecting that to be any closer than 0.05ft at best.
The 0.01ft diff at a monument is exceptional in comparing their RTK to your RTK.
How is it that it never has failed in the history of surveying that the first speech from a new contractor on the job is to ruffle feathers and make the statement that they have to redo everything anyone did before them.
I don't miss that work at all...........gimme some boundary any day.;-)
He's a CAD guy. It's got to agree to double precision... 17 decimal places or the polylines don't join. You'll have to do your survey over again.
paden cash, post: 375391, member: 20 wrote: Those are my sentiments also. I just want to be aware of all the factors before I tell him my survey "is what it is..".
Truth be told the project will be built by three different contractors using three different surveyors whose concern will be their portion of project. A tenth will not be noticed.
However if we are working with steel, a tenth is huge. Sometimes people can't see the big picture.
paden cash, post: 375391, member: 20 wrote: Those are my sentiments also. I just want to be aware of all the factors before I tell him my survey "is what it is..".
What kinds of things were off 0.1? Pavement surveys ought to agree in elevation to about 0.03 or better, and horizontally, you would like to see things like property corners in that range also. But for most stuff, ground shots, poles etc. 0.1 is plenty good.
I am also wondering why SPC would "be confusing" to anybody. Nobody who does not understand SPC should be messing around with projects like this surely? I have done lots of long highway projects, and I would argue that you should stay with SPC (or UTM), Engineers need to understand that the changes in quantities between a "ground distances" project and a SPC (scale distances) project are too tiny to affect the costs of materials in a way that is significant in relation to the total budget. And, as to RW issues, the acreages affected by takings for RW are similarly so tiny that they are of no practical consequence whatever (as far as my experience is concerned).
Some of the very few problems I have encountered with integrating different surveys comes in when people have coordinates that "look like" (seem to be) state plane coordinates, but are actually ground coordinates. The further away you get from the center of the project, the more off the coordinates are (relative to SPC). If the surveyor had merely chopped a couple of digits off of the left side of the N and E coordinates, (and maybe raised or lowered the elevations 600 feet) to make it clear that SPC were not being used, then any subsequent users of the survey would have understood what needed to be done to that data to translate and scale it to SPC.
My guess is that in the case in question, the decision was made to do the project in "Quasi state plane coordinates" (coordinates that look like SPC coordinates) and at least one of the other surveyors did their work in "true SPC" coordinates. I think that this would result in exactly the sort of issues that were encountered in the subject example, although, in a project spanning 3 miles, nothing major is going to go wrong.
I found out the hard way about all this stuff on a 30 mile interstate highway control project, providing the master controls (SPC controls) for four separate engineering design projects (and four or more survey projects to support the design, with later survey projects to lay out new construction). All the problems that did occur over the next decade of that project resulted from engineers who thought they needed to scale their local part of the effort "back to ground" (read "more accurate") distances. The coordinates still "looked like" SPC, and the original control points actually were SPC, so some of the survey companies worked using SPC, and some used ground. Things just did not fit, but were not so far off as to cause major problems, so all the surveyors ended up looking like they didn't know what they were doing (just like the situation in this thread).
The fact that needs to be pounded into everybody's head is that there is NO downside to using SPC on any project large or small, and if you DO go ground, just make it clear to everybody by hashing the coordinates, or localizing them to 5000/5000 to make it inexcusably clear to everybody that you are not on SPC. Problems are very expensive and can be so easily avoided, why not do it one of these two ways?
1. Message to design folks: The quantities are not going to change, (nor their cost) enough for anyone to even notice in relation to the overall budget. For example, if doing the project on ground coordinates means you will need $100,000 of asphalt on a linear project, if the SPC scale factor for that project is 0.999999, then the COMPUTED cost of the asphalt will total $99,999.90 cents. That is a dime! This number is made up, but you can do the math for a real project to get a more real number, and if the difference was $50 it still wouldn't matter at all relative to total cost of the asphalt, much less to the total project cost as a whole.
2. Message for surveyors RE rights of way: Even in Texas, the acreages and boundary distances for individual takings are not going to change enough for anyone to complain or notice. On a project with scale factor 0.999999 a 15' by 1000' taking would "show up as" a 14.999985 foot by 999.999 foot taking. So the dimensions on the RW takings plat would be rounded to, you guessed it, 15' by 1000'. The "correct" (ground)15 X 1000 comes out to 15,000 sq ft, (0.34435261708 acres) while the SPC version would show 14.999985 x 999.999, which comes out to 14999.970000015 sq ft (0.344351928375 acres). So assuming a per acre cost of $10,000 dollars, doing the project using a ground coordinate scheme with "true ground distances" would net the property owner $3,443.52, while using the SPC system would net the landowner "only" $3,443.52. In this example, the deleterious effect of using SPC is zero cents, due to rounding to the nearest cent. In a real project with really large tracts, the effect (per parcel taking) would be measured in pennies, and at most, in a rare situation, still less than a dollar.
3. Message for construction layout surveyors, and their supervisors. On a properly run project, surveyors competent to do so should lay out semi permanent construction control points every 800 feet or so, on either the ground system or the SPC system, depending on what has been done before. The key thing here is, SPC is not a problem, because if the project is SPC, those localized controls spaced at 800 foot intervals will be SPC as well. Then, you can send out an idiot who knows nothing about SPC to do the layout, and he can use ground OR scale distances to do his work. This is because he can lay out 400' of stuff (halfway to the next control point) and then move up to the next control point (lay out the other 400' of stakeout behind him, and the first 400' of staking looking ahead) using ground distances. A more sophisticated layout surveyor would plug the project scale factor in, but really, at 400' it does not matter. BECAUSE at 400' the difference due to SPC is 4 percent of 0.01 feet, and our EDMs have an error bar of around 0.02'. To put this another way, our EDM error is 50 times the size of the "bad measurement" you get because of having the project on SPC.
4. You can combine all the above by realizing that using SPC has an effect on a project that is one to two degrees of magnitude smaller than the effect to the project of the error in our survey instruments. If the error in our instruments is too small to matter, than so is any "error" (read "bad measurement") we might get by using pure SPC on a project (ie. not scaling back to ground distances to make stuff more accurate, as some engineers insist we do).
John Wetzel, post: 375428, member: 11747 wrote: What kinds of things were off 0.1? Pavement surveys ought to agree in elevation to about 0.03 or better, and horizontally, you would like to see things like property corners in that range also. But for most stuff, ground shots, poles etc. 0.1 is plenty good.
I am also wondering why SPC would "be confusing" to anybody. Nobody who does not understand SPC should be messing around with projects like this surely? I have done lots of long highway projects, and I would argue that you should stay with SPC (or UTM), Engineers need to understand that the changes in quantities between a "ground distances" project and a SPC (scale distances) project are too tiny to affect the costs of materials in a way that is significant in relation to the total budget. And, as to RW issues, the acreages affected by takings for RW are similarly so tiny that they are of no practical consequence whatever (as far as my experience is concerned).
Some of the very few problems I have encountered with integrating different surveys comes in when people have coordinates that "look like" (seem to be) state plane coordinates, but are actually ground coordinates. The further away you get from the center of the project, the more off the coordinates are (relative to SPC). If the surveyor had merely chopped a couple of digits off of the left side of the N and E coordinates, (and maybe raised or lowered the elevations 600 feet) to make it clear that SPC were not being used, then any subsequent users of the survey would have understood what needed to be done to that data to translate and scale it to SPC.
My guess is that in the case in question, the decision was made to do the project in "Quasi state plane coordinates" (coordinates that look like SPC coordinates) and at least one of the other surveyors did their work in "true SPC" coordinates. I think that this would result in exactly the sort of issues that were encountered in the subject example, although, in a project spanning 3 miles, nothing major is going to go wrong.
I found out the hard way about all this stuff on a 30 mile interstate highway control project, providing the master controls (SPC controls) for four separate engineering design projects (and four or more survey projects to support the design, with later survey projects to lay out new construction). All the problems that did occur over the next decade of that project resulted from engineers who thought they needed to scale their local part of the effort "back to ground" (read "more accurate") distances. The coordinates still "looked like" SPC, and the original control points actually were SPC, so some of the survey companies worked using SPC, and some used ground. Things just did not fit, but were not so far off as to cause major problems, so all the surveyors ended up looking like they didn't know what they were doing (just like the situation in this thread).
The fact that needs to be pounded into everybody's head is that there is NO downside to using SPC on any project large or small, and if you DO go ground, just make it clear to everybody by hashing the coordinates, or localizing them to 5000/5000 to make it inexcusably clear to everybody that you are not on SPC. Problems are very expensive and can be so easily avoided, why not do it one of these two ways?
1. Message to design folks: The quantities are not going to change, (nor their cost) enough for anyone to even notice in relation to the overall budget. For example, if doing the project on ground coordinates means you will need $100,000 of asphalt on a linear project, if the SPC scale factor for that project is 0.999999, then the COMPUTED cost of the asphalt will total $99,999.90 cents. That is a dime! This number is made up, but you can do the math for a real project to get a more real number, and if the difference was $50 it still wouldn't matter at all relative to total cost of the asphalt, much less to the total project cost as a whole.
2. Message for surveyors RE rights of way: Even in Texas, the acreages and boundary distances for individual takings are not going to change enough for anyone to complain or notice. On a project with scale factor 0.999999 a 15' by 1000' taking would "show up as" a 14.999985 foot by 999.999 foot taking. So the dimensions on the RW takings plat would be rounded to, you guessed it, 15' by 1000'. The "correct" (ground)15 X 1000 comes out to 15,000 sq ft, (0.34435261708 acres) while the SPC version would show 14.999985 x 999.999, which comes out to 14999.970000015 sq ft (0.344351928375 acres). So assuming a per acre cost of $10,000 dollars, doing the project using a ground coordinate scheme with "true ground distances" would net the property owner $3,443.52, while using the SPC system would net the landowner "only" $3,443.52. In this example, the deleterious effect of using SPC is zero cents, due to rounding to the nearest cent. In a real project with really large tracts, the effect (per parcel taking) would be measured in pennies, and at most, in a rare situation, still less than a dollar.
3. Message for construction layout surveyors, and their supervisors. On a properly run project, surveyors competent to do so should lay out semi permanent construction control points every 800 feet or so, on either the ground system or the SPC system, depending on what has been done before. The key thing here is, SPC is not a problem, because if the project is SPC, those localized controls spaced at 800 foot intervals will be SPC as well. Then, you can send out an idiot who knows nothing about SPC to do the layout, and he can use ground OR scale distances to do his work. This is because he can lay out 400' of stuff (halfway to the next control point) and then move up to the next control point (lay out the other 400' of stakeout behind him, and the first 400' of staking looking ahead) using ground distances. A more sophisticated layout surveyor would plug the project scale factor in, but really, at 400' it does not matter. BECAUSE at 400' the difference due to SPC is 4 percent of 0.01 feet, and our EDMs have an error bar of around 0.02'. To put this another way, our EDM error is 50 times the size of the "bad measurement" you get because of having the project on SPC.
4. You can combine all the above by realizing that using SPC has an effect on a project that is one to two degrees of magnitude smaller than the effect to the project of the error in our survey instruments. If the error in our instruments is too small to matter, than so is any "error" (read "bad measurement") we might get by using pure SPC on a project (ie. not scaling back to ground distances to make stuff more accurate, as some engineers insist we do).
CONTINUATION OF MY LAST POST (1000 word limit problem):
So, on tiny projects, mid sized projects and really large projects SPC works just great.
On tiny projects ground coordinates work great, mid sized projects are, well, it depends on how big, and on really large projects, ground coordinates suck, because we live on a sphere, and plane coordinates are on a plane, and they just don't work at all. That is why we have SPC.
So, if SPC always works, and ground coordinates only sometimes work, there is really no reason not to use SPC all the time, and get everybody (read: especially civil engineers) very comfortable with it. And, when we don't use SPC (I often don't notwithstanding the previous sentence), lets make that very clear by using clearly non SPC coordinate systems, like 5000/5000. I have seen massive confusion and problems that result from coordinates that were based on SPC at one point, but then used ground surveys out from that. The problem is not the use of ground distances, but the fact that looking at the coordinate list does not clue you in that this was done. Obviously proper project management and research should uncover that "quasi" SPC coordinates are in use, but why make people dig for that nugget of info when you could just shift to 5000/5000 when you start the project. SPC is good, GROUND is good for small stuff, but QUASI SPC is a recipe for misunderstandings and "failure of different surveyors to agree" that makes us all look bad when we hook seperate projects or surveys together. In this thread, the project as described is a quasi SPC project that fits really well to three "true SPC" control points. IF those three points are near the center of the project, you would expect to get exactly what has happened at the periphery of the project, 0.1' discrepancies that were probably caused (I am guessing) by one or both of the other survey outfits looking at the coordinates, noting that they "look like" true SPC coordinates (which they surely did) and then just running with that. They probably did their work in SPC, and did not bother to check with anybody first to make sure it wasn't a "quasi SPC" project. This is just my guess as to what happened, but it would perfectly explain the failure to agree between the various surveys.
THE CLIFFNOTES VERSION of all this blather is: Go ground/arbitrary coordinates or go SPC, BUT never NEVER go hybrid (quasi SPC) as was done in this project. A three mile size project is just the right size to have problems that are noticeable but not likely to cause problems. If this had been a ten mile project, heads would be rolling because the disagreements between the surveys would be quite large. Just my opinion.....
AND FINALLY, if all this makes sense to you as a surveyor, remember it is your responsibility to educate the folks around you, as early as possible in any given project, especially the design engineers, who are not equipped by their education process to just "know" this. I learned it all the hard way.
John Wetzel, post: 375430, member: 11747 wrote: ...Go ground/arbitrary coordinates or go SPC, BUT never NEVER go hybrid (quasi SPC) as was done in this project. a three mile size project is just the right size to have problems that are noticeable but not likely to cause problems. If this had been a ten mile project, heads would be rolling because the disagreements between the surveys would be quite large. Just my opinion......
I agree. The engineer wanted "poser SPC" because their plant "as built" was in what seemed to be SPC. This was actually debunked before I started in on my part of the contract because a previous surveyor had found a +/- 2.5' difference in some X's in the concrete on this "as built" survey. ANd there are some other things that don't fit too well...but it's what they have in their database. And the engineer wanted to use that coordinate base anyway. OK...whatever...let's move the real world so the engineer's records look right...sheeesh.
And since I posted this I have new email. The third survey is actually the bogey. I have a dgn that I just briefly looked at, but it appears as though the gas line surveyor used a fence corner as a tie in to the property/ right-of-way (it's a couple of feet off at that corner) AND the drawing is oriented to a line that digitally runs due east, but is labeled as S88å¡47'15"E...which is the bearing of the south line of the plant site. The drawing doesn't really fit in anywhere real well. I'm a little tired and Monday will be another day...BUT it looks like the pipeline survey was put together by a 1st. semester cad student. I think there is good data points in his survey, but it was not represented well by the dgn.
In my email reply I suggested they give the pipeline company some solid coords and let them massage their dgn until it fits that control. I have been asked to "incorporate" the pond topo in with my survey (not really a problem)...before it's all said and done, I bet I'm asked to "incorporate" that pipeline survey into mine as well.
Now the hard part is going to be what kind of disclaimers I have all over those parts of my drawing that were actually surveyed by others.
Good Luck with the disclaimers. You might want to talk with a Lawyer as they are the experts at shifting responsibility, not that you should have any for their work, but sounds like you will need all the help you can get.
Tell the guy that mean Mr. Reality says nothing affects the Project and he needs to accept the fact or change his occupation to a theoretical math teacher. In the real world, we have a horrible constrain called a budget that greatly limits our pursuit of perfection. If he is receptive, he may actually finish this project and move on to another.
John Wetzel, post: 375428, member: 11747 wrote: What kinds of things were off 0.1? Pavement surveys ought to agree in elevation to about 0.03 or better, and horizontally, you would like to see things like property corners in that range also. But for most stuff, ground shots, poles etc. 0.1 is plenty good.
I am also wondering why SPC would "be confusing" to anybody. Nobody who does not understand SPC should be messing around with projects like this surely? I have done lots of long highway projects, and I would argue that you should stay with SPC (or UTM), Engineers need to understand that the changes in quantities between a "ground distances" project and a SPC (scale distances) project are too tiny to affect the costs of materials in a way that is significant in relation to the total budget. And, as to RW issues, the acreages affected by takings for RW are similarly so tiny that they are of no practical consequence whatever (as far as my experience is concerned).
Some of the very few problems I have encountered with integrating different surveys comes in when people have coordinates that "look like" (seem to be) state plane coordinates, but are actually ground coordinates. The further away you get from the center of the project, the more off the coordinates are (relative to SPC). If the surveyor had merely chopped a couple of digits off of the left side of the N and E coordinates, (and maybe raised or lowered the elevations 600 feet) to make it clear that SPC were not being used, then any subsequent users of the survey would have understood what needed to be done to that data to translate and scale it to SPC.
My guess is that in the case in question, the decision was made to do the project in "Quasi state plane coordinates" (coordinates that look like SPC coordinates) and at least one of the other surveyors did their work in "true SPC" coordinates. I think that this would result in exactly the sort of issues that were encountered in the subject example, although, in a project spanning 3 miles, nothing major is going to go wrong.
I found out the hard way about all this stuff on a 30 mile interstate highway control project, providing the master controls (SPC controls) for four separate engineering design projects (and four or more survey projects to support the design, with later survey projects to lay out new construction). All the problems that did occur over the next decade of that project resulted from engineers who thought they needed to scale their local part of the effort "back to ground" (read "more accurate") distances. The coordinates still "looked like" SPC, and the original control points actually were SPC, so some of the survey companies worked using SPC, and some used ground. Things just did not fit, but were not so far off as to cause major problems, so all the surveyors ended up looking like they didn't know what they were doing (just like the situation in this thread).
The fact that needs to be pounded into everybody's head is that there is NO downside to using SPC on any project large or small, and if you DO go ground, just make it clear to everybody by hashing the coordinates, or localizing them to 5000/5000 to make it inexcusably clear to everybody that you are not on SPC. Problems are very expensive and can be so easily avoided, why not do it one of these two ways?
1. Message to design folks: The quantities are not going to change, (nor their cost) enough for anyone to even notice in relation to the overall budget. For example, if doing the project on ground coordinates means you will need $100,000 of asphalt on a linear project, if the SPC scale factor for that project is 0.999999, then the COMPUTED cost of the asphalt will total $99,999.90 cents. That is a dime! This number is made up, but you can do the math for a real project to get a more real number, and if the difference was $50 it still wouldn't matter at all relative to total cost of the asphalt, much less to the total project cost as a whole.
2. Message for surveyors RE rights of way: Even in Texas, the acreages and boundary distances for individual takings are not going to change enough for anyone to complain or notice. On a project with scale factor 0.999999 a 15' by 1000' taking would "show up as" a 14.999985 foot by 999.999 foot taking. So the dimensions on the RW takings plat would be rounded to, you guessed it, 15' by 1000'. The "correct" (ground)15 X 1000 comes out to 15,000 sq ft, (0.34435261708 acres) while the SPC version would show 14.999985 x 999.999, which comes out to 14999.970000015 sq ft (0.344351928375 acres). So assuming a per acre cost of $10,000 dollars, doing the project using a ground coordinate scheme with "true ground distances" would net the property owner $3,443.52, while using the SPC system would net the landowner "only" $3,443.52. In this example, the deleterious effect of using SPC is zero cents, due to rounding to the nearest cent. In a real project with really large tracts, the effect (per parcel taking) would be measured in pennies, and at most, in a rare situation, still less than a dollar.
3. Message for construction layout surveyors, and their supervisors. On a properly run project, surveyors competent to do so should lay out semi permanent construction control points every 800 feet or so, on either the ground system or the SPC system, depending on what has been done before. The key thing here is, SPC is not a problem, because if the project is SPC, those localized controls spaced at 800 foot intervals will be SPC as well. Then, you can send out an idiot who knows nothing about SPC to do the layout, and he can use ground OR scale distances to do his work. This is because he can lay out 400' of stuff (halfway to the next control point) and then move up to the next control point (lay out the other 400' of stakeout behind him, and the first 400' of staking looking ahead) using ground distances. A more sophisticated layout surveyor would plug the project scale factor in, but really, at 400' it does not matter. BECAUSE at 400' the difference due to SPC is 4 percent of 0.01 feet, and our EDMs have an error bar of around 0.02'. To put this another way, our EDM error is 50 times the size of the "bad measurement" you get because of having the project on SPC.
4. You can combine all the above by realizing that using SPC has an effect on a project that is one to two degrees of magnitude smaller than the effect to the project of the error in our survey instruments. If the error in our instruments is too small to matter, than so is any "error" (read "bad measurement") we might get by using pure SPC on a project (ie. not scaling back to ground distances to make stuff more accurate, as some engineers insist we do).
I don't think DOT will ever stop using surface coordinates,,,,,,,I've never known any engineers or surveyors locally that had issues with it except one who just wanted to do his own thing so he was removed from the vendors list. Kinda high price to pay.
Frankly modified SPC has been used by DOT for at least 40 years now and many follow along.
But then scales range from .9998 to .9992 locally. Of course a 100' row is 99.92' in state plane which is pretty poor and is the reason for surface coordinates......
Imagine trying to design curves in state plane, just what do you do with the radius of a 1 degree curve?
As Shawn said "CAD guy". Maybe they should take him/her out into the field once in a while.
T.W.