6 ms·

As an example of a problem that can occur: Client wants to survey the elevations of a parcel of land for building a house. The surveyor goes out and collects

As an example of a problem that can occur: Client wants to survey the elevations of a parcel of land for building a house. The surveyor goes out and collects data with gps survey equipment, which produces a .csv file of northing,easting,and elevation values relative to some National Geodetic Survey benchmark. The engineer imports this .csv into his cad software, produces a grading plan .csv file. The earthmoving contractor imports the grading plan into his software and grades the site. Client gets house built, moves in and discovers that the front yard turns into a swamp after every rain. Somewhere in the chain of importing and exporting .csv files, software A was using survey feet and software B was using international feet. This can also cause problems like structures built in the wrong spot, fences built on neighbors land, etc.

abofh 7y ago
The difference is .0002%. your examples would barely add up to an inch over a mile.
- opwieurposiu 7y ago
  Sometimes the benchmark for northing/easting/elev is many miles away from the actual site. This is especially true with gps surveys. It only takes a few inches to wind up with a substantial drainage problem.
  - function_seven 7y ago
    Wouldn't the elevation difference between benchmark and site be the important bit here? If so, then the difference would rarely be more than a mile, and the resulting error would be at most an 1/8"
    - beerandt 7y ago
      It's that you're in the wrong e, n location, which may be at an entirely different elevation than where you want to be.
    - opwieurposiu 7y ago
      It has been 20 years since I worked in this area so likely I have some details wrong. If elevation is distance to the center of the earth 1/8" per mile is 40 feet! From the link below you can see the noaa benchmark is using both elevation in feet above sea level and in meters from the center of the earth. https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=aa3449 https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=aa3449
      - beerandt 7y ago
        If you're not converting H (gps orthometric height) to E (local elevation datum to "sea level") first, you're doing it wrong, for a number of practical reasons. Usually there's not even a reason to use H directly anymore, since elevations should reference a local benchmark. GPS has changed things, but surveyors are still obscenely practical when it comes to procedures for eliminating systematic error.
        1-6 7y ago
        Using an RTK-GNSS requires a gravitational model of the earth to measure mean sea level (MSL). Using your position, you use the GEOID model to figure out the delta of the nav satellite's orbit to the MSL.
        beerandt 7y ago
        Yes, That's what I mean by going straight from H to e first. The geoid is that conversion. Although for rtk, the delta isn't to any of the sats, it's to your base station. The deltas to the sats cancel out of the equations, which is where most of the gains in accuracy come from. But it still needs to be tied into a known elevation benchmark. That said, most field crews will still be shooting a differential to a published benchmark, or plugging the geoid file into the data collector, or dialing into a vrs or cors that already is adjusting for the geoid. For them, H is just an extra data point asking to be plugged into the wrong data field. Unless you're doing static observations or geodesy or manual network adjustments or direct gravity readings, the new way (22) isn't all that different from the old way (83). There's just different underlying theory of MSL and different math to adjust the network. And if you are doing any of those things, then you generally already know what you're doing.
    - 1-6 7y ago
      We're not even getting to the topic of elevation (yet). The US Survey Foot is 1200/3937 which is a not a clean value that terminates after a few decimal places. With high grid coordinates (X,Y), your origin is based at 0,0 as origin. When you scale between metric and US Imperial, if you're not using enough significant digits it impacts how the model scales. This can cause a difference of 1 foot or more from my experience. Floating point operators also become an issue especially in CAD. Standardizing to one number is a great start.
  - beerandt 7y ago
    There's a ppm error there as well, but that's a usually a different problem. If different units were used, it would be obvious when checking in on the benchmark. If it's in northing and easting, the baselines are to the South and West of all coordinates covered by the projection, plus an offset. So it might start at 1000000 instead of zero, to ensure format consistency and to catch blunders. So it's about 2ppm, multiplied by the number of feet between you and the Southern or Western most edge of your state (if single projection state) plus 10^5 or 10^6 or similar. It's designed to make an error large enough to notice. Vertical however is rarely a problem, as it's all based off local benchmarks.
dmtroyer 7y ago
seems like an issue that surveyors and engineers should be well acquainted with.
- drdeadringer 7y ago
  How often does a "should" translate into reality?
  - 1-6 7y ago
    It's entirely possible that they might not be aware. Surveyors are concerned with measuring bearing and distance from an arbitrary location (5000,5000) in some cases. The distances they traverse are not enough to see differences between US Survey foot and Int'l Feet. Cartographers and GIS folks who tie into a photogrammetric basemap are probably more aware. Most Civil engineers are like 'NAD83? Ok, whatever you say~!'
    - Enginerrrd 7y ago
      This is true. I barely understand coordinate systems enough to get by and somehow I know them better than any other engineer I've talked to. Surveyors are 50/50 on whether they know all that stuff or not. Really though, there's no excuse for errors caused by that stuff. Survey data should almost always only be used relative to itself.
clucas 7y ago
I worked as a CAD operator, surveyor, and occasional field crew worker at a civil engineering firm for a few years. We never ran into issues like this - by far the more common issue was the difference between NAVD and NGVD. At the time I was surveying, the FEMA flood insurance maps in our area were only partially updated, so the base flood elevation could be in either datum, and you had to really watch it so you didn't end up about 0.7' off vertically.

abofh 7y ago

The difference is .0002%. your examples would barely add up to an inch over a mile.

opwieurposiu 7y ago
Sometimes the benchmark for northing/easting/elev is many miles away from the actual site. This is especially true with gps surveys. It only takes a few inches to wind up with a substantial drainage problem.
- function_seven 7y ago
  Wouldn't the elevation difference between benchmark and site be the important bit here? If so, then the difference would rarely be more than a mile, and the resulting error would be at most an 1/8"
  - beerandt 7y ago
    It's that you're in the wrong e, n location, which may be at an entirely different elevation than where you want to be.
  - opwieurposiu 7y ago
    It has been 20 years since I worked in this area so likely I have some details wrong. If elevation is distance to the center of the earth 1/8" per mile is 40 feet! From the link below you can see the noaa benchmark is using both elevation in feet above sea level and in meters from the center of the earth. https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=aa3449 https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=aa3449
    - beerandt 7y ago
      If you're not converting H (gps orthometric height) to E (local elevation datum to "sea level") first, you're doing it wrong, for a number of practical reasons. Usually there's not even a reason to use H directly anymore, since elevations should reference a local benchmark. GPS has changed things, but surveyors are still obscenely practical when it comes to procedures for eliminating systematic error.
      - 1-6 7y ago
        Using an RTK-GNSS requires a gravitational model of the earth to measure mean sea level (MSL). Using your position, you use the GEOID model to figure out the delta of the nav satellite's orbit to the MSL.
        beerandt 7y ago
        Yes, That's what I mean by going straight from H to e first. The geoid is that conversion. Although for rtk, the delta isn't to any of the sats, it's to your base station. The deltas to the sats cancel out of the equations, which is where most of the gains in accuracy come from. But it still needs to be tied into a known elevation benchmark. That said, most field crews will still be shooting a differential to a published benchmark, or plugging the geoid file into the data collector, or dialing into a vrs or cors that already is adjusting for the geoid. For them, H is just an extra data point asking to be plugged into the wrong data field. Unless you're doing static observations or geodesy or manual network adjustments or direct gravity readings, the new way (22) isn't all that different from the old way (83). There's just different underlying theory of MSL and different math to adjust the network. And if you are doing any of those things, then you generally already know what you're doing.
  - 1-6 7y ago
    We're not even getting to the topic of elevation (yet). The US Survey Foot is 1200/3937 which is a not a clean value that terminates after a few decimal places. With high grid coordinates (X,Y), your origin is based at 0,0 as origin. When you scale between metric and US Imperial, if you're not using enough significant digits it impacts how the model scales. This can cause a difference of 1 foot or more from my experience. Floating point operators also become an issue especially in CAD. Standardizing to one number is a great start.
- beerandt 7y ago
  There's a ppm error there as well, but that's a usually a different problem. If different units were used, it would be obvious when checking in on the benchmark. If it's in northing and easting, the baselines are to the South and West of all coordinates covered by the projection, plus an offset. So it might start at 1000000 instead of zero, to ensure format consistency and to catch blunders. So it's about 2ppm, multiplied by the number of feet between you and the Southern or Western most edge of your state (if single projection state) plus 10^5 or 10^6 or similar. It's designed to make an error large enough to notice. Vertical however is rarely a problem, as it's all based off local benchmarks.

dmtroyer 7y ago

seems like an issue that surveyors and engineers should be well acquainted with.

drdeadringer 7y ago
How often does a "should" translate into reality?
- 1-6 7y ago
  It's entirely possible that they might not be aware. Surveyors are concerned with measuring bearing and distance from an arbitrary location (5000,5000) in some cases. The distances they traverse are not enough to see differences between US Survey foot and Int'l Feet. Cartographers and GIS folks who tie into a photogrammetric basemap are probably more aware. Most Civil engineers are like 'NAD83? Ok, whatever you say~!'
  - Enginerrrd 7y ago
    This is true. I barely understand coordinate systems enough to get by and somehow I know them better than any other engineer I've talked to. Surveyors are 50/50 on whether they know all that stuff or not. Really though, there's no excuse for errors caused by that stuff. Survey data should almost always only be used relative to itself.

clucas 7y ago

I worked as a CAD operator, surveyor, and occasional field crew worker at a civil engineering firm for a few years. We never ran into issues like this - by far the more common issue was the difference between NAVD and NGVD. At the time I was surveying, the FEMA flood insurance maps in our area were only partially updated, so the base flood elevation could be in either datum, and you had to really watch it so you didn't end up about 0.7' off vertically.