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Surveying Accuracy

Accuracy in Surveying is not a given-it is a discussion which is relative to a particular situation. In this support group, discuss the topic of accuracy in respect to surveying best practices for error reduction in the field.

Members: 30
Latest Activity: Mar 12, 2013

Support Group for Discussing the topic of Accuracy in Land Surveying



Accuracy in Land Surveying is not a given-it is a discussion which is relative to a particular situation, a specific place and set of conditions.. In this support group, discuss the topic of accuracy in respect to best practices, tips and tricks in the field for finding accuracy. Topics may include anything related to errors, corrections,safe measures and wisdom from years in the field.

Discussions on Accuracy in Land Surveying and error reduction




Latest Discussions By Category ▿

Strong and weak geometry.

Started by Scott D. Warner, PLS Aug 8, 2012. 0 Replies

Strong geometry vs. weak geometry is something that should be understood by those of us who are working in the field and using traditional line-of-sight instruments such as total stations.  What is…Read more →

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Subject Tags: surveying, surveying precision, survey accuracy, survey precision, surveying accuracy

Precision vs. Accuracy

Started by Scott D. Warner, PLS. Last reply by LSU Justin Apr 7, 2011. 2 Replies

Hello, I would like to cross-post from a discussion entitled TRAVERSE & BEARING. Doing an "exact" traverse is all about precision and measurement methodology. Since there is error inherent in all…Read more →

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Subject Tags: precision, Accuracy

Differential Leveling Overview

Started by LSU Justin Feb 12, 2011. 0 Replies

   Differential…Read more →

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Subject Tags: Vertical, Datum, closing, loops.methods, NAVD88

Starnet Article

Started by Rich Maher. Last reply by Arnel M. Domag Dec 12, 2010. 7 Replies

This article is dated, but very comprehensive. Written by one of the early users of the program that uses it in ways even the programmer didn't think of initially and helped to steer its…Read more →

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Reply by Michael Elbert on May 15, 2012 at 11:23am

I just finished a large infrastructural project where LIdar,GPS and TPS were involved.

1. To start we establised a GPS base line every 1 km.

2 Then we run a traverse between the GPS points with the 3" Leica T15VIVa.

3.A LIDAR scan over the project was tied to the existing ground control.

4.Road surface measured by a total station was both

a vertical and horizontal benchmark for the LIDAR point cloud.

5.A least square adjustment by my proprietory soft

rendered the data with the mm accuracy in position and

10 mm accuracy in height.

6. This was done foe a major infrastructure upgrade in Israel. The legth of the le g is 50 km(about 31 ml)

7. I can offer a service for anyone interested in high accuracy control adjustments

Reply by Rich Maher on December 5, 2010 at 7:06pm
...dee discussions in this group...
Reply by LSU Justin on December 5, 2010 at 6:02pm

Awesome discussions and exactly what I am hoping for in this group. Please in the future remember to start a new discussion in this group so that all discussions will become categorized. Thanks everyone...keep up the great work! The button to start new discussions is above and similar in every group...

Reply by MARK GREGORY HILL on December 5, 2010 at 1:54pm
Thanks also to you Timothy. Perhaps I should have mentioned that I'm not using the Sokkia in conjunction with the Carlson. That gps data was processed separately in the office (not by me i might add and I do need to check how it was done). I like your idea for checking how it is working and need to try that. I did balance the angles and the closure was a little better in one direction. I actually used a different total station for the remaining 3/4 of the traverse after the bad closure the first time and am wondering if that might have something to do with it. I expect to get the original one back this week and haven't heard the results of the calibration check yet. I did remeasure a few of the distances to compare the two and got anywhere from 2 to 6 hundredths difference in some legs that were 2000 feet and even longer in length. This has been helpful and again thanks for taking the time and giving it some thought.
Reply by MARK GREGORY HILL on December 5, 2010 at 1:38pm
Oh hey thanks Arnel for taking the time and effort to read and think about my problem. I believe I understand what you're saying about the propagation of parallelism. It's not too difficult when working in a 2 dimensional world but when you add the geoid, scale factor, convergence and mix gps with cadastral surveying it gets much more complicated. As for the advise to transform local to grid that is where it gets out of control. I don't know of a method, formula or program to help with that. I think that I can take the northing and easting of each coordinate and apply the scale factor to adjust the ones needed but that gets kind of complicated and time consuming and I'm not really sure that I am doing that correctly. I have several other ideas like creating a duplicate file (to experiment with and not screw up the good file) and trying to change the scale factor to generate new coordinates but haven't tried that yet. I was hoping to find a program that would do that and have been all over the internet hunting unsuccessfully so far. There's some great tools available at the NGS site but none apply to adjusting coordinates from local to grid that I can find. One adjust from UTM to other systems but not local (or in my mind ground). I've got a few weeks to explore and think about it. You are right on with the idea that I need to somehow transform from ground to grid correctly before I can even adjust. Thanks a lot for sharing your knowledge and advise.
Reply by Arnel M. Domag on December 5, 2010 at 8:59am
My point of view is, you need to transform your local or ground coordinates to grid system before you can say that considerably you're off position in relation to GPS control. Managing scale factor is not the only issue with regards to your situation. You also need to manage convergence issues. Please take note that everytime backsight measurement is performed, you are propagating parallelism of orientation project wide contrary to the idea of being local with covergence values everywhere.
Reply by Timothy L. Davis on December 5, 2010 at 8:29am
Mark, I have worked with sokkia gps before and have seen many problems with the gps used with the carlson. The geoid has to be updated constantly or you will bad data. A way to check that is to set the gps on a known usgs monument and check the hieght. If it is, then you may have a Geiod issue. I had to can the Sokkia I was working because of the geiod issue. When it was busy, it made me mad when I had to get job complete and the GPS was not working.

Another thing you can do is check your post process of your control. If your setup allows a lot of poor data then you may have an issue with the post process setup. I don't see this happening, but it could.



With your legg that was out, did you bablance your angles and re-check the closure? If the closure gets worse, then it will be an angle issue. If the closure is better, then it probably is a GPS issue.


This is just my experience. I hope it helps.
Reply by MARK GREGORY HILL on December 5, 2010 at 12:47am
Here's one for ya. A recent actual personal situation. I was contracted to survey approximately 1000 acres to be divided into 8 parcels. It is in 3 sections and there are numerous parcels within that are owned by other individuals therefore leaving several cutouts within (sort of irrelevant really). Two of the sections border a highway and there are dirt roads and trail roads around the 2 sections that are open and drivable. Some of the property is wide open where GPS would work great but most areas are densely wooded and require "conventional" total station traverse methods. My plan was to get GPS coordinates at the beginning and end of the traverse along the open highway on the North side and use the total station to traverse from one to the other and use the GPS control for a closure check. The same procedure in a few "GPS friendly" openings on the South side of the 2 sections. The traverse was about 2 miles long on the North side, along the highway so I set 2 GPS points on the West end at about 2000 feet apart that I used for the beginning and a third at the East end of the 2 miles that I used for the end. I occupied one of the 2 at the beginning and backsighted the other and proceeded to traverse toward the third from West to East. I only had 5 legs in this traverse. When I closed into the third point at the East end I was out about 0.5 feet for line and about 1 foot for distance. Yikes. Although when I thought about it I had about a 1/10,000 closure. Not great but better than the 1/5000 required for "open rural" surveys. I did not set collect or "rep" the angles as I was going to have a closure check at the end and have never had any big problems with accuracy in the past. GPS is giving "grid distance and the total station was giving "ground" distance. Right? So I applied a scale factor calculation to the distances and came in within a few hundredths of a foot. Wow! Didn't help with angular closure and boy I sure wished I had "repped" those angles. Well there was plenty more work still left to do so I thought I would address that situation later and proceeded to run another traverse around a 1 mile square that I could easily run down the open dirt roads. The roads had a few curves and angle points so I ended up with a 19 leg traverse that was run with the total station only. No GPS involved at all. I could have used GPS for some of the traverse but most of it was under dense canopy and I also had to locate the road and various fences so I only used the total station, especially since I only have static GPS equipment. When I made the closure I was out 0.90 feet in a North and South direction and 0.36 feet East and West. Dang it! After the crappy results I got with the first traverse I decided to "rep" those angles and collected 2 sets of "repped" angles at each setup. So that entire traverse was run with only the total station. I'm sure it was functioning well as it had only been about 4 months since it was last calibrated and I take very good care of my instruments. Only the total station was used so therefore the grid factor wasn't an issue. The traverse was about 4 miles in length so I had roughly a 1/24,000 closure. Better than the previous traverse. I can perform an adjustment, and I will, but in the past I have ran traverses that long and closed within a tenth or maybe 3 tenths at the worst (tempting to not even worry with adjustment). Like I said I don't believe there was a problem with the "gun" but have sent it in for a checkup since I do have a "spare". Just wondering if anyone had any tips or tricks or advise? Maybe I'm getting old and I know my eyesight isn't what it once was but I can still see the hair on a fly's leg at 150 feet. HA. Does anyone know about the grid or scale factor issue? Has anyone else experienced the same problem? I have static Sokkia GPS equipment and am sure the processing was done correctly. I know the scale factor can be changed in my data collector to compensate for the ground verses grid distance difference but I've never had that problem before and believed the difference to be a fairly small value. I don't know everything about GPS but I've had the 3 Sokkias for about 5 years now with no problems and have learned quite a bit. I have compared the distances between GPS and the total station before and noticed that it never checks perfectly. I have always gotten anywhere from about 0.06 feet to about 0.10 of a foot difference in 600 foot to 2500 foot lengths when comparing the two but have never actually used GPS combined with the total station for a closure until now. I have also never thought about the scale factor being an issue either. I use a Carlson Explorer data collector with the SurvCE software and always leave the scale factor set at 1.00000 although I do check the box allowing for the curvature of the earth for some reason. I collected 2 sets of doubled angles which actually gives the mean of 4 angles. I suppose I could have collected 6 sets but don't feel that would have helped much. I welcome any comments, advise or even criticism. Adjusting distances from grid to ground or vice versa is fairly simple but does anyone know of a formula or a program that converts coordinates from ground to grid and vice versa?
 

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