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To GPS or not to GPS, that is the question
Posted by Charlie c on January 1, 2017 at 8:58 pmI am considering the purchase of my first RTK unit and am confused by the vendors I have been talking to.
One says RTK cannot be used for urban and suburban lots and is questionable for Elevation Certs. He says it is usable for large projects and control, but the GPS points will not agree with TS results. He says there are ways of making adjustments to compensate for the differences I will get between TS and GPS.
Another says just the opposite, best thing since sliced bread.
Most of my work is suburban lots and acreage, usually 10 acres or less, but occasionally 100 to 500 acres.
The topography is hilly and ranges 400 to 1500 foot elevation with acreage usually having a large number of trees.
There is no other surveyor in my area using GPS, (maybe that should tell me something) so there is no one to ask questions of and advice.
What do you use GPS for?
Do you find differences between GPS and TS distances and how do you reconcile these?
Where would you suggest I go for information?
Happy New Year to all, and thank you for any help you can provide.MightyMoe replied 7 years, 8 months ago 25 Members · 63 Replies -
63 Replies
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I suggest asking for a demonstration from your local dealers.
I am a Javad authorized representative and i try to never sell a system to anyone who I have not given a demonstration.
Make sure that they go to a job site with you to make sure it will work where you work and not just in a parking lot.
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Think of it like this:
Setting your RTK Base in the middle of a project, and running the Rover unit around ÛÏshootingÛ all of the Corners, fences, topo shots, whatever, is pretty much the SAME THING as setting your Total Station (or Robot) on the same point, and doing the same thing with a Prism.
EXCEPT for one thing…
If your are talking about a square mile or two, the RTK should give you better answers than the Total Station (up to a point).
If you are talking about a å? acre Lot, then the Total Station should kick the RTK’s butt.
You are still (pretty much) talking about a ÛÏradial surveyÛ in either case, so the more checks (RTK observations or Angles/Distances) the better. RTK could very well give you best answers between two points a foot apart that are å? mile from the base, but if they are only 50 feet from the Base, the Total Station should win hands down.
Getting RTK (or Static) GPS on the same spatial page as the Total Station (or vice versa), isn’t really all that tough. There is more than one way to do it, some of which are ÛÏbetterÛ than others. By ÛÏbetter,Û I mean MORE REPEATABLE for you or the next guy/gal. State Plane Coordinates is one way to go, Low Distortion Projections another. Personally, I would stay away from ÛÏcalibrations/localizations,Û BUT, when properly done AND quantified (metadata) they will work just fine for many applications (I guess).
Loyal
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I don’t know why the one guy says you will get different answers with RTK and a TS.
A good RTK system will return very tight results, I will use the robot for small acreages and house layouts. The RTK works very well for elevation Cert. you should get within .05′ vertically to your NACD88 control.
Trees and buildings are issues for RTK or static, I wouldn’t buy units that don’t have static.
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MightyMoe, post: 406952, member: 700 wrote: I don’t know why the one guy says you will get different answers with RTK and a TS.
Presumably, he’s referring to the fact that the RTK-derived coordinates may be on some mystery map projection and that grid distances inversed between them will not match the horizontal surface distances measured between them with a total station.
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The work you describe sounds like a perfect candidate for RTK. My robot now sees less than 20 hours of use a year, and the type of work you describe matches mine fairly well.
Please remember that RTK is a tool that requires training. If you invest a few days with a proper equipment representative, you will be well on your way. I will be blunt here….If you do not…you will fail, and cause harm.
Where are you located?
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Kent McMillan, post: 406953, member: 3 wrote: Presumably, he’s referring to the fact that the RTK-derived coordinates may be on some mystery map projection and that grid distances inversed between them will not match the horizontal surface distances measured between them with a total station.
Possibly; then he shouldn’t be selling RTK,,,,,,,,,or anything else really, my TS will be “on” the same projection that my GPS is for any particular job so that all those issues are cancelled out.
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I have opinions. (whodathunk?)
Whatever gear you buy, learn 3 things:
1.) It’s capabilities.
2.) It’s limitations.
3.) How it works, in various envirnments.One thing I’m discovering is that many users don’t take the time to learn the capabities, and limitations of their equipment.
A basic course in state plane coords is a good idea too.N
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MightyMoe, post: 406957, member: 700 wrote: Possibly; then he shouldn’t be selling RTK,,,,,,,,,or anything else really, my TS will be “on” the same projection that my GPS is for any particular job so that all those issues are cancelled out.
Well, I’d wager that probably 90% of the RTK users bought RTK so that they wouldn’t have to mess around with all that “projection” mishmash and just collect good ole coordinates in some system that has nothing to do with them Fancy Dan “projections” you want to fret about.
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Kent McMillan, post: 406963, member: 3 wrote: Well, I’d wager that probably 90% of the RTK users bought RTK so that they wouldn’t have to mess around with all that “projection” mishmash and just collect good ole coordinates in some system that has nothing to do with them Fancy Dan “projections” you want to fret about.
Right, usually spc, which is easily handeled by a TS.
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MightyMoe, post: 406967, member: 700 wrote: Right, usually spc, which is easily handeled by a TS.
Well, ole Handel was before that there RTK, but he did have an idee or two, I reckon.
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GPS RTK is the greatest gold mine of all time, in the right hands. I have repeatedly demonstrated that in open areas an RTK point using a bipod for stability will match a total station to 0.01′ horizontally and 0.025 feet vertically. I have achieved the same results on the Tucson calibrated baseline. My usual procedure is to take every important shot five times, in succession, and the unit averages them.
Don’t use a bipod, orient yourself any which way on different shots, run the rod up and down using a bubble calibrated a year or two ago, and your results will vary. Not may vary – will certainly vary.
Before you buy any unit, most important is a hands on demonstration on one of your typical survey sites, including uploading and downloading.
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All GPS/GNSS, including RTK, outputs coordinates on a projected coordinate system, typically a State Plane system. In a projection system distances between points are scaled to some extent. To what extent depends on where in the projection system relative to the defined origins you are. So the distance you measure between 2 points using RTK will differ from the distance between the same points measured using a total station (or a tape measure) by the scaling, which is around 1 part in 10,000 in the typical case.
There are various ways to deal with this and people do it every day. I use StarNet to deal with combining GPS measurements and Total Station measurements.
I suggest that you need to familiarize yourself with working in state plane first of all. There is lots of stuff online. The book GPS for Land Surveyors is a great resource, but perhaps a bit rough for a beginning.
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Charlie c, post: 406942, member: 6798 wrote: I am considering the purchase of my first RTK unit and am confused by the vendors I have been talking to.
One says RTK cannot be used for urban and suburban lots and is questionable for Elevation Certs. He says it is usable for large projects and control, but the GPS points will not agree with TS results. He says there are ways of making adjustments to compensate for the differences I will get between TS and GPS.
Another says just the opposite, best thing since sliced bread.Hello Charlie. Where you use it really is up to you to figure out by experience. You need to get an idea of the accuracy you are really achieving by comparison and adjustment with other methods.
Lately, by comparison with short static and TPS, I am finding that the old leica 1200 RTK system we use is giving me more reliable results than short static. I have had several bad static vectors processed with 2 – 3 minutes worth of recorded data. This is with both Trimble and Topcon processing software. The same vectors from RTK checked fine against the TPS observations, and always have. I guess it may have to do with the additional time the smart check routines get to run with RTK but I really don’t know. I guess what I’m trying to say is I’m finding RTK observations to be well reliable away from tree canopy, which is the only place I’ve seen bad fixes.
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I have used RTK for at least 12 years for surveys from 1-100 acres. The error in open spaces is about .02-.05 feet depending on the satellite configuration throughout the day. I have been surveying for many years and have never not accepted a monument found for a .10 of a foot. The problem is not RTK but pushing limits without common sense checks especially when accepting solutions in areas with satellite obstructions. If you are in a heavily forested area, conventional methods are more effective but having the ability to set a couple of traverse points in an open area several hundred feet away from a monument you need a mile away can save a lot of traversing.
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GPS and a TS will work seamlessly when set up correctly. The program i use will report distances in ground, ellipsoid and grid between any two points weather shot with RTK or TS. Many years ago it was a PITA to intergrate the two, but that was long ago, its been “hands free” for decades.
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Longer occupation times will improve rtk precision. In the open, [USER=1201]@Bruce Small[/USER] gives precisions that are consistent with my experience.
RTK is capable of working in canopy if proper checks are made.
[USER=8398]@billvhill[/USER] makes a good point. Even on small sites, the practical accuracy of RTK is suitable for cadastral work in most cases.
Precision GNSS measurements, RTK or otherwise, should match very closely with proper precautions, accessories in proper adjustment, proper procedures, proper geodesy.
Be skeptical of anyone who recommends localization or calibration to match total station measurements and rtk measurements. Localization/calibration has its place but I would not recommend it for reconciling total station and rtk measurements. It’s best use is matching coordinate systems (creating transformations).
I sell rtk systems, but I am also a solo operator. U could not do what I do every day without rtk. Simply comparing the measurements of rtk and total station doesn’t quite tell the entire story. Even is small lot subdivisions I find rtk gives me capabilities I would not have otherwise. I locate a lot of additional corners to aid my boundary decisions, quickly and easily. A total station would require traverse in most cases.
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RTK is a big ticket purchase and you sound like you have done no GPS to date. Have you looked a an RTN alternative in your area, at 1/2 the cost or even a pair of basic static receivers, that can be upgraded in the future?
The choice is not just to GPS or not, but how much GPS do you need vs. afford.
Paul in PA
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Shawn Billings, post: 406980, member: 6521 wrote: Be skeptical of anyone who recommends localization or calibration to match total station measurements and rtk measurements. Localization/calibration has its place but I would not recommend it for reconciling total station and rtk measurements. It’s best use is matching coordinate systems (creating transformations).
Nate made a comment in another thread regarding localisations. It turned out, if I remember correctly, that he was not using the terminology as Leica does. I submit that localisations are a perfectly good method to reconcile TPS and GPS measurements. the scale correction will be to a precision sufficient to surpass most people’s TPS measuring accuracy. The orientation can be rigorous when on true or grid, or can be taken from a baseline long enough when doing an custom orientation.
I would like to hear a good reason(s) why localisations are not acceptably accurate. Reasons based on the nature of the localisation procedure.
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Charlie c, post: 406942, member: 6798 wrote: I am considering the purchase of my first RTK unit and am confused by the vendors I have been talking to.
One says RTK cannot be used for urban and suburban lots and is questionable for Elevation Certs. He says it is usable for large projects and control, but the GPS points will not agree with TS results. He says there are ways of making adjustments to compensate for the differences I will get between TS and GPS.
Another says just the opposite, best thing since sliced bread.
Most of my work is suburban lots and acreage, usually 10 acres or less, but occasionally 100 to 500 acres.
The topography is hilly and ranges 400 to 1500 foot elevation with acreage usually having a large number of trees.
There is no other surveyor in my area using GPS, (maybe that should tell me something) so there is no one to ask questions of and advice.
What do you use GPS for?
Do you find differences between GPS and TS distances and how do you reconcile these?
Where would you suggest I go for information?
Happy New Year to all, and thank you for any help you can provide.I also suggest asking for a demo or evaluation before buying.
As for GPS & TS survey agreement, they should agree within the accuracy limits of both systems, as long as the comparison is done correctly. For instance, you cannot compare a MAP distance to a MARK-to-MARK distance without modification; they both have to be reduced to the same surface, be it the map or the geoid/ellipsoid. It is like comparing currencies of different values. Systems should have these mathematical/cartographic reductions built into the software! All you need to be careful with is to use the right term to express the value.
GNSS systems (GPS+GLONASS) should surpass their GPS-only counterparts. The extra cost you pay for a GNSS system is justified especially in less than ideal open-sky environments. RTK systems usually require more than 8 satellites, preferably 12, in view to maintain reliable RTK accuracy. GNSS helps maintain more than 10 satellites in view most of the time.
We manufacture custom systems based on brand-name GNSS boards. We allow a full week free evaluation of the system. We are located in Florida. Our network rover solution starts at $2,850! Email [email][email protected][/email] to arrange for your evaluation unit.
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Get an X90-OPUS and do static for a while to get comfortable with mixing GPS and total station work.
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