The red flag was in the surveyors datum note which states:
"NAD83 NY Western zone (Ground)"
FYI, never apply scale factor to coordinates.?ÿ
Hmmm, no realization, no epoch date, no explanation of what "Ground" means, and no real indication that it's SPCS....yeah that's a red flag all right.
I'd be A-OK with not scaling coordinates, but?ÿI don't see many surveyors (office or field crew) who understand how to correctly set up their software to apply the correct CSF at each setup/observation. Engineers also tend to get twitchy when they see that the design survey and all control work is grid not ground.
Lee, given how much you travel, is that something you run into on a regular basis?
Lee, given how much you travel, is that something you run into on a regular basis?
Yes, like you said many don't understand scale factors. They have a hard time understanding GPS vs EDM measurements. When in reality nearly all modern data collection software can apply the proper CSF to EDM distance on the fly. Which takes no effort. But in the past few years, we have seen fewer problems as many are now using VRS. The projects suggesting SPC datums established 10 or more years ago are the most frustrating.
We have seen errors of 1-4ft due to issues such as wrong parameters in software like: US Survey ft vs. International ft, WGS84 vs NAD83 vs. NAD83_NO_TRANS
Over 100ft in elevation usually points to no GEOID.
When an error is more random from SPC, it can usually point to the surveyor establishing SPC from a coordinate pair far away, then EDM traversing without proper scale factors and/or angle issues.
I have seen some surveyors feel the need to be on True or Geodetic Bearings, with ground distances. They will hold one NGS monument in SPC, then set bearings to Geodetic and scale to 1.000000.
Using localization or calibration with SPC is just wrong.
On all of these projects, I take advantage of CORS, OPUS, Static, VRS, and RTK to establish modern SPC since it is so easy to do and repeatable. Some of my projects in Canada and the Marshal Islands are on UTM coordinates. One interesting project at a gold mine in Kirkland Lake, Ontario, Canada where all work above ground was on UTM meters, while below ground was a local datum in Feet.
?ÿ
?ÿ
That's pretty similar to my experience over the years. Never had to deal with different systems above vs. below ground, but have done a few mine sites with some pretty screwball transformation parameters.
I just helped a colleague one state over review some field data that we thought was bad. After a lot of double-checks, I can find nothing wrong with our data. As best I can tell the ROS/plat that we are retracing applied the inverse of the combined scale factor to get "ground distances", making the record distances between monuments shorter than grid inverses rather than longer, and of course far shorter than our observed ground distances. It's only a theory, but the ratio of record to measured distances is pretty darn close to the average CSF. Of course there's no info on the plat about instrumentation used or reduction procedures...
@rover83?ÿ
I think that was a common problem left over from the pre-electronic calculator days, when multiplication was much easier than division. We know that, in NGS terminology, we multiply ground distances by the combined factor to reduce them to state plane distances and divide state plane distances by the combined factor to convert them to ground distances.
Some software and juristictions, though, calculated the reciprocal of the combined factor and reported it as some unique name. In Texas, it's called a Surface Adjustment Factor. Depending on one's training and familiarity with the software or jurisdiction, errors were bound to occur. Here's a description of the Texas SAF, copied from here: TxDOT Survey Manual: Geodetic Surveying
"Since a user would not use one of these factors without the other, they may as well be combined into one. To derive a combined adjustment factor (CAF), we multiply the scale factor by the elevation factor. TxDOT has calculated a very general combination factor for each county in the state. It is the reciprocal value of the CAF meaning that it is multiplied times a state plane baseline length (or coordinate) to arrive at a surface value. This multiplier, known as a surface adjustment factor (SAF) and a list can be obtained from the district surveyor if the use of project specific factors is not required."
Terminology and the problems it creates.
It's NOT a combined scale factor, that term is reserved for a unique 3d point or the mid point of a 3d line between two 3d points in a projection. To call the TX DOT Surface Adjustment Factor a Combined Scale Factor is incorrect. They are correct with their terminology.
@mightymoe A crescent wrench does not change identities when used as a hammer. Still a crescent wrench.
It is a combined scale factor, it's simply used in a different way.
No its not. Combined Scale Factors are specific to 3d points and lines. Any lat long pair will have one grid scale factor but an infinite number of combined scale factors. The process TX DOT is describing is to scale the surface up, therefore calling it a Surface Adjustment Factor is correct, and using a number larger than 1 is also correct.?ÿ
Yes, when Texas took the reciprocal of the combined factor, they renamed the result. I wasn't there when it happened, but it can be argued reasonably that the name change was meant to eliminate confusion between a ground to grid (CAF) multiplier and a grid to ground (SAF) multiplier. Of course, using NGS terminology (Combined Factor) and remembering that it's used to reduce ground distances to grid distances would seem to be enough, save for the cumbersome division tools of the past.
The Elevation Factor reduces a ground distance to an ellipsoidal distance and the Scale Factor reduces an ellipsoidal distance to a grid distance. The Combined Factor, the product of the Elevation and Scale Factors, reduces a ground distance directly to a grid distance.
At some point. someone (perhaps an engineer?) realized that dividing endpoint State Plane coordinates by an average scale factor and calculating the distance between the adjusted coordinates was mathematically equivalent to calculating the distance between the State Plane coordinates and dividing by an average scale factor. Thus, simple "ground coordinates" were born. Subsequent software developments have refined the exercise into a ubiquitous robotic process, complete with least squares adjustment of the intermediate results.
I think that it's a pretty safe bet that many contractors will subject new 2022 LDP coordinates to the same treatment. The coordinates will be close enough to ground to use as they are, but what the hey, we've always done it that way, so why change now?
?ÿ
?ÿ
?ÿ
The point being is that loooonnngggg time SPC users went down this road 50 plus years ago. All these arguments were debated over and over and DOTs settled on Project/Surface Adjustment Factors after trying other solutions.?ÿ
The argument against doing it today is that all work is computerized and use state plane since no one is really working on a ground system anyway (I will tell you that doing a boundary plat in state plane is tiresome, all the extra calculations). Being one of the highest states this can get messy, particularly near central meridians and high elevations.?ÿ
The nice thing about using a scale factor holding 0,0 is that it's simple to shrink or expand georeferenced files into autocad, TBC and other newer computer programs. Not that DOTs were that forward thinking but it becomes almost seamless.?ÿ
Way back in the early days 1,000,000+ SPC coordinates were almost always shown using the truncated values. Not because of math reasons but because no one wanted to type or deal with the extra place.?ÿ
But today truncating the values looses the connection and makes it more of a PITA to use files with Surface values.?ÿ
Many surveyors today were only introduced to SPC systems with GPS, but that's not true of western surveyors that worked on large projects.?ÿ
The nice thing about using a scale factor holding 0,0 is that it's simple to shrink or expand georeferenced files into autocad, TBC and other newer computer programs. Not that DOTs were that forward thinking but it becomes almost seamless.
It may be simple to import to a single program, but when working with scaled coordinates interoperability between programs is a gigantic pain. I guess it depends on the type of work that is being done, but if coordinates are not in a defined projection the end user loses a lot of functionality. If you're scaling from the origin in SPC, any georeferenced files someone brings in are going to be off (sometimes by a lot), we can't do OTF transformations, and any geodetic location queries (which are not uncommon these days) are going to be off as well.
We typically have to deliver data that will work with GIS software, Bentley products, and Autodesk products. I do a fair amount of custom LDP development; once you select your coordinate system it's a simple matter to publish the parameters and deliver both the data as well as templates with the projections already defined. The engineers and land development people love it, as well as the GIS folks, because they can bring in any other data they need to without having to shift/scale it all and possibly screw it up.
Way back in the early days 1,000,000+ SPC coordinates were almost always shown using the truncated values. Not because of math reasons but because no one wanted to type or deal with the extra place.?ÿ
But today truncating the values looses the connection and makes it more of a PITA to use files with Surface values.?ÿ
Not truncating scaled coordinates (especially scaled from the origin) is asking for trouble on anything that is getting sent to a client, whether internal or external. Once we scale coordinates, we're no longer in state plane and best practice is to make it clear by truncating. Metadata tends to get lost or forgotten.
Sure, some CAD programs have a way to apply a scale factor, but how many clients and end users know how to work with that?
I'm sure I am in the minority here, but scaling coordinates really needs to become a thing of the past. Either run things with a defined projection and apply scalars to conventional observations as @leegreen advocates, or move to region- or project-specific LDPs where applying a scalar isn't necessary. Do both if you really want to dial in your data. Maintain that geodetic link and allow users to easily transform data.
I'm also in the minority when it comes to storing geospatial databases. CAD sucks at it, but that seems to be the prevalent method for keeping survey data. How many of us have been storing our data in geodetic databases that will be easily transformed when the new datums are released and CORS/RTN broadcasts are updated to ITRF? I know my employer hasn't. We've been given a reprieve with the release being delayed, but I doubt we are going to take advantage of it.
@rover83?ÿ
Deliverables have never been an issue with DOTs that I've ever seen. I've worked with dozen's of engineering companies on DOT projects, they've not had issues with this process. As far as bringing in orthos or quads, they are scaled up. Then sending out SPC line work is simple since it's scaled down.?ÿ
I was an early user of LDP's. The issue I've had is they don't play well with other software. I can use them in any workflow I have but it's almost impossible to send them to some of my clients. It got so frustrating I quit using them except in Montana where state plane is very distorted.?ÿ
The NGS wanted imput designing the new zones, I sent in my suggestions. Hopefully it will work where unadjusted state plane systems will be "close enough" in my areas to work un-adjusted. We shall see.?ÿ
But, I'm going to continue to work for clients using the surface adjustment factors. All the nay-sayers can leave that work to me as far as I'm concerned.?ÿ
Sometimes it's hard to remember that State Plane was intended to simplify surveying. Well, Oscar Adams did have this outlandish idea that if every point on every survey had correct coordinates, disputes would be minimized, but that was always a stretch.
The problem is that a sphere and an ellipsoid both have two finite radii of curvature while a plane has two infinite radii of curvature. The intermediate surfaces commonly used, the cone and the cylinder, each have one infinite and one finite radius of curvature.
So, as we model the earth with a sphere or an ellipsoid, we lose ground features, and when we reduce the curved surface to a flat surface, we lose two finite radii of curvature. With these losses of physical reality, the fit from ground to grid is bound to suffer. Thus, adjustments are required at each stage in order to return the data to its state at the previous stage.
And two of those adjustment factors are the Elevation Factor and the Scale Factor. Angles also suffer, so there's convergence as another adjustment.
State Plane maps are rigorous but they're also robust. Many methods of adjustment from ground to grid and back again work very well and declaring one to be superior to the others is somewhat of a fool's errand.
But the rise of GIS software and its ability to function with any rigorously defined and properly coded mapping system may tip the balance toward LDPs or even toward unadjusted State Plane coordinates.?ÿ
The ongoing challenge is to be able to work with any and all of the schemes that translate curves into straight lines. That and delving into what black boxes are doing and that their results square with what we need.
?ÿ
The up and coming datums and associated projections will bring surveying and measurement science closer together. The list of surveying tasks that can be accomplished using technically 'correct' methods will grow. At the same time we will develop tools of mathematical violence that work well enough for the task at hand. The argument over which way is right will continue through infiniti.
In my early career a handful of folks knew enough to package up as many of these tools as they could learn, then select the right one for each task. That number has grown steadily over time. Hopefully we will reach a point where it eclipses the number who hold one method as right.
In construction we are seeing a lot more Engineering performed on top of Georeferenced Orthographic Imagery as a horizontal base map along with State or FEMA funded lidar for vertical information. The data is often more accurate than some topos done by ground survey crew not familiar with SPC. However there are no BM's, just Metadata and datums identified. This is GIS in action, as it should work. Just need our fellow surveyors to learn how to correctly use these datums and learn to verify before construction begins. Many of these projects are now the contractors responsibility to verify upon commitment. On these new projects,?ÿ this is when a licensed surveyor first enters into the equation.?ÿ
When I was recently at las Cruces, we did exactly that and the survey team provided the BM TBM etc for the alignment and then project control.
Was pretty leading edge esp for a municipality of that size.
Takes buyin from the whole team which we definitely had.
Can anyone explain to this ancient surveyor the necessity of constantly changing datums? ???? TIA
It probably doesn't matter which datum you use to lay out house lots.?ÿ But consistency in the larger picture is important. The average surveyor now has measurement capability that is better than the self-consistency of the present datums.
Of particular importance, subsistence has made NAVD88 problematic in many areas and the 2022 datum will allow creation of consistent flood plane data.?ÿ Also, the rate of destruction of bench marks means and the spacing of the old level runs means that there are many areas where it is difficult to find one, and thus get good and consistent elevations.
@flga-2-2?ÿ
>NAD 83 is non-geocentric by about 2.2 meters.
>NAVD 88 is both biased (by about one-half meter) and tilted (about 1 meter coast to coast) relative to the best global geoid models available today.
These issues derive from the fact that both datums were defined primarily using terrestrial surveying techniques at passive geodetic survey marks. This network of survey marks deteriorates over time (both through unchecked physical movement and simple removal), and resources are not available to maintain them.
In other words the new datum is more closely tied to satellite orbits making fewer calculations needed to transform GNSS observations to biased and disappearing ground control.?ÿ
