GPS errors - seeking comments

[mpallamary]

Howdy everyone,

As part of my forensic activities, I am always intrigued by error isolation. Finding things like bad backsights, bad rotations, bad scale adjustments, etc.

I wanted to solicit the community to see if there is a prevalent problem that arises from the use of GPS. I have my own opinions and observations but I do not want to share them for fear of influence. I think it would make for a good brief article for everyone, sort of a "look out for this" kind of a thing.

Is anyone interested in commenting? Obviously, it will be shared with all. As always, thanks for your kind consideration.

Good health to all!

[LS_8750]

One thing I have discovered on several occasions while construction staking is elevation busts on the original topo ranging from about a tenth to a tenth and a half. Normally on the high side.

We will independently carry in the established job benchmark from somewhere off-site via level loop and notice the nearest control point to the benchmark is spot on. Then, when we start taking shots on curb flow lines and other existing improvements we will see problems.

This is especially problematic on flat lots when the engineer's curb flow line slope is 0.3%. Not so bueno when we discover the existing improvements we are tying into as a tenth higher than that shown on the plans.

I am not a fan of GPS topos on flat lots.

[TTaylor]

Mike,

There are some error sources that are potentially prevalent regardless of the type of GPS survey that you are performing (static, fast-static, kinematic, and RTK). The application and required accuracy of the resultant survey data will dictate what the Surveyor will have to consider for the overall error-budget.

From the beginning of GPS surveying the 5 main error sources to consider were Anti-spoofing, Selective Availability, Position Dilution of Precision (PDOP), atmospheric delays, and multi-path. This doesn't include blunders. Nowadays, AS and SA are no longer a factor.

If you are producing CCS83 values then there are a whole slew of other error sources to consider. Such as error in your control, crustal motion, mixing epoch dates, etc.

Plus the type of satellite survey also changes some error considerations.

If need be we can separate out the discussion for vertical from that of horizontal.

I have attached another one of the modules I produced back in 2005 for your use.

I have read with great interest many (all) of your articles in the American Surveyor and always appreciate the efforts you and other authors put forward to produce thought provoking articles. I have noticed that many of the articles have discussions on measurement versus the boundary surveyor.

I think it would be interesting at some point to some for us to have an open and frank discussion of measurement and boundary surveys, particularly the use of GNSS for these surveys. Just a thought. I happen to have some ideas that may be of interest to you and others as I know your ideas are to me.

I realize my answer to your original question is probably a little less than clear, but it is a big subject that doesn't lend itself to a short answer.

[mpallamary]

Hi Tom,

As always, I appreciate your comments. I have been trying very hard to develop a dialogue on this and other important topics. As you may know, I testify rather extensively and I do a lot of standard of care cases as well as defending surveyors and engineers who get sued both locally and across the state. Others have suggested to CLSA that I make some presentations, including one at the upcoming CLSA Conference here in San Diego but every time someone suggests I be placed on the agenda, the slate is always full and in my case, the conference presentations are always full, sometimes a year or two out.

It is unfortunate that the leadership at CLSA is not interested in these topics as I do not believe it benefits the community to discourage discussions like these. It is always a sad day for me when I see another surveyor get sued because he/she does not know how to survey and there is more and more of that going on. Lately, I have seen some very troubling problems with the use of GPS in surveying. As you know (tongue in cheek) because the data is displayed to the nearest thousandth of a foot, it must be correct.

By and large and in most instances, the folks who are getting sued do not have any QA/QC programs in place. In the case of GPS, it is coordinates out of a box. I refer to this problem as "coordinates in a cup." Gee they are pretty but no one knows how they were produced.

I will continue reviewing this topic and continue expressing and publishing my thoughts and observations whenever possible. As an aside and as you may know, the Cal Surveyor stopped accepting any of my articles for apparently the same reasons. I had tried to get articles such as one I am proposing herein published through CLSA but they are not interested. That is fine as I now have a national audience where these kids of discussions are encouraged; people have stopped looking to California for leadership in surveying matters.

Again, many thanks!

[mpallamary]

Tom, your paper is good. I think a good place to maybe start is to find out how many people are using GPS and under what circumstances. A great presentation might be one, perhaps a one day seminar where someone like you would present problems with GPS and the next hour, a presentation of a boundary case where there are problems. The next hour you go to GPS and then follow it with another boundary case. It is amazing how many times I question people about their methodology and the use of redundant measurement and they look at me with glazed eyes. "What are you asking me? The coordinates came out of the machine - they can't be wrong." Yeah, let's go with that one.......

[TTaylor]

Hi Mike,

I know from experience that putting together a conference schedule complete with workshops, side meetings, fund raising activities , etc. is a big undertaking. I helped coordinate the conference for a few years with Crissy and Dorothy as well as at least one Geodetic workshop in 2005 working with Bill Young. (I miss spoke the year in a previous post to the consternation of a very good technician).

I will say that I think this is one of the activities that Crissy and Dorothy do very, very well. The conferences are always successful and typically make money for the Association. All of the administrative efforts they do for the Association are exceptional. As you know, I think that the clarity of roles and responsibilities on Land Surveying professional topics and decisions within the Association should be clarified.

Mike, I agree with you that the subjects of the value of precision (accurate) measurement to surveying, the thought processes of a boundary surveyor, and the relationship between the two would be an interesting topic. I might be wrong. However, maybe others out there would agree and possibly next year you will be on the program. Do not read too much into not being on this year's program.

As you probably know, I have promised a few articles to this forum related to geodetic issues. The holidays sidetracked those efforts but they are on the middle burner. Those articles (already written in the head) have a direct connection to your interest. Namely, your role as an expert witness and your stated opinion that "measurement is dead". I'm assuming those types of comments were hyperbolic with the intent to generate discussion.

Mike, I do not think there is a Land Surveyor out there that does not agree with your concerns about the lack of a scientific foundation for the collection, verification, and analysis of precision surveying data.

From my seat here in the peanut gallery it appears that you have encountered way too many problems with decisions from other surveyors that might not have been the best. This is probably true of all of us. Different opinions is a hallmark of being a professional. Otherwise we could just have a technician do the work.

Again, I appreciate the articles that you write and look forward to more visible presentations by you. I, of course, will help in any way that I can.

[mpallamary]

Thanks Tom! I am good across the board. And yes, my goal is to generate dialogue and to get folks thinking. I have learned a long time ago that the best way to accomplish that is by inviting and encouraging conversation and not censoring it.

I have testified in court or deposition more than a hundred times and that is not to impress anyone. The point is, you tend to see a lot and the court room is the one place you can be guaranteed to hear opposing views. The legal process is a healthy one and for better or worse, those opposing views produce law and that is what we should be guided by.

Life is good, I am good and all is good!

[JRM]

A few common problems I have seen a lot over the years:

-Calibrations

-Grid vs. Ground

-Combined Scale Factor

-Datum mix ups

These are elementary problems but I have found that in many cases surveyors have been trained by the GPS sales guy not other surveyors with geodetic experience.