GPS Surveyors,
Do you rely completely on a network (Trimble VRS/Leica Smartnet), use the network to set a control point set-up your own base station or make a static/rapid static observation and set-up your own base station when GPS Surveying?
Basically, I previously worked for a company and we would use the Trimble VRS network to set a control point or two and then set up our own base station. The company I currently work for relies completely on "network surveying" (Leica Smartnet). Do you prefer one over the other, why?
I know factors come into play such as the length of your baseline etc.? When working on the network, when practical we check into a TxDOT or NGS marker prior to beginning a new job to verify X,Y,Z and have been pleased with the results.
I'm looking at purchasing new GPS equipment soon and I'm torn between buying rovers w/ network capabilities only or should I buy a rover & base set-up.
Thoughts, Suggestions? Thanks in advance!
CJurica
SA, TX
I don't use the VRS a whole lot (or RTK for that matter), as we don't do a lot of that type of work, although I do have a subscription. However, if I am doing topo in an small area, I would prefer to setup my own RTK base. I would process the base data in the office to get the coordinates.
However, for a survey covering a larger area, where a single base radio would not be sufficient, I would just use the VRS. I have used the VRS in a hydro survey, although there are issues with latency. We were having a problem getting reception from the base (located 5' above the water) and the rover in the boat as we got further away (still intervisible).
A lot of what we do is photo control and LIDAR control, typically covering very large areas. A single base of RTK would not cover the area. We always do those as static surveys. By the time I select a point, set a target, make a sketch, etc, I have enough time for a static occupation (15 minutes) against a base receiver within 15 km. On many projects we need multiple bases (every 15 km or so), but don't have to worry about the radio. And these sites are often very isolated, with poor cell phone coverage. But doing it that way lets me tie in benchmarks and other control.
There are many recent opinions in the recent postings about Network RTK accuracies.
Not my area of expertise upon which to opine.
It's Sunil Bismath's though:
Click on attachment
Attachment: cvvd - Utilization of Network RTK GPS in MTO Highway Surveys.pdf
His comments at the end are particularly interesting with regard to comparison to other jurisdictions.
Cheers,
Derek
I think we need to pin down the accuracy 🙂
We don't use VRS, but we are now starting to be asked to carry out work where somebody else has been unable to get "GPS signals". There doesn't seem to be any problem in these areas using RTK, so I would assume that the others have been using VRS and have suffered from poor network signals.
I would suggest that if you go for VRS you also ensure you have a base/rtk capability as well.
I'd slightly disagree with John on the observation times needed. Generally the figure he quotes is enough, but we use 3 minutes plus 3 minutes per kilometre as the guideline figure to ensure a good resolution for static. As noted, if you are fixing control, by the time you have done a sketch, photos, etc. then 15-20 minutes have passed.
If you need to use a total station as well (perhaps wooded areas etc.) an adaptor to let the gps sit on top of the instrument or the target is very useful. We had a couple of types made up - one acts as a signal rod with the standard Trimble pole target, so you can leave it set on an RO tripod and have the GPS base running at the same time. The other screws into the top of our Trimble 5605 so we can get a static fix on the instrument while detailing (see www.SCSsurvey.co.uk/services/powerlines.htm for an illustration).
You could also use this with VRS - as a bonus, this way you would get quite a reasonable time of observation and thus improve the VRS fix.
Movement of the instrument appears to have no effect on the accuracy of the final GPS fix.
Our minimum criteria is 15 minutes for lines up to 15 km, then 1 minute per km over that (minimum). I have been doing several thousand points a year for the last 25 years with that criteria, it works. But, we always do more when setting monuments or control for other survey work, 30 minutes to several hours.
We often have GPS on top of the prism on a tripod, but so far we have not put it on top of the instrument, we just swap them out. The instrument (S6) sits 0.196 m above the tribrach plate (same as a T2). The prisms we use, with the hockey puck adaptor, sits 0.083 m above the plate. The GPS by itself (i.e. no prism) is 0.033 ARP, and on top of a prism is 0.140 m ARP (due to a gap between the top of the prism and the bottom of the antenna. So, knowing the HI we can compute the other HI's pretty easily.
I should add that if we are setting a pair to use the total station (i.e. for shooting a point in the woods, some Lidar projects require a certain number of check points in bare ground, brush, woods, etc), we will have a static base (not RTK) running somewhere. We then put an RTK base unit on the backsight, and set it for HERE, and it begins transmitting. Next, we do a RTK rover position at the instrument (observed control point). This gives a "close" position, and an accurate azimuth. When we shoot the backsight with the total station we get feedback if it is matching the GPS RTK vector (the vertical angle and the distance). Back in the office I process the data collected at the backsight RTK base against the other static base, and then translate the RTK base to the correct position. Recompute, and then all of the observations are on the correct system.
Agreed. I should have said that we also use 15 minutes minimum.
GPS on top of the instrument is very handy when you are in wooded areas (such as corridors cut through tree areas for powerlines) as it means you can run an open ended side traverse into an area where you need information and get a fix on a single traverse station in a clearing to terminate it. This sometimes saves a bit of time(useful at the end of the day when darkness is falling and you realise that you don't want to start from there tomorrow!)
Chris. I would suggest the base/rover setup with cell and radio in each. Then you could do radio when cell coverage is bad and you get the 25 mile range with cell on a mobile base station. Best of both worlds. I don't think the VRS hype is worth the extra money.