Who believes their data collectors?
I have been using a TDS Ranger with Survey Pro software v. 4.11.2 for all my survey jobs since using GNSS receivers. Most of my projects are small (within 3 mile rtk range) using a single base. Most of my projects start from an autonmous base position using assumed ground coordinates (10K, 10K, 100). Elevations never vary more than 100′ even on the edge of the 3 mile. Since most projects are usually in 2D i seldom use geoid model. All sec cor’s,control pints etc are occupied 1-2 min
Sometimes i take the total station out to check a few points that are visible and find that total station distances vary more than i would expect using the inverses produced by data collector coordinates. Is the data controller using the correct origin scale in order to produce ground coordinates?
- 49 Replies
> ….total station distances vary more than i would expect…
You talking hundreths? tenths? or feet?
Its not the data collector giving you the variables, its the GPS data. Or so goes my guess.
We have not yet invoked the power of RTK in our office. In part due to the fact that I have not satisfied myself that it will always provide me with defensible data. I am curious how well GPS would stand up to traditional surveying in a court of law were the minutia of measurement to become a factor in a dispute.
I would recon that your ground data is superior and dont be staking any building corners via the GPSvaries from a few hundreths to a few tenths per 1/2 mile
Do you average epochs? how many? … if you don’t you need to.
Also what are your acceptance precision tolerances set on?
Is the data controller using the correct origin scale in order to produce ground coordinates?
I can’t say from what you have posted, but;
Probably not.Typically, there should be very little difference between RTK GPS (when done properly with multiple observations) and the total station. You need to be sure your GPS and your total station are comparing apples to apples.
It may be that you are projecting to the ellipsoid in GPS and seeing ground distances with your total station. Is the total station distance always longer?
Take your raw Lat, Long and ellipsoid height for two points and calculate the distance between them in your software package using different projections. Hopefully, the software allows you to see the ellipsoid distance and a ground distance. You should be able to puzzle it out.
I would suggest learning how to set up a LDP for these small sites. That will put your GPS on (or very close) to ground distances.
Compare your procedures with those in the linked document. In a nutshell – redundancy, occupation time, multipath, goes double for your base.
Or could you be referring to a difference between inverses on the grid and ground level measurements with your TS?
I set my key control points with RTK, five shots per point, in relatively open areas. I’ve kept track over the years and the difference between RTK and my total station on control points is always under 0.025 feet, and averages 0.009 feet horizontally. Vertical is roughly twice that. If you are getting anything like a tenth discrepancy in open areas something is wrong. Scale factor, whatever, something is wrong.
I am curious how well GPS would stand up to traditional surveying in a court of law were the minutia of measurement to become a factor in a dispute.
Quote from 1972
I am curious how well a distance meter reading would stand up to traditional surveying in a court of law were the minutia of measurement to become a factor in a dispute.Just sayin’
We use RTK and robotics with data collectors for everything…once in a while an OPUS Solution, but usually VRS to get us on State Plane Coordinates.
I trust my business with it…usually RTK GPS pairs check within a few hundredths…I’d say 0.03′ on average.
It sounds like you might have a setting wrong in the collector (scale factor, US Survey Feet, etc.). When everything is properly calibrated, checks are very close.
Are you sure your total station is correct?
And as Loyal mentioned here just the other day, many users of total stations don’t even own a thermometer and barometer.
Or, if they do attempt to account for barometric pressure, they use what the weather report calls barometric pressure which is somehow adjusted to sea level and is always close to 30 inches mercury, even at 7000 feet elevation where the pressure is much closer to 23 inches mercury. Sure wish the weather-report folks would call that number they report “barometric index” or something similar, because it definitely is not an actual pressure.
Sounds like you need to take all your gear to the nearest EDMI calibration base line and see how your GNSS and total-station measurements compare with the published distances.
GB
Are you sure your total station is correct?
Glenn,
I have heard many surveyors say they just set the PPM correction on the total station to ZERO as if that is some sort of average. I ask if they only measure on 60°F days when the pressure is at 30 inchHg. Blank stare. It is both an attitude and ignorance issue with many surveyors.
J.
Are you sure your total station is correct?
THAT IS AN EXCELLANT IDEA! I LIVE WITHIN 100 MI FROM 2 BASELINE COURSES
Are you sure your total station is correct?
> And as Loyal mentioned here just the other day, many users of total stations don’t even own a thermometer and barometer.
>
> Or, if they do attempt to account for barometric pressure, they use what the weather report calls barometric pressure which is somehow adjusted to sea level and is always close to 30 inches mercury, even at 7000 feet elevation where the pressure is much closer to 23 inches mercury. Sure wish the weather-report folks would call that number they report “barometric index” or something similar, because it definitely is not an actual pressure.
>
> Sounds like you need to take all your gear to the nearest EDMI calibration base line and see how your GNSS and total-station measurements compare with the published distances.
>
> GB:good:
Are you sure your total station is correct?
And naturally you will check adjustment of the tribrachs, level vials on poles, and all those things before loading the buggy, right?
Good luck.
GB
Are you sure your total station is correct?
Right on Jerry. I am old enough to remember even chaining corrections. I am just not a geodesy surveyor and therefore i am very skeptical about what’s coming out from a data collector
Are you sure your total station is correct?
Glenn, that’s just what I was thinking. I know if my rtk and robot disagree by more than 4-5 hundredths I’ve got to adjdust my bubbles.
> Quote from 1972
> I am curious how well a distance meter reading would stand up to traditional surveying in a court of law were the minutia of measurement to become a factor in a dispute.
>
> Just sayin’I agree that this same argument was very likely in 1972 and up through the mid 80s but I would add that it is still under consideration and potentially relevant today in some very rare cases. Albeit even a chaining correction can come into question. What takes some bite out of your point are the numerous uncontrollable, immeasurable, and often unnoticeable conditions such as sun spots, frequency interference, PDOP, signal delays and loss of coverage (when using a cell phone or other comm for acquiring corrections), and the fact that most surveyors (myself included) have no clue what sort and amount of data comes down from the heavens to our little magic box. I know, I know, EDMs are not infallible and can provide anomalous measurements due to heat refraction off of roads; signals ‘bending’ around small branches, trees, and bld corners; faulty equipment and incorrect prism offsets; etc. The difference between these and those inherent to GPS is that the EDM errors are noticeable by, and therefore corrected for by the operator and can otherwise be mitigated and eliminated through proper field procedures.
Are you sure your total station is correct?
If I am ever cross examined in court by an aggressive attorney and challenged by an unfavorable expert witness, I understand more than enough about what my total station is doing to allow me to put up a substantial and defensible position. How many of you riding the RTK bandwagon would be able to say the same? Are your procedures solid enough to explain to a jury how you know your GPS data is and was accurate? Do you truly understand enough to sit and be cross examined by an attorney who questions your understanding of the sheer amount of math involved in calculating a GPS signal return? How easily could a hired-gun expert touting a PhD tear your data to shreds? I am not questioning RTK so much as I question those who use it and their procedures and blind reliance upon it. For me, the vegetative state of my region coupled with my level of discomfort offers far too many questions than solutions. Hell, I still question the validity of my post-processed static data considering the numerous answers I can achieve by simply changing a few minor elements in the software. Even if your data is correct and wonderful and perfect you’d better hope your GPS does not become the ‘glove that does not fit’.
Are you sure your total station is correct?
Are your procedures solid enough to explain to a jury how you know your GPS data is and was accurate?
Yes
Do you truly understand enough to sit and be cross examined by an attorney who questions your understanding of the sheer amount of math involved in calculating a GPS signal return?
Just as much so as with any other technical aspect of surveyingHow easily could a hired-gun expert touting a PhD tear your data to shreds?
Not veryMy data collector told me not to listen to you.
Stephen
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