Questions:
1. What is the least understood aspect of plane coordinates or plane coordinate systems?
2. What is the most error-prone aspect of using plane coordinates?
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1. What is the least understood aspect of plane coordinates or plane coordinate systems?
Interesting question. In my experience I would say how the plane is projected from the globe. On the other hand, it might be close to the bottom of the list with respect to the need to understand unless you are in the business of designing them.
2. What is the most error-prone aspect of using plane coordinates?
Without a doubt converting to ground coordinates.
You should not attempt 2 unless you understand 1.
I'd answer both questions at the same time....
The least understood and most error-prone aspect of plane coordinate systems is the fact that while these systems are horizontal systems, ellipsoid height has a (sometimes extreme) effect on the distortion present in a given project.
(Edit: I probably should have clarified that I am talking about plane projections that define a relationship between the global ECEF system and a plane system, rather than simple assumed plane systems, which have their own issues.)
1. What is the least understood aspect of plane coordinates or plane coordinate systems?
Plane Coordinates: Units. (Meters, International Feet, US Survey Feet, Rods, Chains, etc.)
Plane Coordinate Systems: Eventually, they will not be precise/accurate for (very) large areas.
2. What is the most error-prone aspect of using plane coordinates?
Human error. See #1 above.
And metadata, or lack thereof.
3. Is there ever confusion about what the surveying instrument is reporting? For example, are measured distances grid or ground, or are GPS coordinates on a plane, ellipsoid or at ground level?
3. Is there ever confusion about what the surveying instrument is reporting? For example, are measured distances grid or ground, or are GPS coordinates on a plane, ellipsoid or at ground level?
I would expect some confusion because the position can be represented in multiple ways and the user needs to know what has been selected in the equipment.
The electronic distance measurement is a slope distance which the instrument or data collector will usually be set to correct to horizontal ground distance. The data collector may be set to apply a chosen factor to match grid distances for the project.
GNSS works in earth-centered XYZ coordinates and then the answer is converted to the chosen representation of lat-lon-ellipsoidal height or project-specific representation.
Oh, there can definitely be confusion over reporting, if the user does not understand how the software works or what settings have been chosen:
It's great to have the flexibility/customizability, but it can go sideways really quick if there's a lack of knowledge.
The note at the bottom of your picture is interesting:
That makes sense, given that the instrument can only measure actual slope distances. To get ellipsoid distance from a slope distance, one needs an average elevation factor, and that's not available.
But if we go a bit deeper in the manual, to the Coordinate System Settings, we find this:
The default coordinate system for jobs that do not contain data from Trimble Business Center is Scale factor only with a default scale factor of 1. Use this projection type when the job will contain observations only from a conventional instrument and you are using a local scale factor to reduce distances to the local coordinate system. If you are working in a small area and are not sure which coordinate system to use, use the default setting.
This seems to say that the local scale factor reduces slope distances to plane coordinate distances.
But it's the combined factor that does that, not the scale factor.
Is there confusion as to what the three adjustment factors that can be used to reduce a slope distance to a plane distance?
Is this confusion further facilitated by equipment manufacturers using terms loosely?
Wouldn't is be better if everybody used NGS terminology?
This seems to say that the local scale factor reduces slope distances to plane coordinate distances.
But it's the combined factor that does that, not the scale factor.
Is there confusion as to what the three adjustment factors that can be used to reduce a slope distance to a plane distance?
It's mainly that if you are running in a Scale Factor Only job, you're just running a total station in what is almost certainly an assumed coordinate system, and as there is no relationship to a geodetic coordinate system, you are given the option of entering a scale factor if you so choose.
It's extremely rare for us to do so. If there's a defined combined factor for a job, it's almost certainly related to a defined projection and so we will use that projection.
The scale factor. To project the data from ellipsoid to the plane aka mapping projection. Every surveyor needs to understand the limitations of different mapping projections first. Equal area azmuthal etc etc. then understanding how the scale is applied to go from ellipsoid to that plane. Lambert scale is at latitude etc. Mercator bs transverse mercator. Also once you scale the way people do it today gps from grid to ground doesn’t help with the understanding. Because we keep scaling coordinates that remain looking like spc. When i use to teach. I would start with old nad27 and utm data sets. We had no gps. So edm reduced that ground distance to ellipsoid or msl then project to mapping plane . When you have done a few of these traverses long hand it clicks . Like why geodetic azimuth forward and backward is not 180 exact. What a geodetic distance is vs a horizontal distance and surface distance and how to relate. Of course as mentioned above it doesn’t matter what i know if i just push buttons and set the parameters like my boss sais. I use trimble mostly and it drives me nuts that people will not use gps and robot edm in same job file. So i have my crews running grid and if acreage or anything else will be an issue from the mapping distortion i do that on my end and show them. I think the whole grid to ground terminology itself has caused more harm than good. Surveyors need to understand projected coordinates and how the differ from our historical assumed coordinates. When we treat them the same we have placed a bandaid and not cured the reason why we get cut.
One thing that also doesn’t help is the one point factor like TBC uses and DOT’s using one factor at zero zero. And we end up saying and calling that a combined factor and or grid ground factor for project X. Or county X. When we look at where and how the system was originally designed this was not how it was done. Yes a one point can be fine for small projects with little relief for our purpose at times and it makes it quick. The issue is it has become the standard and more of a production mindset vs being checked or correctly applied. In other words we become Lazy. I think every elementary surveying class should besides just doing a lat and departures and computing a traverse on assumed coordinates should go through a complete grid traverse. That means also computing the latitude and longitude and usiing a table or whatever to get the scale factor. All horizontal and zenith angles used and reduced to azimuth or bearings all zenith angles corrected for difference in height. Not elevation nor ellipsoid then horizontal distance computed. Then we can take our starting and ending lats and longs compute the grid coords. Apply the scale factor average at each section on traverse line then apply the average ellipsoid and or msl factor on same compute the project led distance back to plane. Now we can use sine and cosine to compute lat and departures. Go through and then add in a bowditch compass rule after closure etc. thats one part. Take that same data set and compute true azimuth throughout forward and backward at each line. I still remember the dmd method. It never left my brain. When we involve those other senses from turning angles measuring those distances writing them down. Reducing them and engaging in the math directly we learn. Its like paying cash vs debit or credit card there’s a relationship or a connection that takes place in our brains that doesn’t happen from clicking an icon . Yes we can take anyone and say push this first that 2nd that last but they truly learned a process not what is going on. I heard it today. I asked for where a particular raw data and where the project was scaled from. I don’t know it doesn’t matter i draft and just do it this way its the way we always done it. No connection and understanding which at some point blows up and cost big money or when you need them to do something different it doesn’t compute. I found an old US coast and geodetic letter in a old 3 ring binder today. Some of the old folks probably remember this. If you have never read one its a great bit of history. I will try and post it but I scanned it in and its sideways vol 1 I believe. This old binder i use to take with me to read. And learn. Especially doing static surveys when we sat for hours on same point baby sitting and filling out logs every 15 minutes about which svs we were tracking etc.
Sorry I can’t figure this i phone out so someone can fix but this is a old goody. From the old letters
Yes, I have that one and it clearly shows Oscar Adams' ultimate goal. Dr. Adams, though, was a mathematician and an educator. A great mind, a fine designer of calculartion procedures, but premature wirh a mathematical system for surveying not quite ready for prime time.
But if you read Dr. Adams' work carefully and with an eye toward understanding, you know what's going on in the black box. Perhaps not the algorithms, but certainly the inputs and and expected output. The whole purpose of plane map projections is to make the calculations easier, replacing 3d math with 2d; spherical trig with plane trig, etc. Perhaps that's no longer necessary with GPS and unlimited computer power.
NGS, though, obviously thinks that plane surveying is still viable, else they would spend their time and effort in other areas.
Or maybe they've been unduly influenced by Dr. Adams' dream
@mathteacher well most definitely. I have had to have that document in my possession since at-least 1994. I do not know how I got it. It was in with some other documents ngs re bucner and some more math. So I think i probably picked it up at some seminar. I had the geodesy for the layman in my binder as well. The binder has seen better days. I remember sitting on the 5 gallon bucket and organizing my self to read. When i saw that today as i am trying to get my books put up in the office my boss said you have to scan that in and send it to me. I said sure. Yes NGS is probably smart after watching history. This is the year 2023 and that was in the 1937 all thise years to major changes In Datums but the plane state plane coordinates systems and truthfully some are just now starting to use them not because they want to but gps and rtk and the time savings has forced them to. I think heads would explode if we did all earth centered or lat longs everywhere. I talked to surveyors from all over and some still don’t get it. Not that they don’t have the capacity or ability to they are all way more intelligent than i am. Its they just want to do it there way. Or for some reason refuse to see the relationship of projected coordinates vs plane plane coordinates. The math is so easy and most things a surveyor computes is much harder than a basic ratio in all simplest ways we can state it. For grid to ground the famous words. I brought this up not long ago. In VA the regs read reduce all distances to horizontal plane. But most will tell you it sais to ground. Its intent is not what it states. I am not a judge of words because i am terrible at writing. But historically many surveys were done along the earth’s surface. Aka slope distance with poles i many areas. I think it was around 1930’s when they changed it i have not found the exact date yet but i am researching trying to find. They changed to reduce all to horizontal plane. Aka along the surface but horizontal not slope distance. To me other than the acreage for cost on taxes and such. As long as its noted clearly grid vs ground doesn’t matter to much. Its all retraceable and can be followed At some point and we are almost there someone is going to wake up and go hey we are not measuring as good as we thunk we did and we are distorting our distance between monuments no worse than being on a projection trying to assume the world is flat. With gps we are measuring greater distances and doing larger projects. And we all say we can assume the world is flat for 5 miles and we will scale everything to ground. But the project is 20 miles long how does that work. Today people need to know when a one point scale is not going to work. And why leaving a lot of projects on grid is much better than scaling to ground. But its the same thing. Every project scale to ground. No matter what. I wonder what will happen when the new datum comes out.
@rover83 one thing that is needed is to blow up and highlight that definition of ground. This is what surveyors know but do not relate often. All of our so called ground distances are a average or a distorted if we have no difference in heights available. So in theory we create little planes at every observation with the total station. We have not historically seen the distortion Or don’t are or need it. But it’s there. All state plane coordinates do is reduce everything to a common plane. Man i so enjoy reading your comments. As always very well written and great to follow. You have a gift. Keep using it for sure. Seems to me you have figured out that dirty little secret in one understanding. Two allowing the technology to work for you instead of against you. Explanation of the last. On job A vrs rtk established a pair of points on grid nad 83 spc zone x. Then the robot was used from those pairs using scale factor only in Trimble access. Traverse was competed. On job A vrs rtk data downloaded and new coordinates for pairs assigned. Exported to new TBC project now file with robot data brought in delete the grid coordinates use new sudo spc ground coordinates all scale only job. Adjust traverse. Control file given no big deal. Oh wait. Hey client asked for X to be done just use vrs its close enough. That is a standard workflow. And i am not fibbing. I ask where was it scaled from because i can compute it or figure it out if it was one of the original points. Nope somewhere about middle we always do it that way. We can use grid coordinates with robot it doesn’t scale right. I am like what. Keep educating folks. You have a great way with words. I am to blunt and poor grammar. I just keep beating the drum.
