It isn't magic, though
YUP, if a LDP is too warped for you then go ahead and use a SPC which has much, much greater distortion. That will solve it. Nothing like having your bearings something like 5 degrees off also. We shouldn't have started using wheels either.
Kent
You certainly have a point.
For an LDP to really live up to it's potential, you have to define a specific distortion budget, and LIMIT the projection to the area(s) in which “it works” within that budget.
ALL projections are “low” distortion, depending on your definition of “low.”
A threshold (budget) of 20ppm will still let you play in an area with 800 vertical feet of relief, IF you limit the SIZE of the projection to about 16 miles N/S (Lambert) or E/W (Transverse Mercator). That can give you a pretty big frog pond to swim in (or NOT if you are in the mountains).
Loyal
Kent
> A threshold (budget) of 20ppm will still let you play in an area with 800 vertical feet of relief, IF you limit the SIZE of the projection to about 16 miles N/S (Lambert) or E/W (Transverse Mercator).
As long as we keep in mind that the main benefit is so that engineers can draw plans in CAD with a standard coordinate system, fine. From a surveying standpoint, the advantages are quite negligible and the expense is having hundreds of different projections to jack around with across the state.
Kent
Granted...
So why don't we just adopt a geodetic system of “true” (geodetic bearings) and “true” (mean height of each line) distances?
That way there would only be ONE SYSTEM on the whole planet!
Loyal
I agree, but we need some method to remove the confusion of having SPC coordinates and some bastard value that very closely resembles an SPC.
I can't imagine how many times a perfectly good topo has been violated by having an EIT block move and rotate to match some imaginary SPC ground coordinate system.
Kent
> So why don't we just adopt a geodetic system of “true” (geodetic bearings) and “true” (mean height of each line) distances?
The answer of course is that it would be quite useless for the sorts of things that surveyors need to do, but just fine for geographers.
Surveyors, of course, will still be doing rigorous reductions to grid even using the Less Distorted Projections as Mark Mayer alluded to in mentioning programming one of the Oregon mini-projections into Star*Net. However, into the mix you can figure that there will be plenty of surveying done at SF=1.000000, ignoring the fact that the Combined Scale Factor may not even be within 20 ppm of that value.
Kent
“The answer of course is that it would be quite useless for the sorts of things that surveyors need to do, but just fine for geographers.”
That makes no sense whatsoever, I assume you are simply confused.
“Surveyors, of course, will still be doing rigorous reductions to grid even using the Less Distorted Projections as Mark Mayer alluded to in mentioning programming one of the Oregon mini-projections into Star*Net.”
Well that might be your problem in a nut shell.
Maybe you need to get a Least Squares Program that uses a GEODETIC model, as opposed to a grid model. The USC&GS/NGS (and GLO/BLM pretty much) has been in a geodetic system for over 200 years!
In a geodetic model, there doesn't really need to be much in the way of “reduction,” it is what it is.
Loyal
Kent
> “The answer of course is that it would be quite useless for the sorts of things that surveyors need to do, but just fine for geographers.”
>
> That makes no sense whatsoever, I assume you are simply confused.
Well, I realize that you love to spend long winter nights jacking around with computations. Doing geodetic inverse and forward computations would probably give you a thrill! For actual surveying operations involving the retracement of plane surveys run by angular methods, geodetic computations are useless since it is much easier to use an intermediate plane projection. End of report.
I will say that I'd love to watch you calculate up some large project layout from the construction plans just by geodetic calculations. I could tell that story for years and always get a laugh. :>
One Off Projection Debate of about 8 years.
It's quite obvious who won this debate. DOT's are using LDP's, States and Counties are adopting them, NGS has written papers, modern surveyors are using them and many other measurement disciplines.
Some guys won't admit they're dead. Seems they can use a computer to get online but can't run software designed to do all the math. Hardcore if it worked for grandpa no need to progress any more.
One Off Projection Debate of about 8 years.
> It's quite obvious who won this debate. DOT's are using LDP's, States and Counties are adopting them, NGS has written papers ...
Yes, obviously I did if standard projections are being adopted. Thanks for noticing.
> I agree, but we need some method to remove the confusion of having SPC coordinates and some bastard value that very closely resembles an SPC.
I think it's fair to say that as long as engineers are involved in project design, there are going to be some weird and ugly things done with survey data.
> I can't imagine how many times a perfectly good topo has been violated by having an EIT block move and rotate to match some imaginary SPC ground coordinate system.
Yeah, but even if you'd supplied it in some Less Distorted Projection with an SF=1+/-30ppm, there would still be no guarantee that the project coordinate system would remain intact through the design process, would there?
It isn't magic, though
> It's only in the small, green "islands" that scale distortion is held to less than 20 ppm, which seems a pretty thin accomplishment. Obviously, the Low Distortion Projections are pretty much Nearly Level Terrain Projections.
It's those green areas where most of the surveying is done. The blue areas are back in the hills where nobody lives and most all the land is unimproved and government owned. The green is where the highways run and the population is.
It isn't magic, though
> It's those green areas where most of the surveying is done. The blue areas are back in the hills where nobody lives and most all the land is unimproved and government owned. The green is where the highways run and the population is.
Sure, the flat land tends to be developed and highways tend to pick easy grades. It's natural that the highway engineers would optimize their projections for highway work.
I was remarking on the choice not to use red for all of the >50 ppm distortion areas, but to use blue which is very close to green graphically. It's a fudge.
In Texas, we don't have federal public land and the surveying would be done all over the mapping area, not just the easy parts. Unlike the 6 x 6 breakdown of the federal rectangular survey, we have survey systems that extend across multiple counties and that are best dealt with in some common projection.
Loyal
Maybe you need to get a Least Squares Program that uses a GEODETIC model, as opposed to a grid model. The USC&GS/NGS (and GLO/BLM pretty much) has been in a geodetic system for over 200 years!
It is odd that the old surveys are geodetic and with all our new stuff present day surveyors are on planes. It's like we're afraid of even dipping our toes into a geodetic system even though all the GPS equipment operates that way. Of course, turning in a plat to the county with True North bearings and distances calculated in three dimensions might cause some problems, but they can be educated. It would be great if there was an Autocad type program that would do the processing and drafting.
MightyMoe
Well...I agree, but I don't really see that happening any time soon (if ever). I believe that a more realistic approach, would be to return a tabulated list of the points (corners and/or Control) on each Plat (or description) that is easily read by OCR Software.
Something like:
NAD83(2011) Epoch 2010.0000 Meters
1 p114 -1857480.0086 0.0015 -4478243.9953 0.0024 4132542.9512 0.0022
2 p115 -1854170.0892 0.0013 -4492321.8313 0.0026 4119265.6061 0.0025
3 p116 -1822883.5693 0.0010 -4508570.9832 0.0018 4116105.9662 0.0017
A full 3x3 covariance matrix is probably not necessary (and of little value to most folks), so I have just shown the rms values above. The data above is JUST an example, and is NOT real NAD83(2011)...
With this data in your (or THEIR) hot little fist, a transformation to SPC, UTM, a County LDP, or ANY other georeferenced coordinate projection would take a couple of nanoseconds. At this point, the Bearings and Distances returned on the PLAT (or Description) would be pretty much irrelevant, and ASSUMING that you had some reasonable meta-data on your Plat/Description, could be duplicated (checked) in about the same time frame anyway.
Loyal
What Would a Spatial Legal Description Look Like?
Beginning at a point at elevation 202.45’ Thence on a forward azimuth of 45 degrees 30 minutes 16 seconds a ground distance of 1000.00’ to a point having an elevation of 265.23’. Thence on reverse azimuth of 135 degrees 27 minutes 16 seconds etc. etc.
And being 345.9876 surface acres or 34.4578 grid acres of land.
What Would a Spatial Legal Description Look Like?
So what happens to your POB when there's a landslide? 😉
Richard - I believe we're on the same page
I can see your point. But I'm not sure such a solution would be ideal just yet.
If everything is stored internally in an ECEF representation, that means that doing something as simple as a stepped-offset of a curb line would involve some very elaborate calculations "behind the scenes". Such a thing is very easy in a grid projection system, but a lot more complex if we get out of the "2-1/2 D" approach we like best in Civil engineering.
And there would be other repercussions, as well. For example, take stage storage computations. It's very easy to calculate volume to an elevation using a grid projection. However, doing the same thing in an ECEF system is inordinately more complicated.
Similar issues would be felt in other places. In general, I would expect an ECEF system to require at least an order of magnitude more computations. Such a thing may be acceptable in processing GPS data, but it's a very different thing for our Civil design software. We're often frustrated with the speed of such software as it is, and it would only be worse if we expected it to do dramatically more computations for everything.
I could see software like you mention coming up in the future. But I'm not so sure that current hardware would really be up for the task.
Loyal
There are certainly problems. The most obvious is giving coordinates in a description. They don’t really land anywhere in my rules of construction. Montana has written a law defining allowable coordinates to be used in a land description and they (if I remember correctly) can only be NAD83 on the Montana Coordinate System. I can’t imagine using them, but that is an option. Of course you are back to state plane and with Montana’s projection it can get very distorted.
Loyal
> There are certainly problems. The most obvious is giving coordinates in a description. They don’t really land anywhere in my rules of construction.
Well, as a rule, they should land on the actual boundary corners as determined by the survey. If they don't, that's a problem that may well extend to other parts of the description. I think Loyal likes the ECEF coordinates because (a) they have more digits in them and (b) they facilitate creating one-off projections with scale factors close to 1.000000. It's sort of schizophrenic to want to create computational difficulties in order to pretend that the scale factor is exactly 1.000000.
