Baselines and GPS Equipment

The S6 can of course be checked on a baseline, I do it on a regular basis. It will work without station setup if you go into survey basic (or something like that, I can't remember exactly what it is called). Or, you can just setup and give it bogus coordinates and a 0° azimuth. Either way, the most important thing is to have the CORRECT temperature and pressure (station pressure, not sea level), as well as the CORRECT prism offset. Note that the S6 has an on board barometer, which I have found to be pretty accurate. An error or 1° C causes 1 ppm error in distance, as does 0.1" of pressure. You can determine the actual prism offset using a fairly simple 3 point baseline.

As for GPS, it gets its scale (and orientation) from the ephemeris and ultimately from the tracking station's coordinates. I don't see any benefit at all from checking GPS on a baseline.

I have my own baseline out in the front yard - so yes I check equipment regularly.

Checking GPS is a whole different subject. If your GPS collects data and you post process it what do you check? The real issue is the satellites, environment, processing software, etc. You need to pay careful attention to your output reports and learn what to look for. As was suggested yesterday, if you are worried about a specific point you need to do multiple checks over multiple days.

I realize what I'm about to write is unorthodox, but hey, it's a bold new world. If you are consistently getting agreeable results from your gps and total station, I see no need for baseline checks. GPS vectors have become so precise and the methodology so different between GPS and terrestrial observations that having agreement between them is as close to a guarantee of true accuracy as we are likely to ever have.

That is true-I very frequently mix GPS and conventional observations in an adjustment. I can tell right away if there is any problem with the S6, or an incorrect prism, etc. So, I do not feel the need to check the EDM as frequently as in the past. I used to go to a NGS baseline that was quite a ways from here, sometimes when I had work in that area.

Now, I have a 2 pillar baseline on my property that can be used for a quick check. It will verify the scale of the EDM, or the offset, but not both.

I don't disagree Shawn, but some states still require a periodic baseline check of equipment.

I think that horizontal positions are 'in the bag' with GPS.

However, I also think that elevations can be a mixed bag. I try to set a checkpoint at the same elevation as my base point on every job (RTK or Static) and check-in. Just to make sure that the antenna model, the HI and if more than one rover all the rovers are okay.

When I switch from CMR+ to RTCM3 I am always worried that the base offset will get dinked around with.

I would think that a periodic elevation check would be a great calibration check for GPS. Well worth the time to perform.

M

Several years ago on the "other" site someone said that RTK GPS wasn't accurate within a tenth (or something like that), so I was off to our local baseline. My unit was very accurate, both horizontally and vertically. I also regularly compare my control points set by GPS with the later reflectorless distances, and they match very well.

That is where I am. Starting a CDOT job next week and baseline check is part of the requirements.

S6 was easy peasy. Same as "we always did it".

For GPS, I set the base on each of the four monuments and shot the other three with the rover @ 180 epochs each. I had a minimum of 16 satellites and as many as 22, so my dops are all good.

I was just wondering if anybody had developed a "proper procedure" or knew of one.

ahh. Got ya.

What a great website for gaining insights into how others work and the requirements they operate under.

Sounds like you have a good plan there.

Very good point, Mark.

Verticals can be tricky with GPS. Not necessarily because verticals are a terrible weak component (they are a little weaker than horizontal) but because some software (data collectors and post processing software) trips over itself with how it handles instrument heights. Getting the rover close to the base to see if the vertical looks reasonable and occasionally catching a total station shot between a rover and base position to check for vertical errors are good practices in my view. As in most error trapping, big errors jump out pretty quickly, it's the little ones that sleep until the worst possible moment.

GPS on an official baseline may satisfy a requirement, and can be a good check for repeatability, but is still missing some important checks.

I'd say go to a point that has NGS data sheet GPS data and see how closely you can repeat it. That checks antenna height and how the software is treating datum/geoid information.

At least one calibration base line in Massachusetts (Marlborough) has two stations (the 0 and 1300 meters) that are published in the NGS database.

:good:

[sarcasm]As long as I don't hose the site calibration on the actual job.[/sarcasm]