Robotic Total Station Vs. GPS Surveying 1

[JP12]

Anybody have any experience or feedback using a robotic total station Vs. using GPS for surveying (with the GPS we would have access to the CORS network here in NY, really no base station required). We are an agricultural engineering firm looking to have a one man crew to do topographic surveying for base mapping. Any suggestions would be helpful. Thanks.

 

[cg719]

From my experience with both that i recieved during a summer internship, gps is great for open areas and works best when you are using a local base and a rover (1 gps on a known point and 1 gps with the surveyor). this is the most accurate set up.

also gps very useful when used for setting up control

however gps is useless when under tree cover or when close to buildings.

hope this helps. let me know if you have any more questions

 

[civilperson]

Cheap crew of one person is very expensive when the man walking around discovers the vehicle is gone!

 

[dik]

On a current project, the Contractor is using a robotic total station with 3 min accuracy... can't improve on that by much!

Dik

 

[SkyCapitan]

I agree with cg719 regarding tree cover and buildings. Sometimes the gps will find a way to connect under tree cover, but if it doesn't you're out of options.

Also, I was always under the impression that a total station is more accurate if you care about measuring elevations within more than a half inch tolerance.

 

[cg719]

yes, a total station is more accurate for shooting elevations, but if you are just doing topo, getting dead accurate isnt always crucial.
and if you are about to get under tree cover, you can always set 2 points using the gps for a backsight and occupation point, and then all the shots taken with the total station will be on the same coordinate system as the ones you shot earlier with the gps.

 

[dik]

sorry... that should have read 3 second... Dik

 

[DanGalbraith]

The GPS process you are looking at is termed Real Time Kinematic (RTK) GPS. The stated specification in the horizontal is generally in the 5mm (0.017’) range and 15mm (0.05’) range vertically. Please realize that the specifications are only a single part of the error budget, there are several others such as rod measure ups, length of baseline, multi-path and so on.

One Man total station surveying is performed with a Robotic Total Station.

As a rule of thumb, RTK will get you about 0.1’ of accuracy if (big if) there are no obstructions. Trees are a very common source of error and even if you get a lock under the trees, then you must contend with multi-path errors. Multi-path is when the radio signal is received directly by the GPS unit and also a secondary signal which bounces off of other objects. RTK next to buildings and fences are also big issues here.

The length of the baseline further degrades the value of the solution. With VRS you generally have very long baselines.

The vertical element of RTK is the weakest.

Robots are generally very precise and while they do have a line of sight limitation, there are no issues like tree canopies or shot placement next to buildings to contend with.

Depending on the budget, the first equipment to purchase would be a Robot, the second RTK GPS. At least that is how I did it. I am a consulting Land Surveyor and have quite a bit of time doing One Man applications.

 

[jheidt2543]

Dan:

What is the learning curve on these systems?

 

[DanGalbraith]

The learning curve for running the RTK unit is usually very quick, say 1/2 a day. The actual time to produce efficient reliable work might take a few weeks.

How the survey is set up and the checks we build into the survey are one of the keys to quality work. By example, redundant observations to stationary points throughout the survey is very important.

RTK can be likened to an angular side shot. It is a single observation and there is generally no raw data that is collected. Post processing the survey requires raw data.

There are more precise/accurate GPS processes, but the learning curve and expense of the equipment are significanlty increased.

Robotic total stations depend on the operator and if the operator has any experience with stanard total stations.

I started my business 5 1/2 years ago and had a bunch of time getting back in the groove as it regards field instruments. Once I got the robotic capability it took me a few weeks to feel really comfortable with it.

Robots require a direct uninterupted line of sight(I'll call this lock). The robot transmits a laser to the prism. The measurements are transmitted via radio to the data controller. When (not if) the line of sight in interupted we loose lock. Depending on the local conditions regaining lock can be quite the chore.

I did a one man stakeout for a 5 story parking deck a few summers ago. 3 sides of the site had either active vehicular travel or was at grade parking. Lots and lots of reflections off of wind shields. It was very challenging to regain lock as the robot tried to lock onto wind shields and occasionally locked on to a moving vehicle. It will take working in hostile environments to learn how to deal with some things. By the way, the job went great and the entire thing was staked with a 0.02' tolerance horzintally and vertically.

If I had to choose one over the other I would go robotic all the way. In open country with no trees to obstruct the RTK and no wind shields to obstruct the robot both of the processes are about equal in speed with the robot being more precise.

 

[jheidt2543]

About five years ago I talked with a guy who did nothing but construction surveys, mostly building layout and control. He told me he had the best luck setting up his RTS and control points 10' or so above the ground level on long term berms or platforms. That way he shot over most of the work going on. I'm not sure how practical that always is, but it seemed to work for him. Working around active traffic is difficult regardless of the equipment.

 

[DanGalbraith]

That’s good procedure. Anything we can do to increase our sight distance and decrease obstructions is good.

There are differences in the equipment as well. Some manufacturers have an active tracking system where there is a transmitter/receiver mounted on the instrument and the rover rod. These types of systems are said to have a shorter operating range.

One system has a target recognition system in a passive environment. The prism dimensions and other characteristics are programmed into the tracking system which allows the instrument to differentiate the desired prism from other background noise. This system has a 500 to 600m robotic operating range and reflectorless range of 1000m.

I’ve recently purchased the latter system and will put it to the test very quickly.


Daniel E. (Dan) Galbraith

https://www.gulfsurveyors.com/

 

[InDepth]

Call this guy from SOLDATA

Daniel Kippelen
General Manager

Sol Data Inc.
12 McClane Street
Cuddy, PA 15031
Tel: +1 412 677 2292
Fax: +1 412 202 0618

Mobile: +1 412 526 1345

http://www.soldatainc.com