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GPS work
My office is relatively new to the GPS marketplace and so we are trying to determine an appropriate field procedure for GPS work. We are currently using Topcon Hiper V equipment, and use Carlson SurvCE in our mini and Carlson Survey for drafting. Currently, we are starting a procedure where upon arrive on the job site we will set two control points using a MiFi and connected to our state plane coordinates. We then determine we will set up a Base Receiver over one of those coordinates and configure using the point number. We can then double check the coordinates for the second control point for tolerance acceptance, and any other control we have on the property.
We will then continue around the property and shoot in corners, roads, etc.
Does this seem appropriate, and will this procedure, once processed and adjusted using the SurvNET program in Carlson, keep our coordinates in state plane coordinates? In the past we have set up using Read GPS (unknown points), and then localized on the MiFi/State Plane points. When doing this, after the adjustment process, if we returned and shot only using the Rover and not setting up and localizing, the points would differ by around 6-15 feet (it is my understanding this is because we did not set the Base over a known State Plane point).
Thank you for all your help and suggestions.
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25 Replies
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Do you know what bearings you are on?
Nate
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So, we turn on the vectors from the Base to the Rover. Is that what you mean? So, we have the vectors from the Base in order toe adjust the points.
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Are you applying theta, or not?
I’ve used Carlson data collector before. As I remember, you have an option to use state plane grid, or ground. Are you using either of those?
I used to use ground, THEN apply theta. To get Grid brngs, ground scale, at the base.
In an attempt to make field crew feel like kings, some of the calculations are hidden. This has the bad effect of hiding important metadata.
I’ve not used Carlson in years, so I’d hazard a guess that it has changed some… But I don’t know for sure.
I’ve migrated to Javad, and it gives me metadata, as I like.
N
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Run least squares by any chance?
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I did see the ground to grid option today. I will look into that.
We run the lease squares data and that seems to be when the error occurs. When I run the unadjusted coordinates into the software it matches up the the MiFi Rover shots, but once adjusted it goes off around 9 feet. Not sure why.
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Doesn’t the least squares report tell you what measurement is in conflict with the rest to give such a discrepancy?
Depending on where you are relative to your state plane zone, the discrepancy could be which kind of foot is selected?
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I did see the ground to grid option today. I will look into that.
We run the lease squares data and that seems to be when the error occurs. When I run the unadjusted coordinates into the software it matches up the the MiFi Rover shots, but once adjusted it goes off around 9 feet. Not sure why.
Don’t take this the wrong way but you would be smart to get some formal training.
The knowledge of the forum is great but you need to understand what’s going on from the theoretical level on up… and it’s highly unlikely that this is the place that you’ll get the thorough foundation needed to develop sound processes, debug problems, and reliably deliver what your client needs.
Without that foundation, you’ll have lots of sleepless nights wondering if your data is valid or not…
This isn’t meant to discourage you – it’s to point you towards hooking up with a good trainer for 3 or 4 week long sessions where you get to explore the topics being mentioned here. While that sounds like a big investment – it’s nothing compared to the stress and cost of seeing poured concrete curing in the wrong location.
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I like what @JKinAK said.
The fact that you posted the 1st post in this thread is a good indicator that your on the right track, and that your conscious is already niggling you. That’s a good sign.
There is a good pile of knowledge to be learned. Many of us have a few feet of error somewhere…. From the learning process. Some of us have it, and still don’t know where it is!
Nate
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So, we turn on the vectors from the Base to the Rover. Is that what you mean? So, we have the vectors from the Base in order toe adjust the points.
No, it’s a little bigger than that.
It’s the theoreticaly perfect north, that your particular “grid” is based on. You can use any north you want… Within some reason. And, you can make your gps do it.
Some folks in the early days used “true north by gps”. Problem is that true north is no longer true north. If you move east or west. True north is a series of converging lines, in the northern hemisphere. So, really, there is no such thing as true north, without telling us WHERE it’s true. Move 1/4 mile east, or west, and you are not on true north. The lines that point at the north pole are converging. True north is ambiguous, without telling us WHERE it’s true. We’re surveyors. We get hired, to remove ambiguity, not inject into the title chain.
Nate
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“Currently, we are starting a procedure where upon arrive on the job site we will set two control points using a MiFi and connected to our state plane coordinates. We then determine we will set up a Base Receiver over one of those coordinates and configure using the point number. We can then double check the coordinates for the second control point for tolerance acceptance, and any other control we have on the property.
We will then continue around the property and shoot in corners, roads, etc.”
This is the method I have used since about 2001. It works well.
I have returned to projects years later and easily reproduce my results. I have shared coordinates on section corners with other surveyors and they have checked and matched my results.
It’s a good idea to check into a NGS monument with good horizontal to confirm you have your data collector settings correct.
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I frequently use GPS but rarely use localization. Set up your job to work in State Plane (or other appropriate grid projection system). Then just work in State Plane. Use localization only when you are obliged to adopt some legacy local coordinate system.
As an aside, I rarely use localization but I often use resection to locate my base or total station. I set my control in convenient places around the perimeter of the site where it will be safe from destruction, then set my base or TS where it is convenient for the work at hand. But I insist on using 3 points to resect. So, in your work flow, I’d be setting 3 MIFI points.
Aside #2 – it is really important to get your base in a really open area. That doesn’t have to be right on site. As long as it is within radio range, you’ll be good. Public parks, school yards (when school is out), freeway cloverleaf infields (well back from traffic) all make good base locations.
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Also, suggest you always collect static at the BAse while performing RTK. This will give you a check on your VRS or RTN established base point, using OPUS.
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As JKinAK stated, please obtain some formal training either by another survey(preferred) or elsewhere. You’re on the right track by posting here but the intricacies of each GPS setup, software and hardware, will be important to master.
I would place understanding the grid to ground coordinate conversion and how your set addresses this at the top of the things to learn. I have seen some nightmare coordinate setups on 90% finished projects. the project was designed on grid but the control was on ground and also the other way around. It all stemmed from the surveyor’s lack of understanding of grid vs ground coordinates and how they worked with their equipment.
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Formal training is great if you can get it. But I only ever got bits and pieces of it over the years. Some I got was really good (gpstraining .com -now defunct), and a lot of it was a waste of time and money. If you were to ask me where to go to get it today, I couldn’t really tell you.
So read all you can. Go youtubing, there is a lot there. Ask questions here. If some training opportunities present themselves, by all means, go for it.
A couple of reading resources: Javad GPS Tutorial; NGS – Real Time User Guidelines ; GPS For Land Surveyors
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I got my first training with Pro Marks from watching Mark Silver’s you tube productions….I especially like the one where the wind is blowing so hard you can barely hear Mark…. ???? there are many you tube videos out there for different flavor GPS…seek and find on google….
Ronald W. Berry -
I think about control points and equilateral triangles when deciding where to put the control points and where I am going to do my rover shots.
Static, my points are like between 3mi to 5mi apart and I rover inside the triangle.
RTK is a whole other animal.
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Formal training is great if you can get it.
You can get it – recognize that it is STEP #1 in the process of using GPS gear to survey.
Your dealer should be able to hook you up with solid equipment-specific training.
There are on-line college courses that will give you a solid foundation in geodesy, geopositioning, spatial data adjustments, spatial analysis, and much more.
Maine and others also offer on-line courses.
You have to understand basic geodesy and projections to measure competently with static or RTK GPS.
You won’t even know when (or why) it is or isn’t appropriate to use State Plane coordinates without this knowledge (let alone explain it to your boss or a client).
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Not meant to disagree with or challenge what others have said, but here are a few suggestions I have.
I started in 2001. I was fortunate that a friend I went to school with was the Denver area Leica sales rep. He gave me 3 days of training on how to use the equipment, set up configurations, use the software and load coordinates into the instrument so I could seek and find “things”. You already have the equipment and software, but having a dealer that will answer you phone call while in the field and provides excellent tech support is invaluable. Your NGS regional advisor is also someone that can be very helpful, esp. for online resources, publications, training and webinars offered by the NGS
An old friend gave me the best advice I ever received for using RTK, “Never walk past a point that you’ve shot before.” When I first started, I double-checked my work with conventional gear to know that it was giving me answers I could depend upon. You may wish to also double-check shots by coming back a second day at a different time and set your base on a different control point.
I have never used a localization, even for projects with a local mine grid. Some dealers taught it to nicky-new-guys as an easy way for them to not make mistakes, but I agree with others that having the results in geodetic coordinates is superior to coordinates that have been mashed by a localization procedure. As others have stated, collect static data at the base and learn how to use OPUS (it’s easy).
A good primer on geodesy can be found on this Defense Mapping Agency web site:
And the one book on GPS I recommend is Jan Van Sickle’s book, “GPS for Land Surveyors”. Jan is a good writer and it is well organized.
If you have post-processing software, I suggest checking your RTK shots with 15 to 30 minute static sessions between your base and rover. There isn’t much out there about how to evaluate residual plots. For the most part I will look for a very “noisy” satellite, but often turning that satellite off and reprocessing doesn’t change the answer much if at all. I usually will look a the DOP values out of curiosity. Having the GDOP below 6 is a good rule of thumb.
In my opinion you don’t have to know a lot about geodesy (certainly not a college course in geodesy where calculus and calculus based physics courses are prerequisites). Map projections is another matter. Have your whole staff attend a CE course at the next local conference. If possible attend any course that Dave Doyle teaches!
Until you develop confidence in the reproducibility of your RTK surveys always double-check your work. Sh*t does happen with black-box technological wonders!
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the reason i localize GPS jobs is so i can also use conventional instrument on it. if i setup the job and keep it in state plane, then trying to use the total station on those points never works. im always off on my setups. maybe due to scale factor or something. im sure im doing something wrong.
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If you do not set, check and traverse through at least three points, you have done no check at all.
Paul in PA
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