California Surveyors Forums

From the responses I read, it may be NEVER ! . . . .
Because it really doesn't isolate the "big" random error (garbage in/out) and it doesn't point out poor workmanship. Therefore if one were to use good workmanship, care and repetition, the closure error for the original traverse would be sufficient. Why do we need to see a one in a million adjustment, when one in 400,000 is pretty dang good.

Did we "least squares" our way to the moon ? really ? . . .

When we retrace an old document that was surveyed in the 40's , lets say,... the adjustment method of the day was "Compass rule" or maybe "transit rule" . I don't think that a least squares adjustment helps to relocate lost or obliterated points any better than a compass rule.

As far as the "May" part, perhaps a good adjustment will be effective when doing a statewide network for highway control. New Stuff . . .

For the Young Cohorts, I see far too many of them with one hand on a text book, the other on a Law book, their nose in a computer and ABSOLUTELY no clue as to what they are doing.
Land Surveying is not a game for fancy toys, it is a common sense measurement of Land and how its allocated parts relate to deeds, maps, Rights-of-way and many many more.

I have in my hand, a map prepared by (*) who indicates that all found monuments are off [this way and that] ALL of them.
So what was proven there ?

Riddikaluss just plain riddikaluss. . .

LOL "Good"

My Cohort
already understands the necessity for calm logical thought. the search for the best possible solution and the reasons to keep ones self interests out of the decision making process of everyday boundary problems.

There is hope on the horizon for the next generation of Land Surveyors.
As long as this discussion board continues to be, there is truely hope.

Stay thirsty my friends.

XX
"good"

Countydude-

In my opinion, I wouldn't say it's necessary to go to that extra work (especially if you aren't using a program to expedite the mathematics) if you are being prudent in your field procedures and achieving results like those you are describing.

Considering the scope of work that you've indicated, closing to within 0.02 - 0.03 isn't unreasonable, and you won't be gaining a great deal by perfoming a LS adjustment.

If you wished to, you could do a simple compass rule adjustment to tighten things up a bit, but even that may be overkill for the purposes of a topographic survey provided you're closing well.

Now, if you're planning to demolish an existing structure and construct a new townhouse in San Francisco, then I'd certainly recommend you take the extra time to weight each setup and perform the adjustment, while maintaining the good field procedures you already adhere to.

A Least Squares Adjustment is a method for dealing with residuals in a system represented by a math model. Residuals are the differences between the measured values and the adjusted values. Residuals only occur when there is redundancy in the measurements. Redundancy occurs when there are two or more independent observations of the same measurements. Redundancy occurs when there are more measurements than the minimum necessary to compute the positions of all points in a network or traverse. Given sufficient redundancy in a network, least squares will put the residuals in the place where they most likely occurred. How does this work. It is a bit of calculus. The solution is in the adjustments (residuals) to the measurements. A least squares solution is unique, in that for a given set of measurements, the “sum of the residuals squaredâ€￾ is a minimum and no other solution will satisfy this condition. For example, a traverse around a rectangular lot represents a minimally determined system if only three of the courses and two angles are measured to determine the coordinates of the four corners. Applying least squares changes nothing when compared to a coordinate geometry program. If the fourth course is measured then there is some redundancy. Least squares is not very useful for distributing errors or finding a blunder with minimal redundancy and the results of an adjustment may not meaningful. However, measure the two diagonals and least squares becomes an amazing tool in the surveyors arsenal. Why, because now there are a multitude of separate pathways for which the measurements can be simultaneously compared and analyzed. This is an “over determinedâ€￾ system and least squares gives more weight to those measurements that fit better, putting errors or blunders where they most likely occurred. An added benefit of least squares is that the residuals resulting from an adjustment can be used to derive statistical information about the quality of the survey. In summary, the application of least squares to field surveys with sufficient redundancy is very useful tool that helps the surveyor isolate blunders , distribute random errors where the most likely belong and assess the precision of their surveys.

Excellent summary, Michael.....

One minor quibble with an otherwise excellent description:

"least squares gives more weight to those measurements that fit better"

The above sentence can be misleading, as it implies that a given observation is weighted more heavily than others in an adjustment *because* it fits the adjusted value better. In fact, the reverse is true: the adjusted value is influenced by the weight assigned to an observation by the user in the form of a standard error. When that weight is realistic and accurately reflects the error of the observation, the observation residual -- the difference between the observed and adjusted values -- will be small.

Since weighting is not inherent to an observation -- it's metadata provided by the user -- inappropriate weighting can lead to inaccurate results. For example, if an angle turned with a T-2 to fixed targets is given a standard error of 1 degree by the user, it will have less influence on the value of the adjusted angle than one measured with a compass to plumbed lath but given a standard error of 3 arc-seconds. The T-2 angle may be far more accurate than the compass angle, but unless the assigned weights realistically reflect the observation errors, the adjustment may produce an inaccurate result.

.

Does anyone know of any links to free Least Squares adjustment programs? I think Peter Ehlert mentioned that he did. Thanks.

Gary

LSA is not just a fancy adjustment method. It is a great way to process different types of measurements simultaneously. It allows you to use your tools to best advantage. You can close your traverse with a GPS vector, for example, and LSA can process it. You can put anything in there including a 15' measurement with a pocket tape. You can add in old traverses you may have on file done with transit and tape, just assign realistic error estimates.

and weight the measurements accordinly.

Whenever running a loop traverse, more than around a block you should build triangles of measurements. if possible
I remember a job where the 1sec inst crew ran a ~10 point loop traverse along a dam and over the outlet area and several hundred feet outside the dam/outlet area, like a big oval.
I know the company did a LS adjustment but apparently no one thought to make some X ties between the CPs. A edm distance between cps from the dam to the other side, basically cutting the oval in half had 0.35ft diff than the coordinates. The adj flattened the oval. :(