In Shawn Billings recent post it appears he wants to elevate the coordinates he derived from given Bearings/Distances to a higher class of dignity than the source of those coordinates. I disagree and more importantly Kent McMillan seems to vigorously disagree. Shawn thinks that mathematically massaging given data makes it even better. I am sorry but the more you manipulate data the more it degrades.
After all Least Squares is merely the degradation of allegedly precise observations to an agreeably uniform data set. We accept that compromise because we are unable to determine exactly which of our precise observations was not. Least squares then gives us a reasonable estimate of how much each of our coordinates is in error.
It is not necessary to do Least Squares to show the errors of our coordinates, we could instead do a second independent traverse encompassing our found points. We don't do that because either the economics of the situation or we are lazy.
What an interesting concept, measure twice. Such a concept was intrinsic with the PLSS surveying instructions; "measure twice, then adjust". But alas the woes that have occurred from those ancient land surveyors who did not have it in the budget or were too lazy to measure twice.
Now let me address COORDINATES. COORDINATES to me implies a second independent source of data. In todays world one most often finds COORDINATES in reference to survey requirements to use a state plane system. In most cases those COORDINATES were derived from independent GPS observations and are valid for the points observed. Whether or not they are valid for the points shown on the plans is a separate discussion. However given the COORDINATES and the Map I can usually figure it out. I can also figure out when the surveyor in question, took an adjacent or farther removed Map with COORDINATES thousands of feet away and calculated by Bearings/Directions coordinates for points far removed. That surveyor confused coordinates with COORDINATES and signed and sealed to something that was not.
In years past COORDINATES were most often derived from Geodetic Survey Monuments. Alas too many of the following surveyors were not trained in Geodetic Surveying and hence confused their coordinates with COORDINATES.
In your professional opinion what are COORDINATES and/or coordinates?
Paul in PA
In my professional opinion, COORDINATES are what Ted Dura Dura uses.
Just playing devils advocate for the sake of this discussion and a continuance of Shawn's post.
The "DIGNITY OF CALLS", as stated in Texas;
1= Intent, 2= Monument, natural then man made.
Just asking, if the legal description or in Texas, field notes state, beginning at a 1" iron pipe at N10000 and E10000; Thence North 5000 feet to a 1" iron pipe set at N15000 E10000; Thence East 5000 feet to a 1" iron pipe set at N15000 E15000; Thence South 5000 feet to a 1" iron pipe set at N10000 E15000;
Would the intent not be conveyed by the monument and then the coordinate? If you went out and found 3 of the 4 pipes, the coordinates were reproducible, could/would you not reset the corner by coordinate? Don't the coordinates represent his intent?
Paul - I think you've read Shawn backwards. What he really says are that his bearings and distances come from coordinates. He notes that he very seldom measures from corner to corner directly, but rather locates those corners randomly (as most of us do) and then inverses the coordinates (again, as most of us do) to get a bearing and distance.
:good:
I've never read of a distinction between COORDINATES and coordinates. All grid systems have an origin and rotation. Geodetic grid systems are attached to the ellipsoid at the grid origin.
The "manipulation" I've outlined is a simple polar to rectangular conversion found on most scientific calculators.
Least squares is used by Helmert to determine the best rotation, translation and scale between two geometries but does not affect the relationship of points in their respective geometries.
Least squares adjustmentioned doesn't figure into what I was describing.
That's a very helpful simplified example, Greg. Thanks.
A few minor points...
Shawn was very clearly talking about the 'coordinates' which are inversed to obtain the bearings and distances shown on Plats. His point that the coordinates had been through at least one less mutilation wasn't lost on most of us. The fact that Kent made a tangential connection to another issue is irrelevant.
As for least squares we are world's apart. If you measure a properly designed network you don't need to measure the entire thing on a separate one occasion. Interdependent and redundant measurements will already be built in. The work of least squares is not to refine or degrade the measurements but rather to quantify the reliability of the network through standard methods. In the end if you understand what you are looking at you can make a reasonable choice of what (if anything) needs measured again.
Back to the 'coordinates' as Shawn described. By the time the GLO made it to my neck of the woods the Surveyors were taking excellent notes (for the most part). When they ran an offset or busted a triangle it was right there in the notes. Those field notes have helped me recover more than one rock in a place the Plat would never have led me. In essence those are similar to the coordinates of two pins I would inverse to obtain the line for my map. We can chicken egg it all day but one fact remains. Shawn had a point that was worth pondering. Why others can't see that their points are related rather than contradictory is beyond me...
The priority of calls has not kept up with technology.
A lot of measurements were made in times past with compass and chain. So what people became used to seeing on the plat was bearing and distance.
Then there came angle measurements that were better than the difference in compass bearings. At that point it would have been smart to switch to recording angles and distances, since (except for the occasional isolated solar or Polaris sight) that was what was measured. But tradition was preserved and bearing and distance still put on the plat despite the fact that bearings were derived from angle measurements and thus subject to recombination of error, computational blunders, and scriveners' errors.
If a plat isn't self consistent, it would be a lot easier to isolate an angle error than to decide which bearings needed corresponding corrections.
At that time, SPC coordinates based on some distant monument weren't very accurate in an absolute sense. So coordinates were placed at the bottom of the list.
Those SPC or other coordinates WERE pretty good in a relative sense, and could have been used to recalculate the distance and bearing between monuments. But there was little advantage to that, and the disadvantage that they were a step removed from what was actually measured.
Now we have GPS measurements that are reduced to coordinates or vectors from a distant point. Their absolute as well as relative accuracy approaches (or for large parcels perhaps exceeds) the accuracy of angle and distance measurements with a total station. It would make a lot of sense to record and rely on those coordinates rather than bearing and distance derived from them. When you translate them into bearing and distance you are blurring the errors. But tradition keeps us from switching.
I would tend to favor recording the earliest data that is directly meaningful to the parcel. If you measure angles, don't give me computed bearings. If you measure accurate coordinates (relative or absolute), give them to me, probably in addition to computed angles and distances. Those are still needed for the ease of users who are not doing a retracement.
Very well said. Thank you.
:good:
You must be aware that those calls are only part of an actual statement used in Texas descriptions.
I do and have seen simple calls like that for over 40yrs and will continue to see them, they are incomplete.
One important element missing is the adjoiner information. That information deals with intent, senior and junior rights and can not be eliminated.
To establish a coordinate, there must be known distances, bearings and/or vectors or a combination of them.
Then there are those monuments that were set by the surveyor with the tape that was 99.85ft long or the one that never reduced slope distances.
Once we have located monuments with today's technology, the reliance of coordinates is strong to replace them when they become missing.
To use coordinates only to replace something that was last located decades ago is a stretch and tends to be deed staking and possibly ignoring other facts.
The Dignity of Calls are mostly our guide and certainly not always the law. They are given weight by their reliability and when it is known that source is not that reliable, another more reliable certainty is used in its place.
That is something each surveyor must decide on their own, what is or what is not that we sign our name to.
0.02
You make a very good point, Bill, but I don't think the dignity of calls should be updated with new surveying technology. The dignity of calls were not created for surveyors. Any legal description created by a surveyor should be unambiguous enough to not have to resort to them. When they are needed is when a property owner or a lawyer writes a bad description.
I certainly would only use coordinates provided by a non-surveyor as a last resort.
> The priority of calls has not kept up with technology.
> [...]
> Now we have GPS measurements that are reduced to coordinates or vectors from a distant point. Their absolute as well as relative accuracy approaches (or for large parcels perhaps exceeds) the accuracy of angle and distance measurements with a total station. It would make a lot of sense to record and rely on those coordinates rather than bearing and distance derived from them. When you translate them into bearing and distance you are blurring the errors.
When you consider that total stations commonly available can give local accuracies on the order of +/-2mm or better, it would not be correct to say that +/-2cm GPS technology is even close to as good.
It's obviously true that as separation distances increase, the GPS will tend to win out, but for small separations, not.
It's also obviously true that the problem of representing coordinate accuracy is one that is beyond the grasp of many users. Without realistic estimates of coordinate accuracy, both local and network, the bare numbers really aren't that spatially useful except as maybe decimeter-level navigation tools.
>it would not be correct to say that +/-2cm GPS technology is even close to as good.
From the discussions here, not everyone achieves 2 mm accuracy per setup when you consider instrument accuracy, whether they set temperature and pressure accurately, target centering, and instrument centering. GPS might be as good when someone with average quality equipment and average level of care is retracing a section with hills or trees preventing long sight lines, and requiring many setups, each with its own centering error.
> >it would not be correct to say that +/-2cm GPS technology is even close to as good.
>
> From the discussions here, not everyone achieves 2 mm accuracy per setup when you consider instrument accuracy, whether they set temperature and pressure accurately, target centering, and instrument centering.
Yes, but the same folks who can't figure out how to center a prism and target aren't going to suddenly know how to center a GPS antenna on a 2m rover pole set in a bipod. :>
It definitely is true that local accuracy that is easily possible with even an ordinary modern total station is much better than can be had for comparable effort with GPS. A great case in point is a small lot subdivision. No surveyor in his or her right mind would think about staking out lots via RTK because a total station working from an adjusted control network will do a more accurate job of it.
5000/5000 Are "coordinates" But Not "COORDINATES"
There are millions of 5000/5000 coordinates all over the surveying world, but only one of each true value COORDINATES.
"COORDINATES" are unambiguous.
Paul in PA
Bearings And Distance Are Easily Created Without Coordinates
Field observed Bearings and Distances can be readily placed on a map, and have been for eons without coordinates. To rather suitable precision I might add.
Latitudes and departures were first used to simplify the calculation of the areas surveyed and later used to calculate coordinates.
The entirety of PLSS is based on bearings and distances, for hundreds of years without a need for coordinates.
Paul in PA
Dave said it.
:good: :woot: :good:
"No surveyor in his or her right mind would think about staking out lots via RTK because a total station working from an adjusted control network will do a more accurate job of it."
There are a whole lot of surveyors in my area who are not in their right minds. I have to compete with them constantly... They have no clue or concern about accuracy, only about $$$!
