Hey there, doing a topo job tomorrow using gps but having to bring in vertical via level loop, taking about an hour. I know it can be done but am a bit shaky on the process. My thoughts are to set the base on a known NGS benchmark then use the rover to shoot 4 other NGS points to box in the site, then having keyed in the SPC already do a site calibration, check one more point inside or very near the box and then go into the site and set control. Using OPUS for post processing is not going to be possible, as well the GPS unit is an older Z-Max and I just don't know what sort of accuracy can be expected from it. The crew chief isn't familiar with doing this but seemed open to the idea with some reservations as to time required.
tl;dr, can I use GPS to bring in control in under an hour and get good accuracy?
Let me see if I follow you. You want to RTK all your control for horizontal and run a level loop to establish the vertical on the control?
Nope, RTK for the whole shebang. To my knowledge OPUS can make it happen, but as far as RTK (with slightly longer observation times of 5 minutes each) I am not sure.
> My thoughts are to set the base on a known NGS benchmark then use the rover to shoot 4 other NGS points to box in the site, then having keyed in the SPC already do a site calibration, check one more point inside or very near the box and then go into the site and set control.
The procedure you describe sounds perfectly suited to the task, unless you have to meet one of those pesky accuracy standards. In the latter case, you might want to review NOS NGS 58 and NOS NGS 59.
Would it be better to set the base on a random point and use the NGS benchmark in the site calibration and then check into another point horizontally and vertically. Have done this myself and after the calibration, set control around the site for topo.
You didn't say what accuracy you need but with RTK no matter how you come up with the value for the base the RTK vertical is going to be +/- 0.1' on a good day and 0.2 most other days. Anytime you use a calibration or localization all you are doing is averaging the error in the NGS points and transferring them to your network. If you have a mixture of 1st 2nd and 3rd order control points your solution won't be as good as just selecting the highest order and setting the base on that or just setting the base on a random point in the work area and doing a single point localization to the highest order control point. Then you can go to the other points and take shots and compare to the published to see if your localization is good.
While your base is running have it collect static data and send that to opus for another check.
I doubt you will do better than a tenth vertically with the Z-Max. At least that is my experience with the two Z-Max units we have. Good luck with the site calibration, I've had less than ideal results when doing one with TDS.
So you could bring in the vertical with a level loop in about an hour but you'd rather try it with RTK in a procedure you're not sure of that will result in poorer results even if it's completely successful? Just checking if I understand you correctly.
Instead of a 4 point site calibration can't you use a Geiod Model to calc the vertical? Then you could check into some local bench marks and adjust the whole topo up or down to match.
I agree with this. In fact, I will go one step further. If you are close enough to do single baseline RTK from a high order monument with a recently leveled orthometric height, and it seems you must think you are, I'd just forget the localization unless you suspect a major GEOID model problem. Take check shots on some of the other monuments. RTK each of your control points for 3 minutes. Now move your base to another control monument (hopefully you found one that checked well). Do your check shots again and RTK all your control again for 3 minutes. Average together your control coordinates, and do your topo.
@Steve- Yup, learning is the major reason and my thought is that a basic topo is a decent enough place.
@LRDay - Well that pretty much is what the site calibration will do, correct?
@Joe M - Are you still using the Z-Max?
@Jim Frame - Thanks for the links, never hurts to read more on this stuff!
The control in the area is first order stuff but in doing some pre-planning I am finding a disturbing lack of sufficient control, as well even for topo it seems our GPS just isn't up to snuff for what I was planning. Thank you for your educated input to my feeble mind!
We only use the Z-Max units to do static control now.
Steve,
Couldn't have stated that any better! I love my RTK unit, and it does a decent job vertically, but I wouldn't use it alone to determine elevation Control. Dirt work, storm catch basins.. maybe.
buildings and sewer and paving all need optical observations.
Personally, if you are looking to check out the limits of the equipment, you should run the vertical in conventionally, THEN do the GPS work on top of that. You can play around with the calibration, do one point first and check residuals to your level loop. Keep constraining your vertical plane, one point at a time and recheck your residuals. After your fourth control point, you should see some residuals from your software, since 3 points make a perfect plane.
It would only take another hour or so and you'd have learned some valuable information about the limits of your own equipment.
Good luck, and come back to tell us how the project went!
Andy
Using one vertical point as fixed and applying the geoid model and checking into other vertical control points is NOT the same thing as calibrating to 4 or more vertical control points.
Calibrating creates an inclined plane through the points, and you do not want to go outside of the box created by those points.
Holding one (hopefully your best and highest order point) fixed and using the geoid model to check into others is often a better method.
We don't ever, never, ever, use RTK for control. The accuracies are just not there for control. It can be done, but not with RTK. With RTK, each measurement is independant of every other measurement. There is absolutely no redundancy....a biblical word in the world of control.
On most projects, we set several of our local control points in the project area, and, then using static sessions tied to CORS Stations, we will occupy two to four benchmarks with about 15-20 minute sessions. We would then perform a least squares adjustment of the GPS data holding one CORS Station and one benchmark to see how everything fits together, sort of a free adjustment. Repeat as necessary. Based upon the results, we then perform a final adjustment holding the appropriate points.
That's the last time we use the CORS Stations, from now on we will use our local control points to position aerial targets, centerline monuments, or additional design survey control points. We have had very reliable results using these procedures.
Don't listen to the GPS salesmen when they say RTK is the answer, you only need two GPS units, etc. It's not! We use RTK for some topo, but that's it. After hearing alot of surveyors, and not all of them young, talk about how they use GPS and RTK, it scares me. We have a void in our education and training concerning what control is and how to use it properly.
Having been brought up in the world of 1st and 2nd Order Control, my credo is "Control is Control is Control". Strength of figure is still very important in control work, regardless of the methods utilized.
I guess I'll get off my soapbox now. I just couldn't stand it anymore!
If I want to base our survey on NAVD datum, I will use GPS to bring in the Datum.
I will then use conventional means to run all my control from those points.
Passive benchmarks are going away fast, and there is no money in anybody's budget to replace them.
change your rtk setting to static, and post process.
more time is good time, esp. for vertical qualities. use stations with open horizon and collect data to 5 degree elevation mask. the 5 deg is a relatively new concept. the expectation is it will improve the vertical coordinate. those two ngs books mentioned above are very good resources
If you think collecting GPS data with a Z-Max with a 5 degree elevation mask is a good idea... well... you obviously don't have a Z-Max.
""" If you think collecting GPS data with a Z-Max with a 5 degree elevation mask is a good idea... well... you obviously don't have a Z-Max."""
no, i do not. what's the problem? zmax don't handle low elevation signals well?
keep in mind this is for static only. when you post process, you can adjust available signals.
right now, i am racking my brain to remember where i read about the 5 degree mask. does this sound familiar to anyone?
5 seems a bit low, I want to say I usually use around 10-13 degrees.
